Compact neutron sources for Nuclear Physics: from accelerator-based to laser-driven neutron beams

Compact neutron sources for Nuclear Physics: from accelerator-based to laser-driven neutron beams PhD dissertation of: María de los Ángeles Millán Callado Dpto. Física Atómica, Molecular y Nuclear Centro Nacional de Aceleradores Programa de doctorado en Ciencias y Tecnologías Físicas August 2023 Advisors: Dr. Carlos Guerrero Sánchez Dr. Begoña Fernández Martínez Tutor: Prof. José Manuel Quesada Molina Para Adolfo. Sin él no hubiera emprendido este Viaje. Cuando emprendas tu viaje a Ítaca pide que el camino sea largo. lleno de aventuras, lleno de experiencias. No temas a los lestrigones ni a los cíclopes ni al colérico Poseidón, seres tales jamás hallarás en tu camino, si tu pensar es elevado, si selecta es la emoción que toca tu espíritu y tu cuerpo. Ni a los lestrigones ni a los cíclopes ni al colérico Poseidón encontrarás, si no los llevas dentro de tu alma, si no los yergue tu alma ante ti. Pide que el camino sea largo. Que muchas sean las mañanas de verano en que llegues -¡con qué placer y alegría!- a puertos nunca vistos antes. Detente en los emporios de Fenicia y hazte con hermosas mercancías, nácar y coral, ámbar y ébano y toda suerte de perfumes sensuales, cuantos más abundantes perfumes sensuales puedas. Ve a muchas ciudades egipcias a aprender, a aprender de sus sabios. Ten siempre a Ítaca en tu mente. Llegar allí es tu destino. Mas no apresures nunca el viaje. Mejor que dure muchos años y atracar, viejo ya, en la isla, enriquecido de cuanto ganaste en el camino sin aguantar a que Ítaca te enriquezca. Ítaca te brindó tan hermoso viaje. Sin ella no habrías emprendido el camino. Pero no tiene ya nada que darte. Aunque la halles pobre, Ítaca no te ha engañado. Así, sabio como te has vuelto, con tanta experiencia... As you set out for Ithaka hope your road is a long one, full of adventure, full of discovery. Laistrygonians, Cyclops, angry Poseidon—don’t be afraid of them: you’ll never find things like that on your way as long as you keep your thoughts raised high, as long as a rare excitement stirs your spirit and your body. Laistrygonians, Cyclops, wild Poseidon—you won’t encounter them unless you bring them along inside your soul, unless your soul sets them up in front of you. Hope your road is a long one. May there be many summer mornings when, with what pleasure, what joy, you enter harbors you’re seeing for the first time; may you stop at Phoenician trading stations to buy fine things, mother of pearl and coral, amber and ebony, sensual perfume of every kind— as many sensual perfumes as you can; and may you visit many Egyptian cities to learn and go on learning from their scholars. Keep Ithaka always in your mind. Arriving there is what you’re destined for. But don’t hurry the journey at all. Better if it lasts for years, so you’re old by the time you reach the island, wealthy with all you’ve gained on the way, not expecting Ithaka to make you rich. Ithaka gave you the marvelous journey. Without her you wouldn’t have set out. She has nothing left to give you now. And if you find her poor, Ithaka won’t have fooled you. Wise as you will have become, so full of experience... i Acknowledgements / Agradecimientos Cuando inicié este Viaje supe que no lo haría sola. He tenido el privilegio, a lo largo de todos estos años, de haber estado rodeada de personas maravillosas que, cada una a su manera, han conseguido que este trabajo haya sido posible. Algunas me han acompañado desde el principio y a otras he tenido la suerte de conocerlas a lo largo del camino. Ha sido una etapa importante en la que he crecido en muchos aspectos, tanto profesionales como personales, y en gran parte ha sido gracias a ellos. A las experiencias compartidas, a las conversaciones mantenidas, al apoyo, al consejo, a la mano que me han tendido cuando la travesía se volvía más turbulenta. Y aunque no hay papel suficiente para reflejar todo lo que siento, con estas palabras quiero, al menos, reconocerles y agradecerles su valiosa contribución. Sin ellas no habría viaje, ni camino, ni barco... ... sin ellas no habría Ítaca. Quiero empezar dando las gracias a mis directores de tesis, Carlos Guerrero y Begoña Fernández, y a mi tutor, José Manuel Quesada, por darme la oportunidad de hacer este trabajo, pero, sobre todo, por la confianza que me han depositado en todo momento (incluso abriéndome las puertas de su casa cuando aún era una extraña). He aprendido mucho de ellos, no sólo a nivel académico y profesional. Me han ayudado a crecer como persona, a ser más madura, a poner en valor mi trabajo. Han creído en mí incluso cuando yo misma dudaba. Quiero darles las gracias por toda la comprensión, el cariño y la paciencia. Por la mano amiga y las risas, que no están reñidas con la profesionalidad. Por crear la confianza para poder acudir a ellos en todo momento, ya fuera profesional o personalmente. Y aunque a veces ha sido inevitable tener puntos de vista diferentes, siempre han puesto en valor mi opinión y mi criterio. Gracias por guiarme, por abrir el camino, pero dejar que lo recorriera sola, por haber sido un faro durante todos estos años (y los que quedan por llegar). Throughout this journey, I have also had the opportunity to meet people from all over the world. An enriching and exciting experience. And although sometimes it can be scary to find yourself alone in a foreign country where you don’t know the language, I have been lucky enough to meet ii |Acknowledgements / Agradecimientos wonderful people who have made me feel welcomed and supported. I would like to start with the people at HZDR in Dresden, Germany, where much of this work has taken place and who, without a doubt, have been the cornerstone of everything that has been built here. I would especially like to thank Dr. Arnd Junghans and Prof. Ulrich Schramm for believing in this project when we presented it to them. For supporting it in all aspects: providing the work centre, part of the equipment, and granting us the help of the magnificent working group they have behind them. I would like to thank Roland Beyer, Sebastian Urlass, Florian Kroll, Karl Zeil, Irene Prencipe, Stefan Assenbaum, Josefine Metzkes-Ng, and the rest of the Laser Particle Acceleration and Nuclear Physics teams for all their effort and dedication, for the long hours spent at the laser and the accelerator, for answering all my questions and solving all my doubts, for the shared coffees, the laughs, and for all the work they did for me. And thanks to all of you who have continued to work with me afterwards, from afar, through countless emails and virtual meetings, right up to the last minute. Thank you, Flo, for your feedback on the writing of this manuscript. I can’t forget to mention the people of TUD: Stefan Scheuren and Jonas Kohl. We have travelled much of this journey together and they know that it has not always been easy to be "the students". Thank you for the teamwork, for all that you have contributed to this analysis, and especially for being much more than colleagues. Thank you for the beers and the confessions (in Germany and in Spain), and thank you for this friendship that goes far beyond the work that is reflected on these lines. In the same vein, I would like to thank Dr. Christina Weiss and Dr. Erich Griesmayer for providing us with the "crown jewel" of this work: the diamond detector. Also for working with us side by side in setting up the detector, fighting with the electromagnetic noise, teaching me how to work with this equipment and these electronics that I had not faced before, for following the whole analysis process. For your valuable advice and recommendations. Quiero agradecer también a mi familia científica gallega. Aquellos con los que empecé la aventura de los láseres y con los primeros con los que tuve el placer de asomarme al maravilloso mundo de la óptica y los plasmas: el equipo de laboratorio L2A2 de la Universidad de Santiago de Compostela. Muchísimas gracias a José Benlliure y a Dolores Cortina por abrirme las puertas de su “casa” científica y permitirme formarme en ella. Muchas gracias por esta maravillosa y fructífera colaboración y por habernos acompañado dentro y fuera de España con vuestros equipos, con vuestra gente, con vuestra experiencia y toda vuestra hospitalidad. Muchas gracias a todo el equipo de Santiago de Compostela, especialmente a Juan Peñas, Lucía Martín y Aaron Alejo, que son el claro ejemplo de que el trabajo y el placer son perfectamente compatibles. Las estancias en Santiago de Compostela las guardo con un cariño especial por todo lo que me han aportado profesional y personalmente. Algún día veremos neutrones en STELA (y podremos hacernos el París-Dakar). |Acknowledgements / Agradecimientos iii I want also to mention the people from the n_TOF collaboration and the people that I met in Geneva, especially to Enrico Chiaveri for supporting the research stay. Although this work has not been developed there, the stay in n_TOF, at the very beginning of my phD, was really useful as training in neutron production and detection, and also for providing me with valuable tools and skills to work in a highly experimental environment. I want to thank Federica Mingrone, Toby Wright and the STEFF team, and César Domingo and the IFIC team for taking me on their experiments. Also thank Massimo Barbagallo for being my contact person, being the first to receive me, and to take care of me during the whole stay. A mis españolas, Marta y Bea, por recibirme en su casa, llevarme a hacer turismo, darme de comer, cuidarme y esforzarse porque no me sintiera sola en ningún momento. I want to thank all the beautiful people I met there: Adhitya, Sam, Sebastian, Veatriki, Zina, Mario, Thanos, Michi, the Victors, Laurent, Luis, Ion, and the rest of this marvelous crew. Quiero darle las gracias también a Ariel, por haber sido una enriquecedora constante desde aquel primer congreso de protonterapia y después el CERN, los CPAN, la escuela de HiSPANoS, Salamanca, Murcia... Gracias por las interesantes conversaciones, tus preguntas en cada charla, las ideas, los consejos y el intercambio de puntos de vista. Vuelvo a “casa” para dar las gracias al departamento de FAMN de la Universidad de Sevilla, donde siempre me he sentido querida, recibida y bien acogida. Dar las gracias por esas conversaciones en los pasillos, las pausas para el cafelito, la hora de las comidas y esas puertas, siempre abiertas, a las que poder acudir. Gracias a Miguel Cortés por su ayuda con Geant4 y por aceptar que irrumpiera en cualquier momento con una libreta en su despacho porque no me funcionaba tal o cual script. También agradecer a José Miguel Arias por su inestimable ayuda y todo su apoyo a lo largo del camino. Muchas gracias a Maribel, por su cariño y su ternura, por sus historias maravillosas, por dejarme compartir con ella nuestra pasión por la divulgación y por haberme arropado tanto desde que llegué a Sevilla. Muchas gracias a Miguel Ángel Respaldiza por haberme dado la oportunidad de unir mis dos pasiones y por toda la confianza que ha mostrado en el proyecto de radiografía, que tan personal es y tantísimo significa para mí. Quiero dar especialmente las gracias a Joaquín Gómez por haber tenido la parte tal vez menos gratificante, pero sin duda esencial, de este trabajo con la gestión de los proyectos de investigación, el papeleo y la burocracia. Gracias por la paciencia infinita que ha tenido en ese aspecto conmigo, con mis preguntas siempre a última hora (soy un desastre con las deadlines) pero, sobre todo, gracias por las constantes palabras de ánimo y por esa conversación en la fuente de agua que llegó cuando más la necesitaba. Quiero extender mi agradecimiento a toda la gente del CNA, mi otra “casa” durante estos años. A Javier Ferrer, por arrancarnos siempre una sonrisa en los cafés y las comidas, por siempre acordarse de nosotros con sus pastelitos y dulces, por las interesantísimas charlas, por su compañía y sus siempre acertados consejos. A las nuevas adquisiciones del grupo, Pablo y David, iv |Acknowledgements / Agradecimientos a los que les deseo todo lo mejor en el futuro. A Enrique, aunque ahora haya emprendido su propio viaje en la otra punta de España, por haber sacado el trabajo de imagen adelante cuando yo ya no podía sola. Gracias por todo lo compartido en estos años. A los técnicos del tándem: Ángel Romero, Miguel A. Seller y David Pascual, por su excelente trabajo, por su implicación, por las horas invertidas, por su buena disposición a pesar del cansancio y por esos maravillosos pulsos de HiSPANoS que no serían posibles sin sus manos. A todos aquellos que, a pesar de no haber realizado un trabajo científico, hacéis que todo funcione, que todo salga adelante y que todo sea posible. Nadia, Ento, Patricia y el resto de administración por toda su ayuda con las dietas, los viajes y el papeleo. A Mercedes, Teresa, Guti, Luis, Enrique, Jesús y Paco, por tener siempre una palabra amable y un gesto de cariño, porque gracias a ellos siempre empiezo y acabo la jornada con una sonrisa. Quiero dar las gracias a las nuevas amistades que he podido hacer en Sevilla, que me recibieron con los brazos abiertos en su “hermandad” desde el primer día y que se han vuelto incondicionales e imprescindibles desde entonces. En primer lugar, a mis “hermanos mayores” científicos, a Jorge y Miguel, por abrir el camino, por compartir su experiencia, por enseñarme y ayudarme en todo momento, por dejar que me subiera a sus hombros para ver más lejos. A Mario, Carlos Vivo, Carlos Plata, Mercedes, Victoria, Elena, Laura, Andrés, Cristina, José y a todos los que han ido pasando por la “Mongolfiera”, gracias por las cenas de los jueves, las sevillanas y las ferias, las comidas, las fiestas, las risas, las experiencias, los abrazos y todos los maravillosos momentos que hemos compartido juntos. A Elena Grao, que aunque ha sido la última en llegar, a llegado con fuerza para quedarse. A mis compañeros del “despachito” de becarios, Anna y Damián, por vuestro apoyo, vuestros consejos y vuestro cariño. A Tere, por ser mucho más que una amiga. Nuestros caminos han estado unidos desde el primer día y desde entonces hemos ido en tándem para todo. Hemos crecido juntas, madurado juntas, luchado juntas... Gracias por haber estado presente en cada momento, en cada viaje, en cada cambio, en cada acontecimiento importante en los últimos años. Porque entre esas dos pavas que empezaron y las mujeres en las que nos hemos convertido caben muchas risas, muchas lágrimas, muchas experiencias, muchos consejos, muchas confesiones, muchos recuerdos. A mis amigos los “Jumanji”, los murcianos y Eddy; y Bea, Dani y Miguel de Granada, porque han estado ahí desde antes de empezar esta aventura y han seguido ahí, incondicionales, a lo largo del camino y en la distancia, a pesar de las muchas ausencias, del tiempo sin vernos, de desaparecer durante semanas. Por ser el punto de desconexión durante todos estos años. Por las quedadas, los viajes, los roles y las escape room. Por las videollamadas y las conversaciones en “wasap”. Por todas las veces que he acudido a la “mente colmena” y han estado ahí para escucharme, darme su opinión y aceptar que acabara haciendo lo que me daba la gana. Por seguir alimentando mi lado friki y no dejar que me perdiera en los momentos de más estrés. Porque con ellos tengo la seguridad de que, no importa lo que pase, ni cuánto tiempo pase, siempre van a estar ahí para mostrarme el camino de vuelta. |Acknowledgements / Agradecimientos v A Adolfo Bastida, por haber sido mi mentor todos estos años. Porque sin él nunca hubiera llegado hasta aquí. Por verme cuando yo no me veía, por creer en mí, por apoyarme, animarme, alentarme, por darme el empujón que necesitaba, por darme las fuerzas cuando a mí más me faltaban. A Cristina Moreno, por estar ahí, con los pies descalzos, para mí. A Rafa y Adela, por los desayunos de los domingos y por asegurarme de que nunca me falten ni queso ni aceitunas ni cerezas. Rafa, gracias por conseguir que algo grande se convierta en pequeño y aún así siga siendo valioso ;) . Gracias por todo vuestro cariño y apoyo, y por toda la ayuda que me habéis prestado en estos años. A Alberto, mi compañero, el “doctorando consorte”. Porque ha sido el que ha vivido (y sufrido) día a día este proceso. Por aguantarme estresada y agobiada, por el abrazo siempre a tiempo, por los mensajes en los “tupper”, por garantizar mi supervivencia, por los besos, el cariño, el apoyo, la confianza. Por esa habilidad única de entender lo que me pasa antes siquiera de que yo misma lo entienda. Por adelantarte a mis necesidades. Por ser mi refugio. Por creer en mí y creer en nosotros. Ya por último me queda dar las gracias a mi familia, que son y siempre han sido todo para mí. Por ser la red de seguridad que me da valor para arriesgarme, por el apoyo incondicional, por todo el orgullo que me muestran y me hacen sentir, por haberme apoyado en cada paso y haberme alentado a perseguir mis objetivos, aunque eso implicase alejarme de ellos. Por compartir el café, los buenos días, las comidas y los viajes para hacernos sentir que estamos juntos en la distancia. Por esas conversaciones de horas al teléfono y las flores cada mañana. Por celebrar cada victoria como si fuese propia. Porque me han escuchado durante horas a pesar de que no se hayan enterado de nada, pero a ellos les da igual y siguen ahí porque saben que para mí es importante. Porque les debo mi educación, mi orden de prioridades, mi sensibilidad para ciertas cosas. Porque han alimentado mis inquietudes y mi curiosidad. Porque sin ellos no sería quién soy. Porque somos una piña rara, pero es nuestra piña y la quiero. Porque si todo lo demás desapareciera, me seguirían quedando ellos. Especialmente tengo que darle las gracias a Lo, a mi hermana, por ser además mi consejera, por haber ejercido como traductora y correctora de todas y cada una de las cosas que he escrito en estos años (incluyendo esta tesis). Porque siempre está ahí cuando la necesito, sea de día o de noche, y por peregrino que sea el tema. A todos, de nuevo, Gracias. xiii Contents Acknowledgements / Agradecimientos i Abstract vii Resumen ix Contents xiii List of Figures xvii List of Tables xxv Why laser-driven neutrons? 1 1 Introduction 3 1.1 Neutrons in nuclear physics experiments ................... 3 1.1.1 Neutron energies ............................ 3 1.1.2 The time-of-flight technique ...................... 5 1.1.3 Example of nuclear physics experiment with neutron beams .... 6 1.2 Common neutron production strategies .................... 8 1.2.1 Nuclear reactors: fission neutrons ................... 8 1.2.2 High energy electron accelerators: Bremsstrahlung and photoreactions 9 1.2.3 High energy ion accelerators: spallation ................ 10 1.2.4 Low energy ion accelerators: nuclear reactions ............ 12 1.3 Neutron production by laser: a promising new approach .......... 14 1.3.1 Types of high-power lasers ....................... 15 1.3.2 Ion acceleration ............................. 16 1.3.3 Neutron production schemes ...................... 20 1.4 State of the art, prospects and objectives ................... 23 2 Neutrons at the CNA accelerator-based HiSPANoS neutron source 27 xiv |Contents 2.1 The CNA research center ............................ 28 2.2 HiSPANoS facility description ......................... 29 2.2.1 3 MV accelerator system ........................ 30 2.2.2 Chopper and buncher pulsing system ................. 31 2.2.3 Beamline and experimental area .................... 31 2.2.4 Neutron production reactions and targets ............... 33 2.3 Experimental setup for neutron detection ................... 34 2.3.1 Detection system ............................ 34 2.3.2 Detector efficiency ........................... 36 2.3.3 Data Acquisition System and PSD .................. 37 2.4 Quasi-monoenergetic neutron beams ..................... 40 2.4.1 Time-of-flight (TOF) spectra ..................... 41 2.4.2 Background characterization ...................... 42 2.4.3 Energy distributions .......................... 45 2.4.4 Energy resolution ............................ 48 2.4.5 Available beams and neutron flux ................... 49 2.5 Broad white neutron beams .......................... 52 2.5.1 TOF/Enspectra and scattering background ............. 53 2.5.2 Discussion: available beams and neutron flux ............. 55 3 Neutrons at the HZDR PW-laser facility DRACO 59 3.1 The HZDR research center ........................... 60 3.2 Set-up for ion acceleration at the DRACO laser system ........... 61 3.2.1 Laser parameters ............................ 62 3.2.2 Acceleration diagnostics ........................ 64 3.2.3 Ion beam diagnostics .......................... 65 3.3 Set-up for neutron production and detection at the DRACO laser ..... 66 3.3.1 Pitcher-catcher setup .......................... 67 3.3.2 Neutron detectors ............................ 69 3.3.3 Data acquisition system ........................ 76 3.4 Data analysis and results from standard detectors in laser-driven experiments 76 3.4.1 Ion production: Flux and energy ................... 76 3.4.2 Integral neutron production from bubble detectors .......... 82 3.5 Data analysis and results from the diamond detector ............ 86 3.5.1 Electromagnetic noise suppression ................... 88 3.5.2 Redefinition of the baseline ...................... 89 3.5.3 Identification and analysis of single neutron signals ......... 92 3.5.4 Limitations of the Pulse Shape Discrimination (PSD) ........ 95 3.5.5 Detection efficiency and energy calibration .............. 97 3.5.6 Background subtraction ........................102 3.5.7 Measured neutron flux .........................105 3.6 Discussion ....................................107 3.6.1 Simulation of the neutron production at DRACO ..........107 3.6.2 Background simulations ........................110 3.6.3 Comparison between simulation and measurements .........113 4 Conclusions and future perspectives 117 4.1 Comissioning of the HiSPANoS neutron source at CNA ...........118 4.2 Laser-driven neutrons at DRACO .......................119 4.3 Future perspectives ...............................120 4.3.1 Lessons from DRACO .........................120 4.3.2 HiSPANoS vs. DRACO: can neutron induced reactions be studied at a laser-driven neutron source? ...................122 125 Bibliography 127 xvii List of Figures 1.1 Common neutron beam applications according to the energy range and suitable facilities for their production. Image credit: F. Gunsing and the n_TOF collaboration. ..... 4 1.2 Diagram of the basic principles of the time-of-fight technique. ............. 6 1.3 Diagram of the STEFF detector in the four arm configuration used in n_TOF [1]. . . . 7 1.4 Schematic representation of the fission reaction. ................... 8 1.5 Schematic representation of the two-step process for photoneutrons production with Bremsstrahlung. .................................. 10 1.6 Schematic representation of the spallation process. .................. 11 1.7 Schematic representation of fusion and (p,n) nuclear reactions. ............ 12 1.8 Scheme of the TNSA ion acceleration process and intensity profile of the laser pulse. . . 18 1.9 Scheme of neutron production in the pitcher-catcher double-target configuration based on the TNSA ion acceleration process. ........................ 22 2.1 Schematic view (not in scale) of the CNA Tandem accelerator, with the HiSPANoS neutron beamline at the far left end. The position of the neutron detectors and the electronic chain used during the commissioning is also indicated. ........... 29 2.2 Picture of the HiSPANoS beamline presenting the position of the main components. . . 32 2.3 View of the experimental area corner from the target showing the two closest walls (∼ 4 m). ....................................... 32 2.4 Picture of the different targets available at HiSPANoS for fast neutron production. The water-cooled target holder (left) is empty in the picture. The fresh Li disc should be placed in the central cavity. ............................ 34 2.5 A sketch of a scintillator detector mechanism. .................... 35 2.6 Neutron efficiency for an EJ-309 2"x2" organic liquid detector from Pino et al. [2] (with 7% accuracy), for the two energy thresholds used (200 and 300 keVee). The solid lines correspond to analytical fits to 9th degree polynomials. ................ 36 2.7 Difference between neutron and γ-ray signals in the EJ-301 detector and definition of the signal parameters returned by the DAQ. ..................... 37 xviii |List of Figures 2.8 Top: calibration spectra of 137Cs (left) and 60Co (right) for one of the scintillators with the position of the Compton edge labeled. Bottom: Calibration lines for the three scintillators used in the experimental campaign. ................... 38 2.9 Measured distribution of the PSD values obtained as a function of the signal energy measured for the 2H(d,n) reaction with Ed=1,5 MeV. The solid line represents the analytical function separating both particle types. .................. 39 2.10 PSD distribution for signals between 200 and 2500 keVee (projected from Fig. 2.9). The green line corresponds to a fit to two Gaussian distributions. ............. 39 2.11 Picture of the experimental set-up at HiSPANoS. .................. 40 2.12 TOF spectra for 2H(d,n) at 1,0 MeV beam energy measured at 150 cm in the forward direction. The blue line corresponds to neutron signals (see Section 2.3.3). ....... 41 2.13 Time-of-flight distribution for Ed=1,5 MeV in the forward direction, illustrating the wall effect (neutron scattering background) depending on the flight path. The spectra are normalized by L−2in meters. ............................ 42 2.14 PE + Pb shadow bar covering the direct component of neutrons arriving at the detector. The beam comes from the right side of the picture. ................. 43 2.15 Time-of-flight spectra for three different energies showing the background contribution due to the reactions in the dummy target and the scattered neutrons in the room. . . . 44 2.16 Reconstructed neutron energy spectra measured with the D/Ti (with and without shadow bar) and dummy targets for Ed= 1,0MeV (top) and Ed= 3,0MeV (bottom). The aimed quasi-monoenergetic contribution from the 2H(d,n) reaction is obtained after the background subtraction. The contribution of different reactions is labeled in the figure. 46 2.17 Dependence of the neutron energy on the ion beam energy (top) and angle (bottom). The lines correspond to the data of Liskien and Paulsen [3], retrieved with NeuSDesc [4] (which also provides an estimation of the uncertainty). The uncertainties of the experimental data correspond to the measured resolution in Y and, in X, to the detector positioning (bottom) or the accelerator calibration (top). ............... 47 2.18 Dependence of the neutron energy resolution on the flight path distance, measured at 60ºfor 3,6 MeV neutrons. The expected resolution is shown as a solid line, with the dashed lines corresponding to the partial contributions. ................ 48 2.19 Neutron flux measured at different distances (at 0º) and angles (at 150 cm) with a deuteron beam of 1,5 MeV impacting on the D/Ti target. The solid lines correspond to the data of Liskien and Paulsen [3] retrieved with NeuSDesc [4]. ........... 50 2.20 Collection of monoenergetic neutron beams available at HiSPANoS covering the energy range between 2,2 and 6,1 MeV. ........................... 51 2.21 Neutron yields for different thick targets as a function of the energy of the ion beam. Figure retrieved from Y. Zuo et al. [5]. ....................... 52 |List of Figures xix 2.22 Time-of-flight (top), neutron energy (middle) and Ed+Q−En(bottom) spectra for Ed=5,75 MeV on the thick Li (right) and Be (left) targets measured forward at 150 cm. 54 2.23 Neutron production in the thick Be target at different beam energies. The shadow bar spectra and the clean Be(d,n) reaction are also displayed. .............. 55 2.24 Neutron yields at Ed= 5,75 MeV and a time-of-flight distance of 150 cm in the forward direction for the Li and Be thick targets. The blue line corresponds to the data from Meadows [6] for 5,8 MeV deuteron beams at 0ºand the yellow line corresponds to the data from Weaver [7] at 5,0 MeV and 3,5º(Retrieved from EXFOR [8] F0218.015). . . 56 3.1 Layout of the ELBE Center for High-Power Radiation Sources [9]. The laser areas are shaded in different colors. The experiments summarized in this work were carried out in the laser ion acceleration experimental area (at the bottom right). ......... 61 3.2 Outside (left) and inside (right) look into the interaction chamber at the laser DRACO ion acceleration laboratory. On the inside view, the target holders can be identified at the center of the picture and the focusing parabola corresponds to the biggest component at the left. ..................................... 62 3.3 Intensity contrast of the DRACO laser pulse on different temporal scales. Left: nsrange and 100 ps zoom. Right: ps-range with (red) and without (black) plasma mirror. Retrieved from [10]. ................................ 63 3.4 Already used plasma mirror showing circular abrasions produced by each laser shot. . . 63 3.5 Layout of the main acceleration and ion beam diagnostics inside and outside the interaction chamber. .................................. 64 3.6 Images from the proton profiler lanex screen in the rear surface of a Cu catcher for aligned (left) and misaligned (right) shots. The screen attached to the rear surface of the catchers is visible in Fig. 3.7. .......................... 66 3.7 Top: sketch of the pitcher-catcher configuration. Bottom: pictures of the pitcher-catcher arrangement (left), the pitcher holder with some pitchers (center), and the Cu and LiF catchers (right). .................................. 68 3.8 Bubble detectors before (bottom) and after (top) being irradiated in a neutron source. For these devices, each bubble corresponds to ∼0,4 µSv. ............... 70 3.9 EJ232Q-USC scintillator signal for 200 nm formvar pitcher and LiF catcher. ...... 72 3.10 A sketch of the reaction chamber and the detectors placed around in the experimental room with pictures of the experimental setup. The detectors are distributed along a TOF line at 15ºfrom the proton acceleration direction. ............... 73 xx |List of Figures 3.11 Simplified layout of a diamond detector. The ionizing radiation (direct or indirect) produces electron-hole pairs that are driven to the electrodes thanks to the HV, generating an electrical pulse proportional to the deposited energy. The ionization density and spatial distribution characteristics of each kind of interaction determine the shape of the output signal. ................................. 74 3.12 Diamond detectors before (left) and after (right) electromagnetic shielding. The noise reduction includes also Al wrapping around cable connectors, noise suppressors in cables, and electrical isolation between electronic modules with plastic foils and foams. . . . . 75 3.13 Results from the RCF stack for an average laser shot in DRACO. Left: energy spectrum showing the exponential profile typical in TNSA acceleration. The last point at 56 MeV defines the cut-off energy. Right: divergence of the ion beam showing a linear dependence on the energy, ±20ºbeing the maximum divergence, established by the aperture in the first film. ..................................... 77 3.14 MCP and lanex images provided by the TPS. The cut-off energy for the proton beam is obtained from the MCP whereas the number of particles is obtained from the lanex. A TPS spectrum can then be reconstructed from the combined information, extrapolating the interrupted spectrum from the lanex up to the cut-off energy using an exponential fit from Eq. 3.2. .................................. 78 3.15 Top: comparison of the TPS spectra of three different shots with the RCF spectrum. The differences in the slope can be related to the misalignment between the ion beam and the pinhole. Bottom: the image of the selected beams in the proton profiler. . . . 79 3.16 Proton production information for different shots on different days retrieved by the TPS. The average value (solid line) and the standard deviation (dashed lines) are shown in the image. Outliers are easily identified and then discarded. ............. 81 3.17 Top: Measurements at 0º, 60º, and 120ºin different runs for p+Cu (left) and p+LiF (right) with DRACO formvar standard pitcher targets. The average production (solid line) and standard deviation (dashed lines) are included in the figures. Bottom: Angular mapping for p+Cu (red circles and blue squares) and p+LiF (yellow stars) pitchercatcher configurations also with standard pitcher targets. Each measurement was made on a different day. The uncertainties for each run (dashed area) were established in a 30% 84 3.18 Left: Diamond traces illustrating the beneficial effect of the 5 cm lead filter: it reduces the recovery time in half and also the undershooting in the baseline. Right: Diamond traces showing the difficulty of identifying detector signals on top of the γ-flash and the EMP ringing. ................................... 87 3.19 EMP and γ-flash in the diamond signal. Long cables connected to the oscilloscope delayed the signal long enough to allow the attenuation of the EMP. ......... 88 |List of Figures xxi 3.20 Some of the control measures to reduce the EMP noise effect. Left: the watertight shielding for the diamond detectors and the amplifiers, and the clamp-on ferrite bead in cables. Center: the HV cables were picked up in such a way that the formation of coils is prevented. Right: some aluminum shielding around connections in the electronics acts as a Faraday cage. ................................. 89 3.21 Effect of the EM shielding in the detector signals, reducing the noise amplitude (left panel). The noise level changes on different days, but is independent of the catcher (Right panel). ................................... 90 3.22 Top: individual traces for the same data set (solid lines) and the corresponding average trace (dashed line). Bottom: a selected raw trace (yellow) with the corresponding average baseline (blue) and the clean trace after the average baseline subtraction (red). The circles point to the signals which were barely visible before and clearly visible after the baseline subtraction. Times shorter than the temporal threshold (20 ns, shaded region) correspond with the γ-flash tail, where the subtraction is unreliable. ...... 91 3.23 Left: clean trace with the noise level indicated as dashed lines. Right: distribution of noise levels for all the shots in the same data set. The solid line indicates the average noise level and the dashed line corresponds to the threshold established for signal identification (∼two standard deviations). ...................... 91 3.24 Description of the variables used in the signal analysis for diamond detectors. At ∼70 ns, the structure marked with the circle is identified as two different signals with the same t30 and one with two times the FWHM of the other. .............. 92 3.25 Issues in the signal analysis caused by the EM ringing in the baseline, inducing false pileup signals (see text for details). ......................... 93 3.26 Diamond featuring a significant undershoot after the γ-flash and illustrating that the baseline is neither constant nor at zero (left). A redefinition of the baseline in a signalby-signal basis solves the issue (right, see text for details). .............. 93 3.27 Re-definition of all the variables used in the signal analysis for diamond detectors and effect of the corrections in the maximum amplitude of the signal and the FWHM. . . . 94 3.28 2D plot of the FWHM of the signal vs the amplitude for different amplitude thresholds. The noise is identified as the region with 2 ns FWHM and around 4-5 mV amplitude. . 94 3.29 Zoom in the 2D plot of the FWHM of the signal vs the amplitude for Ampth =3 mV showing the region of the EM noise signals. ..................... 95 3.30 Distribution of the form factor F applying different Ampth............... 96 3.31 Signals for different F factors. Top: Low distorted square signal with F < 1,6 as expected. Bottom: Distorted signals due to the EM ringing with F > 2. Black squares indicate the integration range for the area calculation. ................ 97 2|Why laser-driven neutrons? Laboratory [35] or SINQ at Paul Scherrer Institute (PSI) [36]. These factors limit the transfer of knowledge and the potential applications of neutron beams outside the main research centers. This is especially relevant in the case of medical and industrial applications or for educational and training purposes, since hospitals, business, universities, and local research centers cannot afford to have one of these neutron sources, or even access to the already available ones. In this context, the International Atomic Energy Agency (IAEA) spotlighted the need of developing compact neutron sources, not only as an alternative to largescale neutron facilities, but also as complement to them in different areas such as basic research, technical developments, material analysis, instrumentation, and training programs [11,30]. The most extended compact neutron sources worldwide are the ones hosted in medium-size ion accelerators. CANS (Compact Accelerator-based Neutron Sources) are based on low energy (∼MeV to tens of MeV) ion beams which induce nuclear reactions. Of course, while the costs of building and maintaining these "laboratory-sized" accelerators are much more affordable than those of large spallation sources, they feature lower intensities and neutron energies. As an alternative to conventional CANS, high-power lasers appear as a promising alternative to conventional accelerators. Typically, the architecture of a laser fits in a single room, and the shielding requirements are much less than in conventional accelerators, potentially reducing the cost of their construction. Lasers can provide a broad neutron spectrum of particles up to tens of MeV, useful for a variety of applications [37]. In particular, since the laser operates producing high-intensity ultra-short bunches, it is specially well suited for nuclear physics experiments that require time-of-flight. Furthermore, while the increase in the neutron flux delivered by conventional neutron sources (spallation and research reactors) has been at best an order of magnitude over the last decades, high-power laser technology is a very fast developing field and, correspondingly, the neutron production has increased several orders of magnitude in the last two decades. Nowadays, the neutron output efficiency grows with the laser intensity [38] and there is work in progress [39] to further increase the neutron production. The research field of neutron production with high power laser is very active, and many initiatives are ongoing [40]. However the large majority of the works published are focused only on the neutron production itself. Although there are several recent works about neutron production by laser reaching fluxes per pulse comparable to those of conventional neutron sources [41], there is a lack of studies in terms of the applications of these neutrons, especially in the field of nuclear physics experiments. In this context, this work intends to be a first step in order to assess to what extent laserdriven neutrons can be useful for the nuclear physics community, also including a discussion based on experimental evidence about their current limitations and future requirements. The laser facility of choice is the ∼PW laser facility DRACO [42] at HZDR (Germany), while the HiSPANoS neutron source at the 3MV tandem accelerator at CNA (Spain) [43] has been taken as reference, to provide a comparison with a conventional CANS. 3 1| Introduction Experiments using neutron beams are very diverse in terms of the energy of the neutrons involved, the spatial and temporal structure of the neutron beam, and the kind of detectors and diagnostics employed. Everything depends on the specific neutron-induced reaction or process that is to be studied. This chapter offers a brief overview of the relevant characteristics and classifications of neutron beams with particular emphasis in the strategy adopted for their production. The possibility of using high power laser instead of conventional particle accelerators is then presented as a promising alternative for the particular case of conducting neutroninduced nuclear physics experiments. In this context, the state-of-the-art and prospects in laser-driven neutron sources (LDNS) are discussed and the objectives of this PhD thesis are presented. 1.1. Neutrons in nuclear physics experiments There are different criteria that can be used to classify and described neutrons sources based either on the kind of facilities that produce the named neutrons (research reactors, accelerators, lasers...) or on the characteristics of the resulting beam (energy range, pulsed/continuous operation). This section is devoted to these characteristics relevant to neutrons in nuclear physics experiments. 1.1.1. Neutron energies According to their energy range, neutrons can be classified as high energy (> 20 MeV), fast (100s keV to 20 MeV), epithermal (eV to 100s keV), thermal (water moderated at room temperature, ∼25 meV), and cold (< 25 meV) neutrons. Each energy range is suitable for different kinds of applications (see Fig. 1.1): •Due to the longer neutron wavelength, in the scale of molecular structures and interatomic distances, thermal and cold neutrons are applied in life sciences, radiography, 41| Introduction medical isotope production, and diffraction techniques, in addition to applications related to water-moderated nuclear reactors. •The epithermal range is the range of the nuclear resonances and has a special interest in nuclear reactors, astrophysics (as is the energy range of the stellar processes of nucleosynthesis), and medical applications such as BNCT (Boron Neutron Capture Therapy). •Fast neutron beams are very penetrating and mainly produced by fission and fusion reactions, having an interest for energy production, irradiation, medical isotope production, material analysis, and industrial applications. •High-energy neutrons are produced in laboratories with high-energy accelerators and in nature through cosmic rays entering the atmosphere, irradiation and dosimetry are their main application. It is worth emphasizing that some applications such as nuclear energy or neutron radiography are associated with neutrons covering the full energy ranges discussed herein. Figure 1.1: Common neutron beam applications according to the energy range and suitable facilities for their production. Image credit: F. Gunsing and the n_TOF collaboration. 1| Introduction 5 1.1.2. The time-of-flight technique According to the time structure of the neutron beams, these can be classified as pulsed or continuous beams. Continuous beams are adequate for irradiation studies, activation analysis, production of radioisotopes, dosimetry, and applications in which integral effects are considered. However, pulsed beams are necessary for the kind of applications in which the energy of the neutrons is relevant, for instance in differential cross-section measurements, neutron resonance imaging, detector characterization, metrology, etc. In these cases, because neutron detectors are very limited in terms of spectroscopic capabilities (if any), the time-of-flight technique needs to be implemented using pulsed neutron beams. The neutron time-of-flight technique (nTOF) is the most extended method in nuclear physics experiments for neutron spectroscopy, either for beam characterization (see for instance [44–46]) or to determine the energy at which a particular reaction of interest or event occurs (see [47–49] for examples). The history of this technique for neutron measurements and characterization is as old as the discovery of the neutron itself, as illustrated by Dunning et al. [50] in 1935. To implement this technique, in an accelerator-based facility (or eventually a laser one) neutrons are produced through a primary pulsed beam of ions or electrons implying in neutron production target. The neutron pulse that is generated instantaneously has a certain distribution of energy (which can be tuned via moderation) and a temporal width that inherits from the ion/electron pulse. Subsequently, neutrons take a specific time (tTOF ) to travel the distance LT OF to reach the specific detector or sample under study. The more energetic neutrons arrive sooner and are separated from the less energetic ones, extending the duration of the pulse arriving at the detector or sample. This is sketched in Fig. 1.2, which illustrates a typical time-of-flight setup. The time-of-flight distribution shown on the right side features a contribution from neutrons but also from γ-rays, produced when the beam hits the neutron production targets, traveling at the speed of light. Thus, knowing the time (t0) of the neutron interaction with the detector or the sample with respect to that for γ-rays (tγ) and the distance (LT OF ) that they have to cover, it is possible to assess their energy. For neutrons in the range of tens of MeV, the kinetic energy can be determined by the classical expression 1.1: En=1 2mnLTOF tTOF 2 =1 2mnLTOF t0−(tγ−LTOF /c)2 =(1.1) 61| Introduction Figure 1.2: Diagram of the basic principles of the time-of-fight technique. For a distance measured in meters and a time-of-flight measured in ns, the energy of the neutron in MeV is given by expression 1.2: En(MeV ) = 72,29 ·LT OF (m) tTOF (ns)2 (1.2) 1.1.3. Example of nuclear physics experiment with neutron beams Nuclear physics experiments aim at a better understanding of the processes occurring in the nuclei of atoms, including the fundamental mechanisms of nuclear reactions, the nuclear structure of the atom, or the process of nucleosynthesis, with implications for different applications such as nuclear energy, fusion, astrophysics or medicine, but also laying the foundations for new ones. Such experiments need, in general, a wide energy range and pulsed beams to be able to characterize the relevant process in terms of the incident neutron energy by time-of- flight techniques. As the objective is to retrieve the most complete information possible, detectors are used to study reactions by looking at a full set of observables individually, rather than integrally, to be able to look for correlations and coincidences between different events in order to reconstruct the details of the whole process. A complete and illustrative example of the peculiarities of a neutron-induced nuclear physics experiment and the way in which the nuclear physics detectors of different kinds involved are operating in this research field can be found in the experiments with STEFF (SpecTrometer for Exotic Fission Fragments), built at the Manchester University. The objective of STEFF is to provide a complete data set for understanding and modeling 1| Introduction 7 the nuclear fission process by looking for correlations between multiple parameters including the correlated fission fragments’ charges and masses, the total energy release in the process, and neutron and gamma multiplicities in each individual fission event [51]. To achieve this, STEFF is built as two/four time-of-flight arms, coupled each with an ionization chamber, and a thin fissile sample in the middle (see Fig. 1.3). The arms allow for measuring both the velocity and the energy of correlated fission fragments in different directions, thus providing the mass and the kinetic energy of the fragments. An array of γ-ray and neutron detectors around the radioactive sample measure simultaneously the emission of prompt γ-ray and neutrons from the fission. Figure 1.3: Diagram of the STEFF detector in the four arm configuration used in n_TOF [1]. This detector has been successfully used to measure all these observable on a 235U sample using thermal neutron from the ILL reactor [51,52]. However, the data provided by this kind of experiment are also necessary at higher neutron energy and as a function of such neutron energy. This required a good neutron energy resolution, a high instantaneous neutron flux to maximize the fission signal to radioactive background ratio, and a high average neutron flux to achieve enough statistics in a reasonable time. Thus, the experiment was carried out with a 239Pu sample [1] at the CERN n_TOF facility and lasted for more than 20 months; illustrating the need to have access to very intense pulsed neutron beams and the need to be able to operate very different particle, γ-rays and neutron detectors looking at individual events for each nuclear reaction. 81| Introduction 1.2. Common neutron production strategies Different criteria can be used to classify and describe neutron sources based either on the kind of facilities that produce the named neutrons (research reactors, accelerators, lasers...) or on the characteristics of the resulting beam (energy range, pulsed/continuous operation). The latter are driven by the neutron production method involved. In this work, the attention is focused on nuclear physics applications for which a pulsed broad spectrum of energies in the fast energy range (100s of keV up to several MeV) is needed. 1.2.1. Nuclear reactors: fission neutrons Although nuclear reactors do not produce the pulsed neutron beams of interest in this work, they are featured herein because of the very important role than they have played in the field of nuclear physics research with neutron beams. The fission reaction consists of the fragmentation of heavy nuclei into two fragments of approximately equal mass releasing in the process γ-rays and an average number of 2,5 neutrons. The fission fragments are β−unstable and emit afterward electrons and antineutrinos which can be followed by the emission of delayed neutrons (Fig. 1.4). The process is usually exothermic, releasing around ∼180 MeV, part of which goes to the neutrons, which show a Maxwellian shape with tails up to tens of MeV and a mean value around 1-2 MeV [53]. Figure 1.4: Schematic representation of the fission reaction. 1| Introduction 9 The fission reactions for neutron production usually take place in nuclear research reactors where, as in conventional nuclear power plants, the fission reaction in a fuel element (typically 235Uor 239Pu) is maintained by spending one of the resulting neutrons for inducing a new reaction. Although 99% of the neutrons are produced as a product of the fission reaction, as said above, there is a delayed neutron component that comes from the β-decay of some of the fission fragments [31]. The neutron fluxes produced are very intense (ILL features a world record of ∼1015 n/cm2/s in the core), usually continuous and covering the thermal and cold range of neutron energies with the application mainly focus on neutron scattering and and diffraction, production of isotopes for medical applications, radiography, material analysis, and nuclear physics. Nevertheless, fast fluxes [54] and pulsed beams [55] can be also extracted from reactors. The spontaneous fission of heavy nuclei (heavier than thorium) can be also used as a neutron source. This is done mainly for calibration purposes, as is the case of the 252Cf, the only isotope able to provide a useful neutron source [56]. 1.2.2. High energy electron accelerators: Bremsstrahlung and photoreactions The Bremsstrahlung produced by accelerated light charged particles such as electrons impinging in a high-Z medium can be used to induce photonuclear reactions producing neutrons. The photons produced by these mechanisms show a continuum spectrum of energies up to the maximum energy of the incident particle which, in the case of electrons in a conventional linear accelerator (LINAC), is up to tens of MeV. These photons, redirected or already produced, in a suitable photo-neutron target can induce (γ,n) reactions in non-fissile heavy metals such as lead, or induce photo-fission in fissile targets such as uranium (Fig. 1.5). Higher Z implies a higher threshold for the photonuclear reactions, but also a higher neutron yield. The yield also increases with the energy of the electron beam up to 30-45 MeV, reaching a plateau [31]. Uranium is the element that produces the highest yields, but also the neutron spectrum with higher energies. Nevertheless, due to the toxicity of uranium, the high price, and the low availability of the material, other options such as W (the next element with a higher cross-section) or Pb are considered [11]. Bremsstrahlung is a rather inefficient production process, yielding 0,05 neutrons per incident electron and releasing large amounts of energy into the target (2 GeV per emitted 10 1| Introduction Figure 1.5: Schematic representation of the two-step process for photoneutrons production with Bremsstrahlung. neutron) and a strong γ-ray background [53]. The photonuclear reaction produces neutrons in a quasi-isotropic evaporation spectrum with a maximum of around 1-2 MeV. The facilities exploiting this production mechanism are based on high-energy electron LINAC. Although electron LINAC is a technology relatively accessible and easy to operate, the large amount of heat dissipated in the target per neutron produced severe limits on the potential performance becoming an engineering challenge for these kinds of facilities and limiting the maximum power that can be applied to the target to about 50 kW to prevent the material from melting [31]. This forces ingenious solutions such as the nELBE liquid lead target [44]. Together with the already named nELBE at HZDR, other neutron sources based on this mechanism are GELINA [45], at the JRC in Geel, Belgium; ORELA [57], at the Oak Ridge Laboratory, or the 25 MeV electron LINAC at Centro Atómico Bariloche [58] (CNEA, Bariloche, Argentina), which can be considered an actual CANS. Most of these neutron sources are devoted to time-of-flight measurements. 1.2.3. High energy ion accelerators: spallation The term spallation refers to a two-step process involving different nuclear reactions that take place in a heavy nucleus (Such as Pb, Ta or W) after being hit by a light particle (usually protons) at high energy (hundreds of MeV to tens of GeV). In the first step, the energy of the beam is deposited in the target nucleus, leaving it in an excited state and emitting some light fragments and high-energy neutrons (intranuclear cascade). In the second step, the nucleus decays either by the evaporation process, in which it emits relatively low-energy (< 20 MeV) charged particles, photons and neutrons; by fission or 1| Introduction 11 by statistical multi-fragmentation [59]. Fig. 1.6 provides a schematic view of this process. Figure 1.6: Schematic representation of the spallation process. The evaporated neutrons produce the low energy part of the spectrum emitted in an isotropic distribution similar to the one obtained from a fission reaction. Nevertheless, the small fraction (∼3%) of prompt neutrons emitted in the first step of the spallation process reach energies up to the incident energy of the beam (hundreds of MeV to ∼GeV) and are emitted in the forward direction. This process is fast enough to not disturb the original temporal length of the primary beam [60]. The spallation process is endothermic, depositing roughly tens of MeV per neutron produced and with a neutron multiplicity of tens of neutrons per incident proton. In addition, the neutron yield increases linearly with the incident energy of the ion beam [53]. All these factors make spallation facilities the preferred alternative to research reactors to produce high-intense neutron sources with a wide spectrum of energies, extending the useful range of the neutrons to new applications and overcoming some of the drawbacks related to reactors as the spent fuel management or the limit achievable in the neutron production. Spallation sources for neutron beam research are built around large proton accelerators with energies in the range of the GeV, with either pulsed or continuous operation. Pulsed facilities as n_TOF at CERN [46] or DANCE at LANSCE (Los Alamos Neutron Science Center, Los Alamos, New Mexico, USA.) [61] are mostly dedicated to the measurement of fundamental physics and nuclear reactions meanwhile facilities as the Spallation Neutron 18 1| Introduction Figure 1.8: Scheme of the TNSA ion acceleration process and intensity profile of the laser pulse. The acceleration ocurrs in the target normal direction and the spectrum observed for the ions in this acceleration mechanism is a broad white spectrum with an exponential profile up to a cut-off energy of usually tens of MeV [38,79] with an energy-dependant divergence of tens of degrees. The beams are bright, featuring 1012 - 1013 ions per shot [75]. Although several factors, such as the target characteristics, influence the maximum energy available, in general, higher laser intensities on targets produce higher cut-off energies. Also, a thinner target can enhance the maximum energy available [102]. However, as said before, the minimum target thickness is limited by the laser characteristics. A detailed description of the ion source characteristic for this acceleration method can be found in [38,80]. Radiation Pressure Acceleration (RPA) The momentum transported by an electromagnetic wave to the surface of a non-transparent (reflective or absorbing) medium generates a pressure known as radiation pressure and an associated radiation pressure force. The radiation pressure that the laser pulse exerts over a dense plasma in the front surface of the target is able to push the electron and ions into the target, producing a high-density electron layer accelerated towards the forward direction (along the laser axis). Due to the different masses, a charge-separation profile is formed in the bunch that produces the intense electric field that accelerates the ions through the target. If the radiation pressure force is enough to counteract the restoring force due to the charge separation field, the whole volume propagates in a double-front neutral bunch. All the particles are moving at the same velocity and feel the same accel- 1| Introduction 19 eration force from the laser, producing virtually monoenergetic beams [103]. Regarding the thickness of the ion production targets (in relation to the laser wave penetration depth), slightly different types of RPA can be distinguished. •Hole-boring acceleration In thick targets, the double-layer bunch (electrons and ions) from the front surface plasma is pushed inside the bulk of the target, spreading the initial perturbation to the unperturbed regions of the target, acting as a piston that deploys the rest of the ions in the target until the bunch leaves the target rear surface, progressively increasing the ion energy, the number of accelerated ions and the ion density [75]. •Shock-wave acceleration Another emerging mechanism for thick targets related to the laser radiation pressure force is the shock-wave acceleration mechanism. In the case of high-density and low-temperature plasmas (low plasma velocity) [81], the electromagnetic wave of the laser produces a strong high-Match electrostatic shock (the laser velocity is much higher than the plasma velocity) that propagates in the bulk of the target reflecting the ions from the wavefront [104]. •Light sail acceleration In the case of ultra-thin targets, the whole target is compressed in a thin plasma that is accelerated at once in a double-layer bunch (electrons and ions) and driven into the vacuum after the target [79,105] (reminded the sail of a boat, hence the name). The target has to be opaque to the laser radiation, hence all the target momentum can be efficiently transferred into the foil producing a high energy quasi-monoenergetic spectrum [79]. Usually, intermediate stages occur, and part of the laser force is spent in the acceleration of the bunch inside the target, once the rear surface is reached, the rest of the force accelerates the bunch in free space. RPA can compete in the same target with the TNSA process, producing ion components in different acceleration directions [106]. The main differences between RPA and TNSA are that RPA produces much more intense pulses (as all the target is virtually accelerated, not only the rear surface), higher ion energies (hundred of MeV [107,108]) with a quasimonoenergetic spectrum [109,110], and better efficiency accelerating other heavier ion species as long as they were present in the target [108]. However, RPA needs a more intense (dominated mechanism at 1023 W/cm2[111]) and highly spatially homogeneous laser profile of circularly polarized light at normal incidence to minimize early target heating and expansion and not destroy the ion beam before reaching the optimal acceleration [80,112]. These demanding operational conditions 20 1| Introduction impede the application of this acceleration mechanism. However, the effective use of RPA is anticipated to become available in the near future [75], as in the ELI-NP infrastructure in Măgurele, Romania [113–115], providing intensities up to 1024 W/cm2(currently in commissioning phase). Relativistic transparency regimes: Enhanced TNSA and breakout-afterburner (BOA). Relativistic transparency regimes offer the advantage of providing ultra-high intensity bunches of ions and a better conversion efficiency from laser to ion energy similar to the RPA regimes but at laser intensities comparable with the TNSA (1021 W/cm2) and easily achievable in current laser facilities [116]. However, this mechanism calls for ultra-thin targets (whose thickness depends on the laser pulse for an appropriate coupling) so a high-contrast laser is needed to not destroy the target before the arrival of the main pulse [117]. In this mechanism, an enhanced version of TNSA occurs. As the target is ultra-thin, all the electrons available in the focal volume are accelerated to relativistic velocities to generate the sheath field. Then, the hot electron plasma expands sufficiently to become underdense [116], the plasma frequency decreases below the laser frequency and the target becomes transparent to the incident laser pulse [117], which can penetrate deeply into the target. The longitudinal electric field produced by the laser peak couples with the ions in the plasma that move together beyond the target rear surface (BOA). The laser interacts with the whole target, independent of the charge-to-mass ratio [118], so no target cleaning is required for heavier ion acceleration. This mechanism produces, as TNSA, a broad energy spectrum but improves the conversion efficiency over TNSA yielding more intense ion bunches with energies predicted in the hundreds of MeV [116,119]. 1.3.3. Neutron production schemes As it has been discussed at the beginning of this chapter, the growing interest and need for compact neutron sources as a complement to major facilities has been developing in the same context than the high-power laser technology and its application to the generation of intense, pulsed, multi-MeV ion and electron beams. Leaving aside the mechanism for generating these beams, whether it is a medium-size accelerator or a laser facility, neutrons can be produced using one of the reactions discussed in section 1.2. Laser-driven ion beams can bring unique properties to the portfolio of neutron sources that are difficult to obtain using conventional acceleration facilities and that can be of interest 1| Introduction 21 to specific neutron beam applications. These are, for instance, the high instantaneous flux, the intrinsic directionality, and the ultra-short pulse already discussed above, in addition to a compact production process and a reduced cost of operation and maintenance in comparison with a conventional large-scale accelerator. This section briefly describes the main neutron production schemes used currently in laser facilities for the production of neutrons. More information can be found in [37,40] and the references herein. Photonuclear reactions Photonuclear reactions (see section 1.2.2), both (γ,n) or photofission, are possibly the most intuitive production mechanism to produce neutrons in a laser facility as, whatever the laser-acceleration process, the laser-plasma interaction always releases an intense bunch of relativistic electrons. These relativistic electrons can collide in a thick heavy material to produce Bremsstrahlung and induce photonuclear reactions in the same target. As an example of this production scheme, in 2014 Pomerant et al. [120] used the Texas Petawatt laser facility [121] (90 J, 150 fs) and micrometric plastic foils to produce a Maxwelian spectrum electron beam of 10,5 MeV which collided in a stack of natural copper plates. The result was an intense neutron source with 50 ps length and 1,2·109 n/shot (5,5·107n/cm2/shot) emitted isotropically. In the same work, the application of this neutron source for high-resolution Fast Neutron Resonance Radiography (FNRR) [122] is discussed. Bulk target production Bulk target production refers to the production scheme in which the laser acceleration mechanism and the nuclear reactions that produce neutrons occur in the same thick target. This scheme is used in the case of light ions inducing fusion reactions 1.2.4. The usual scheme is the acceleration of ions (usually deuterons) in the front surface of a deuterated target by means of the hole-boring acceleration process (see section 1.3.2). As the acceleration ions are pushed into the target due to the radiation pressure, they are inducing D(d,n) reactions in the target itself. The main advantage of the bulk target production method is the low energy requirements to induce the nuclear reaction D(d,n), that features no threshold. Also, the study of these mechanisms, especially in the shape of deuterated-clusters laser-induced implosions [123], has high interest for nuclear fusion studies. 22 1| Introduction Pitcher-catcher The double-target pitcher-catcher configuration is the most used scheme to produce neutrons and the most similar to the neutron production in a conventional accelerator-driven source. In a pitcher-catcher setup, ions (usually protons) are produced and accelerated by the laser in a thin target called pitcher by means of one of the mechanisms discussed above (section 1.3.2) although the habitual choice is the TNSA process as is the less demanding and robust mechanism for the current status of laser facilities. A secondary target, called catcher, is designed to produce neutrons by means of nuclear reactions (Usually Li or Be target, see section 1.2.4) and placed downstream in the preferred ion acceleration direction (target-normal in the case of TNSA). In Fig. 1.9 a schematic view of a pitcher-catcher setup is illustrated. Figure 1.9: Scheme of neutron production in the pitcher-catcher double-target configuration based on the TNSA ion acceleration process. In this work, the pitcher-catcher configuration was used in the experimental campaign carried out at the DRACO PW laser system at HZDR where different pitcher and catcher compositions were tested. Detailed information about the neutron production setup and results can be found in Chapter 3. 1| Introduction 23 Pitcher-catcher and bulk target comparison Comparisons between bulk target production and TNSA 1.3.2 can be found in the work of Willingale et al. [124] and Zulick et al. [125]. The used laser systems have very different properties: meanwhile in Willingale et al. the laser has a long pulse (400 fs) and higher energy (6 J) than in Zulick et al. (1,1 J and 40 fs), the peak intensity on target (2,6·1019 W/cm2) is two orders of magnitude lower than in Zulick et al. (2,0·1021 W/cm2). Using the bulk target configuration, Willingale et al. reported up to 5·105n/sr meanwhile Zulick et al. the yield reported is 1,2·105n/sr but, the 40 fs laser of Zulick et al. allow for high repetition rate (nominal, up to 10 Hz). Although the conclusion in Willingale et al. is that the bulk target approach enables higher yield for the same D(d,n) reaction, it is noted that the TNSA process in pitchercatcher configuration for another reaction as Li(p,n) can be more productive. Zulick et al. agree with this conclusion reporting 1·107n/sr for the Li(p,n) reaction in pitchercatcher configuration. In the same line, S. Kar et al. [126] reported 5·107n/sr for bulk CD target and 1·109n/sr for pitcher-catcher configuration using a D(d,n) reaction. The discrepancies between Willingale et al. and Kar et al. can be explained by the difference in energy and intensity of both laser systems. In Kar et al., 200 J of laser energy and 3·1020 W/cm2were used. 1.4. State of the art, prospects and objectives With the fast development of PW lasers worldwide [92] and the arrival of multi-PW laser opening the door for new and promising ion acceleration mechanisms, laser-driven neutron sources as a competitive alternative for conventional acceleration-based neutron sources could become a reality in the next decades. Although neutron production in a laser facility was already reported in 1970 [127], the first laser-driven neutron sources were based on thermonuclear fuel implosion in large facilities [128]. It was in 1998, with the work of Pretzler et al. [129], when table-top high-repetition rate production of laser-driven neutrons from a deuterated polyethylene target is reported, achieving 140 neutrons per shot of ∼2-3 MeV from the D(d,n) reaction measured by TOF with 1 m flight path. The experiment was conducted using a 200 mJ 160 fs laser at 10 Hz. Since this pioneering work, neutron production by laser rapidly scaled to higher fluxes. In 2002, Roth et al. [101] reported 2,8·107n/shot with a 30 J 300 fs laser and a pitchercatcher scheme of deuterated plastics. In 2010, Higginson et al. [130] increased the 24 1| Introduction neutron flux reporting 2·109n/shot with a Cu pitcher and LiF catcher. Just a year later, the same research group produced 1010 n/shot up to 18 MeV using CD2targets as pitchers and a LiF catcher. In 2013, the work of Roth et al. [118] claimed the record neutron pulse achieved nowadays with a peak intensity of 1010 n/shot, up to 170 MeV energy. The pulse was achieved using submicrometer deuterated targets and a Be converter for neutrons in the 200 TW TRIDENT laser facility of Los Alamos National Laboratory (60 J and 600 fs) [131]. The excellent contrast ratio of TRIDENT (10−10) allows relativistic transparency regimes for acceleration with an intensity of 1021 W/cm2. In the work of Klir et al. [132] is available a collection of neutron production works, with the maximum neutron flux achieved as a function of the laser energy and the laser intensity. The Roth et al. work from 2013 was produced in a single shot mode but, as was discussed in previous sections, PW-class lasers at a high repetition rate were already commercially available. Emerging acceleration regimes, based on RPA or relativistic transparency, will be also accessible with the new class of multi-PW laser facilities such as APOLLON or ELI, which also promise multi-shot operation. Taking this into account, a pulse with the same characteristic as the Roth et al. from 2013 can be produced in the next years at a high repetition rate. In the work of Guerrero et al. [41], a calculation of the neutron field at 1 m flight path obtained for a pulse of similar characteristics of Roth et al., produced at 10 Hz and moderated by 10 mm of water is compared with conventional large-scale accelerator-based neutron sources such as n_TOF at CERN [46] or GELINA [45], indicating an equal or superior performance of the laser. In the same line, Roth et al. [38] in 2017 estimated the neutron flux for a 200 J and 20 Hz of about 1013 n/shot and forward directed, i.e. not far from the world-class 6·1014 n/shot of the LANSCE spallation source in USA. However, although the production of neutrons has been intensively studied and reported, the literature regarding the applications of these neutrons is limited and mainly only focused on the kind of applications to which integral or current-mode measurements are sufficient. So, regardless of if the new generation of large laser facilities could be competitive with the large acceleration-based facilities in the near future, it is still interesting to question whether laser-driven neutron sources can be competitive with medium-sized accelerator facilities nowadays, in terms of some applications. Maybe the most studied application has been neutron imaging. After Roth et al. [118, 133] demonstrated laser-based neutron radiography in 2013, in 2019 Kishon et al. [134] claimed the first laser-based neutron absorption spectra with a focus on future material 1| Introduction 25 analysis through fast neutron resonance radiography (FNRR). In the same context, in 2022, Zimmer et al. [135] demonstrated three different techniques using a moderated laserdriven neutron beam: neutron resonance spectroscopy (NRS), neutron radiography and neutron resonance imaging (NRI) in the thermal range. The same range of applications is explored also in Refs. [136–138] and even specific instrumentation has been developed to adapt to the distinctive features of a laser-driven neutron source [135,139]. Regarding applications of interest in nuclear technology, Brenner et al. [140] reviews the activities in the laser system VULCAN for differential die-away analysis (DDAA) of nuclear waste and materials, while Perkins et al. [141] reports the used of laser-driven neutrons for material damage studied related to fusion. The exploitation of laser-driven neutrons for medical applications is also discussed, for instance, in Kasesaz et al. [142] for BNCT applications and in Mori et al. [143] for radioisotope production. Regarding nuclear physics applications, although expectations are high, only very little has been published. In Mirfayzi et al. [144] the calibration of time-of-flight scintillators in a laser-driven neutron source is presented. As an illustrative example, one of the ELI facilities, the so-called ELI-NP in Romania [91], has been designed especially for nuclear physics. Among others, neutron reaction cross-section measurements for astrophysical interest and plasma studies are also considered as a future application of ELI-NP [145] and the role of neutron and charged particle detectors in such a scenario is considered. Nevertheless, although the measurement of neutron-induced reactions is planned and considered in future laser facilities, not a single attempt or feasibility study has been found in the literature. Furthermore, the measurement of the neutrons produced in laser facilities has been always made through integral and or current-mode TOF measurements. None of these techniques are suitable for neutron-induced reaction measurements. This requires measuring individual reactions, individual neutron signals related to a single selected reaction product, etc. This of course calls for specific detection systems and electronics which are widely used at conventional accelerator facilities, but it will be certainly far from trivial to operate them in the very harsh environment of a high-power laser. This thesis work represents a first attempt to assess the feasibility of measuring individual neutrons at a high-power laser facility, as a proxy for eventually measuring neutroninduced reactions. The work has been carried out using the time-of-flight technique at the HZDR 1 PW DRACO laser and was complemented by measurements at the CNA accelerator-driven HISPANOS neutron source facility. 27 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source In the introduction, it has been mentioned the importance of developing and exploiting compact sources of pulsed fast neutron beams as a complement, and in some cases as an alternative, of state-of-the-art neutron facilities based on very large-scale accelerators. In this context, this thesis assesses the problem by looking in detail into two very different alternatives. The more straightforward approach is to upgrade a conventional low-energy ion accelerator with a pulsing system and optimize the experimental set-up to achieve the highest neutron yield and best resolution possible. The more innovative and quite disruptive approach is the use of high-power laser systems for particle acceleration and then neutron production. This chapter is devoted to the first approach and it deals with the commissioning and characterization of the fast pulsed neutron beams produced at the HiSPANoS neutron source hosted at the Centro Nacional de Aceleradores (CNA). Conventional fast neutron scintillators and the time-of-flight technique have been used to measure the neutron fields that can be obtained with different neutron production targets, offering a wide variety of beams suited for different purposes. Two goals have been achieved: a new compact pulsed fast neutron facility has been fully characterized and is now available to the user community; it is a perfect case to compare against the laser-driven neutron source discussed in the next chapter. 34 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 3 mm thick aluminum backing (D/Ti = 1,6). In this case, the beam traverses the thin D/Ti mesh losing just a fraction of the incident energy and, depending on this beam energy, can produce spurious neutrons through reactions in both Ti and Al. To subtract those from the 2H(d,n) neutrons, a blank "dummy" target has been made. In Fig. 2.4 is shown a picture of the different targets above mentioned. Figure 2.4: Picture of the different targets available at HiSPANoS for fast neutron production. The water-cooled target holder (left) is empty in the picture. The fresh Li disc should be placed in the central cavity. 2.3. Experimental setup for neutron detection The experiments carried out for the beam commissioning and the corresponding results are divided into 2H(d,n) reactions yielding quasi-monoenergetic neutrons and thick target experiments yielding broad energy neutron beams. The experimental setup was based on liquid organic scintillators for the detection of neutrons and a fast digitizer for data acquisition. In this section, the experimental setup, common to both monoenergetic and white beams, is described. 2.3.1. Detection system Among the different types of fast neutron detectors available, organic liquid scintillators of 2"x2" by SCIONIX have been chosen for commissioning HiSPANoS, as fast organic scintillators are the conventional choice for neutron detectors for TOF experiments and most nuclear physics applications. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 35 The principle of operation of scintillator detectors is based on the property of scintillating materials to emit visible or near visible light when interacting with ionizing radiation. Then, these scintillating photons produce primary electrons in a photocathode that are multiplied in a dynode array inside the photomultiplier tube. The electron current generated is collected in the anode inducing an electric signal. Fig. 2.5 illustrates the scintillator mechanism. Figure 2.5: A sketch of a scintillator detector mechanism. As neutral particles, neutrons do not have the capacity to directly produce ionization when they come through a medium. The interaction of neutrons with matter is then produced by means of scattering or nuclear reactions. The detection of neutrons consists then of detecting the indirect ionization of the products of these interactions, so the chosen material has to be a high probability of interaction with neutrons. For fast neutron detection, hydrogen-rich materials are preferred, in which the interaction is produced via elastic scattering with the H and C nuclei [152] or nuclear reactions in C. The products of the reactions or the recoil of the H and C ions in each case are responsible to transfer the energy to the medium. Plastic and organic scintillators have a fast response that is particularly useful for time-of- flight measurements in pulsed beams (See section 1.1.2) but, on the other hand, organic scintillators have a high gamma sensitivity, with a broad deposited energy spectrum with usually overlaps with the one produced by means of neutron interactions. Nevertheless, for mixing gamma/neutron environments, as can be the case of HiSPANoS, some organic detectors have the possibility of performing pulse-shape discrimination analysis [153]. In these materials, the difference in the ionization process between gammas and neutrons produces a different light response in the scintillating components, producing pulses with different shapes (see section 2.3.3). A total of three liquid organic scintillators, one EJ-301 (Tadeo-1) and two EJ-309 (Tadeo- 2 and Tadeo-3), all coupled to fast photomultipliers Model No. 9214 from ET Enterprises, 36 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source have been used at different distances and angles from the neutron source. These detectors have a short temporal component (3,2 and 3,5 ns for EJ-301 and EJ-309 respectively) and excellent PSD capabilities. The main difference between EJ-309 material over EJ-301 is a slightly longer temporal component but it performs a safer operation due to a higher flash point (which prevents the fire hazard associated with liquid scintillators), low vapor pressure and low chemical toxicity [154]. The most relevant characteristics of a neutron detector used in nuclear physics experiments are its neutron detection efficiency and its capacity of distinguishing between neutron and γ-ray signals using Pulse Shape Discrimination (PSD) [155], a capability of some scintillators related to the different linear energy transfer (LET) of the nuclear recoils and electrons resulting from the neutron and γ-ray interactions, respectively. 2.3.2. Detector efficiency As the signal amplitude in a scintillator depends on the energy of the incoming neutron, so does the detection efficiency, which is significantly affected by the detection threshold applied. Since the detectors used for the measurements have not been absolutely calibrated in a reference laboratory, the efficiency reported by Pino et al. [2] for a similar detector (with a reported accuracy of 7%) has been assumed. The efficiency curves for 200 and 300 keVee thresholds are displayed in Fig. 2.6, together with the 9th degree polynomial functions used for interpolation. Neutron energy 0 1 2 3 4 5 6 7 8 9 Efficiency (%) 0 5 10 15 20 25 30 35 Pino et al. (2014) = 200 keVee th E = 300 keVee th E Figure 2.6: Neutron efficiency for an EJ-309 2"x2" organic liquid detector from Pino et al. [2] (with 7% accuracy), for the two energy thresholds used (200 and 300 keVee). The solid lines correspond to analytical fits to 9th degree polynomials. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 37 2.3.3. Data Acquisition System and PSD The detectors, the clock signal from the chopper, and the output of the current integrator are connected directly to the data acquisition system, without further electronic manipulation of the signal. This data acquisition system consists of a CAEN V1751 digitizer featuring 8 channels with a 10 bits resolution and a sampling rate of 1 GS/s with the DPP-PSD firmware installed, allowing the digital pulse processing for charge integration and pulse shape discrimination. The CAEN software CoMPASS was used for configuring the different channels and for operating the DAQ in coincidence mode: the system analyzes the detector signals only when they are within a time coincidence window with respect to a reference channel, which is the chopper clock, a logic signal produced each time the chopper allows the beam pass. Time (ns) 0 10 20 30 40 50 60 70 Amp (a.u.) 2− 10 1− 10 1 long Q Neutron -raysγ Timestamp short Q Figure 2.7: Difference between neutron and γ-ray signals in the EJ-301 detector and definition of the signal parameters returned by the DAQ. For each signal, the DAQ returns a timestamp (corresponding to signals rising point above 25% of the maximum amplitude, i.e., a digital Constant Fraction Discriminator, CFD) and two values corresponding to the total (Qlong) or partial (Qshort) integrals of the signal. The start time of both integrals is defined 5 ns before the signal starts, then the partial integral (Qshort) stops slightly after the signal maximum (∼7 ns), beyond which the γrays and neutron signal shapes become different. The total integral (Qlong) last up to 70 ns, when the longer signals ends. This is illustrated in Fig. 2.7, where the mentioned timestamp and the integration intervals are depicted. The relative difference between the two integrals, called PSD and given by Eq. 2.1, provides the means to differentiate between neutron and γ-ray signals by Pulse Shape Discrimination (PSD). 38 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source PSD =Qlong −Qshort Qlong (2.1) The distribution of the PSD values recorded from 2H(d,n) reactions with a 1,5 MeV deuteron beam is represented in Fig. 2.9 as a function of the signal energy in light equivalent keVee units, obtained from the calibration with 137Cs, and 60Co sources. As photons do not produce photoelectric reactions in organic materials, the spectra in an organic scintillator for gamma sources do not have photopeaks. The calibration was made then using as a reference the Compton edge energy (477,34 keV for 137Cs and 963,39 and 1118,11 for both gammas of 60Co). The position of the Compton edge was defined as 70% of the falling edge (see Fig. 2.8). Channel 0 200 400 600 800 Counts 0 500 1000 1500 2000 2500 3000 557 Channel 0 500 1000 1500 2000 Counts 0 100 200 300 400 500 1271 1473 Compton edge energy (keVee) 600 800 1000 1200 1400 Channel 400 500 600 700 800 900 1000 1100 1200 1300 Calibration points Tadeo 1 Tadeo 2 Tadeo 3 Calibration lines Tadeo 1 Tadeo 2 Tadeo 3 Figure 2.8: Top: calibration spectra of 137Cs (left) and 60Co (right) for one of the scintillators with the position of the Compton edge labeled. Bottom: Calibration lines for the three scintillators used in the experimental campaign. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 39 Figure 2.9: Measured distribution of the PSD values obtained as a function of the signal energy measured for the 2H(d,n) reaction with Ed=1,5 MeV. The solid line represents the analytical function separating both particle types. PSD 0 0.1 0.2 0.3 0.4 0.5 CµCounts/ 0 0.02 0.04 0.06 0.08 0.1 0.12 γn/ D γ FWHM n FWHM Neutrons -raysγ Figure 2.10: PSD distribution for signals between 200 and 2500 keVee (projected from Fig. 2.9). The green line corresponds to a fit to two Gaussian distributions. In Fig. 2.9, the data are clearly distributed along two branches corresponding to neutrons (upper branch) and γ-ray (lower branch) signals. The separation between the two types of signals has been modeled by the 4th-degree polynomial, shown as a solid line. At low deposited energy, below ∼200 keVee, the small size of the signals combined with the limited resolution of the digitizer and the non-negligible noise level prevents clear particle discrimination. The upward bending of the two distributions above 1300 keVee is related 40 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source to the non-linear behavior of the voltage divider for larger signals, which affects more the partial (Qshort) than the total (Qlong) signal integrals. This limits the PSD capabilities to up to 2500 keVee. Overall, γ-ray and neutron signals are very well separated, as it is illustrated in Fig. 2.10. The PSD features a figure-of-merit (FoM) of 1,5, where FoM is defined by Eq. 2.2, as in Guerrero et al. [153]. FoM =Dn/γ FWHMn+FWHMγ (2.2) 2.4. Quasi-monoenergetic neutron beams In this section, the study of the energies and fluxes of quasi-monoenergetic neutrons that can be produced at HiSPANoS by means of the 2H(d,n) reaction is carried out. This is interesting for future experiments requiring such beams, but also a very valuable tool for characterizing the energy resolution of the facility, as well as for studying the scattering neutron background inherent to a non-collimated facility such as HiSPANoS. Leaving the monitor detector (Tadeo-1) fixed at an angle of -30ºat a distance of 100 cm from the neutron source, the other two detectors (Tadeo-2 and Tadeo-3) have been placed at time-of-flight distances between 1 and 4 m and angles between 0ºand 120º(See Fig. 2.11, upper graph). The deuterium beam energy has been changed between 750 and 3000 keV, producing quasi-monoenergetic beams between 2 and 6 MeV. The results obtained will be detailed in the following subsections. Figure 2.11: Picture of the experimental set-up at HiSPANoS. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 41 2.4.1. Time-of-flight (TOF) spectra In each experiment, the output returned by the DAQ is a list of signals characterized by their time, Qshort and Qlong. The corresponding time-of-flight (TOF) histograms are produced by taking the chopper clock signal as a reference start time. The measured TOF is calibrated so that the γ-flash (a pronounced peak corresponding to the arrival of prompt γ-rays produced in the interaction of the pulsed ion beam with the target) is set to the time required by photons to travel the corresponding flight path, i.e. tflash =LT OF /c, where LTOF is defined as the distance between the neutron production target and the center of the detector active volume (not the front face). time-of-flight (ns) 0 20 40 60 80 100 Cµ counts/ 1− 10 1 10 2 10 All Neutrons -raysγ Neutron peak -flashγ FWHM = 2,35 ns ° = 1 MeV & 0 d E Figure 2.12: TOF spectra for 2H(d,n) at 1,0 MeV beam energy measured at 150 cm in the forward direction. The blue line corresponds to neutron signals (see Section 2.3.3). Fig. 2.12 displays the result of the configuration of a 1,0 MeV deuterium beam hitting the D/Ti target measured with Tadeo-2 at 150 cm distance in the forward direction. The histograms correspond to all (red) and neutron-only (blue) signals, illustrating the excellent PSD capabilities achieved. The γ-ray TOF distribution (green) features a constant level (environmental background), a peak corresponding to the γ-flash, and a second peak of gammas coming at the same TOF as neutrons that only can be removed due to the PSD capabilities of the detector. The γ-flash has a width of 2,35 ns corresponding to the time spread of the ion beam and, as mentioned above, serves as an accurate reference for the neutron production time. The neutron TOF distribution features a pronounced peak corresponding to neutrons of very well-defined energy, i.e. the quasi-monoenergetic neutrons resulting from the 2H(d,n) reaction on the thin D target. The tail observed at 42 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source longer TOF values, two orders of magnitude below the main neutron peak, is related to the deuterium diffusion into the backing [156]. 2.4.2. Background characterization Despite the neutron energy resolution being improved by the increasing flight path, before setting up the experiment, the limited size of the experimental area is an important factor to consider. Geometrically speaking, the dimensions of the tandem hall allow up to 3,0 m of flight path at 0ºand more than 4 m at 60º. However, when the detector is close to the walls, structures start to appear in the measured spectra due to scattered neutrons, as shown in Fig. 2.13. The corresponding neutron time-of-flight distributions of two different flight path distances in the forward direction for Ed=1,5 MeV are represented in the figure. At 3 m, the detector is only 75 cm from the wall, and thus a sizable neutron scattered structure appears, in this case, in the region between 40 and 200 ns. A similar structure appears when the detector is only 1,5 m from the source, hence further from the wall, but it is much more compact. Although these structures have to be taken into account, specially for higher energies in which the scattered component and the direct component can overlap, the contribution of the scattered neutrons can be subtracted by a shadow bar measurement [157]. time-of-flight (ns) 50−050 100 150 200 -2 C/mµ Counts/ 1− 10 1 10 2 10 = 300 cm tof L = 150 cm tof L Wall effect Figure 2.13: Time-of-flight distribution for Ed=1,5 MeV in the forward direction, illustrating the wall effect (neutron scattering background) depending on the flight path. The spectra are normalized by L−2 in meters. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 43 A shadow bar (10 cm of Pb followed by 50 cm of polyethylene, both with a diameter of 20 cm) has been used to estimate the neutron scattering background in different configurations (Fig. 2.14). The shadow bar works by shielding and scattering out the direct component of the neutrons coming from the target so, the signal measured in the detector corresponds to the fraction of neutrons that come from other directions, i.e., the neutrons coming back into the detector after being scatter in the walls, floor or ceiling, producing neutron background. This neutron component can be then subtracted from the measurement to obtain a clean result. Figure 2.14: PE + Pb shadow bar covering the direct component of neutrons arriving at the detector. The beam comes from the right side of the picture. In the case of the 2H(d,n) reactions, also the mentioned dummy target (Fig. 2.4) has been used to study the neutrons produced in the Ti and Al present in the D/Ti target assembly which cause another significant source of background. In Fig. 2.15 is shown the time- of-flight distribution (yellow line) and the background components due to the scattered neutrons (red line) and the target backing (green line) for three different energies for the ion beam (1, 2 and 3 MeV). At lower energies, both components are practically negligible meanwhile, as the energy is increasing, also the production in the backing because more reaction channels are opened. The worse case is for Ed= 3 MeV, where the reactions in the backing become the main component. The scattered fraction of neutrons dominates at longer times due to the extra time needed to arrive at the walls or floor and come back to the detector. The level of scattered neutrons also increases proportionally with the energy of the beams (i.e., with the overall neutron production). This study of the different background sources will allow us to clean the experimental TOF spectra obtained during the fast neutron commissioning at HiSPANoS. 50 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source dependence with the square of the distance, the subtraction of the background, which has been shown to increase with the distance to the neutron source, is thus validated. The bottom graph in Fig. 2.19 also shows a variation up to a factor of six of the neutron flux with the angle. This is another proof of the correct background subtraction, since the neutrons scattered lose the angular information from the source and the experimental values are in very good agreement with the reference data. (m) TOF L 0 1 2 3 4 5 Cµ/ 2 n/cm 1 10 2 10 L&P (1973) This work behaviour -2 L °0 °60 Angle (deg) 20−020 40 60 80 100 120 140 Cµ/ 2 n/cm 0 20 40 60 80 100 120 140 L&P (1973) This work Figure 2.19: Neutron flux measured at different distances (at 0º) and angles (at 150 cm) with a deuteron beam of 1,5 MeV impacting on the D/Ti target. The solid lines correspond to the data of Liskien and Paulsen [3] retrieved with NeuSDesc [4]. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 51 Overall, Table 2.2 summarizes the configurations (beam energy and angle) available and the corresponding fluxes for quasi-monoenergetic neutron beams between 2 and 6 MeV, which are represented in Fig. 2.20. The beam energy of Ed=1,5 MeV has been chosen for producing low-energy beams because, although neutron production is larger at higher energies, the background contribution from the target assembly is still negligible at this energy. In order to ease the beam time and counting rate estimation, the neutron flux in terms of n/cm2/s for both continuous and pulsed beams is given, considering a reference distance of 150 cm, a 5µion beam current in continuous mode and a 4%duty cycle. The corresponding source neutron flux values in n/sr/µC are also included in the table. The uncertainty in the flux is about 10%, considering the uncertainties in the detector efficiency (7%[2]), the finite thickness of the detectors at this distance (3%), and the reproducibility of the results from the neutron monitor (7%), which also includes a 3% accuracy in the beam current measurement. Other error sources have been deemed negligible. Figure 2.20: Collection of monoenergetic neutron beams available at HiSPANoS covering the energy range between 2,2 and 6,1 MeV. 52 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 2.5. Broad white neutron beams The reactions from deuterium beams impacting on the thick Li and Be targets, which produce neutron beams with a broad energy distribution, have been studied only with Tadeo-2 in the forward direction at a fixed time-of-flight distance of 150 cm. This is considered a reference position that provides a reasonable energy resolution while keeping the wall effects negligible. As opposed to the narrow energy beams produced in the interaction of the deuteron beam with the thin 2H target, when the same beam impacts a thick Li or Be target it produces a broad energy neutron beam with much higher flux [5] (see Fig. 2.21). Figure 2.21: Neutron yields for different thick targets as a function of the energy of the ion beam. Figure retrieved from Y. Zuo et al. [5]. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 53 Due to the energy range of the deuteron beam available at HiSPANoS, the reactions on thick Li and Be targets yield the highest neutron production as it is shown in Fig. 2.21. The neutron energies, according to the positive Q-values reported in Table 2.1, are up to ∼10 MeV for the Be target and exceeding 15 MeV for Li. Regarding the latter, a practical aspect to be considered is that while the melting point of Be is 986 ºC, that of Li is only 180 ºC, thus requiring forced cooling when high beam currents are applied. At HiSPANoS, this is achieved by means of the water-cooled target holder mentioned in Section 2.2.4 (see Fig. 2.4). 2.5.1. TOF/Enspectra and scattering background With the same procedure that has been explained in section 2.4.1, the time-of-flight spectra corresponding to the Li and Be targets when bombarded with the highest energy available (5,75 MeV) deuteron beams are displayed in Fig. 2.22. In contrast to the case of the thin D/Ti target, the thick targets stop the ions completely, avoiding neutron production elsewhere and thus making a dummy target background subtraction unnecessary. Hence, only the background from scattered neutrons needs to be measured, with the help of the aforementioned shadow bar (detailed in section 2.4.2). This contribution to the time-of-flight and neutron energy distributions measured is illustrated in Fig. 2.22, in the form of green lines. The neutron scattering background is especially relevant at long TOF (>110 ns), corresponding to low neutron energy (<1 MeV). As the detection efficiency for direct neutrons in this energy range is smaller and eventually null, the neutrons detected in this temporal region correspond with scattered neutrons with high energies but arrive at the detector at longer times. However, the shadow bar method works well and allows an accurate subtraction of the neutron scattering background. The time-of-flight and neutron energy distributions (displayed in Fig. 2.22) feature structures that are worth being discussed. Although all neutrons are produced by means of (d,n) reactions, when the deuteron brings enough energy to the system the residual nucleus (8Be in case of Li and 10B in case of Be) can reach different excited states, which leaves less energy available to the emitted neutrons. Hence, there is a correspondence between the structures observed and the energy of these excited states, which is illustrated in the bottom graph of Fig. 2.22. In this figure, the neutron energy (En) is subtracted from the total energy available (Ed+Q). The result demonstrates the correspondence between the observed structures and the well-known nuclear levels [159] (labeled in the figure) in 8Be and 10B. 54 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source time-of-flight (ns) 0 50 100 150 200 Cµ counts/ 10 2 10 3 10 4 10 htof_neu_CH2_copy Entries 2417741 Mean 65.22 Std Dev 19.64 Excited states structures Thick Be target Be + shadow bar Clean Be(d,n) -flashγ time-of-flight (ns) 0 50 100 150 200 Cµ counts/ 10 2 10 3 10 4 10 htof_neu_CH2_copy Entries 2417741 Mean 65.22 Std Dev 19.64 Excited states structures Thick Li target Li + shadow bar Clean Li(d,n) -flashγ Neutron energy (MeV) 1− 10×512 3 4 5 67 10 20 30 Cµcounts/ 10 2 10 3 10 4 10 Thick Be target Be + shadow bar Clean Be(d,n) Neutron energy (MeV) 1− 10×512 3 4 5 6 10 20 30 Cµcounts/ 10 2 10 3 10 4 10 Thick Li target Li + shadow bar Clean Li(d,n) (MeV) n + Q - E d E 0 1 2 3 4 5 6 7 8 9 10 Cµcounts/ 1 10 2 10 3 10 4 10 0.72 1.74 3.59 4.77 5.11 5.16 5.18 5.92 6.03 6.13 6.88 Be(d,n) B levels 10 (MeV) n + Q - E d E 0 2 4 6 8 10 12 14 16 18 20 Cµcounts/ 1 10 2 10 3 10 4 10 3.03 11.35 16.63 16.92 Li(d,n) Be levels 8 18.15 Figure 2.22: Time-of-flight (top), neutron energy (middle) and Ed+Q−En(bottom) spectra for Ed=5,75 MeV on the thick Li (right) and Be (left) targets measured forward at 150 cm. Although for potential applications, a white spectrum always would be chosen with the maximum energy to improve the neutron flux and the maximum energy available, the Be thick target was irradiated at other beam energies to study the behavior of the neutron field produced. The result is presented in Fig. 2.23. The effect of the energy resolution is lower at lower beam energies, as can be noticed in the spectrum at Ed= 2 MeV, where the curve shows a sharp and well-defined edge while, at higher beam energies, the curves are less sharp, showing neutron energies beyond the maximum energy available due to the 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 55 reaction Q-value (see Tab. 2.1). The structure of levels also changes between the curves, showing that an important contribution in the final neutron flux is due to channels opened beyond the 2 MeV energy beam, which increase the neutron production by more than an order of magnitude between 1 MeV and 3 MeV at Ed= 4 MeV, and extended up to 5 MeV for Ed= 5,75 MeV. Neutron energy (MeV) 1− 10×312 3 4 5 6 7 8 910 Cµcounts/ 1 10 2 10 3 10 4 10 5 10 = 2 MeV d E Thick Be target Be + shadow bar Clean Be(d,n) = 4,0 MeV d E Thick Be target Be + shadow bar Clean Be(d,n) = 5,75 MeV d E Thick Be target Be + shadow bar Clean Be(d,n) Figure 2.23: Neutron production in the thick Be target at different beam energies. The shadow bar spectra and the clean Be(d,n) reaction are also displayed. 2.5.2. Discussion: available beams and neutron flux The background-subtracted neutron energy distributions corresponding to (d,n) reactions on Li and Be have been divided by the efficiency, the solid angle covered by the detector, and the integrated current in order to be transformed into units of neutron flux (n/sr/µC/MeV). The result is displayed in Fig. 2.24, with the uncertainty in the neutron energy calculated from Eq. 2.3 and the 10% uncertainty in the neutron flux discussed in Section 2.4.5. The figure also includes a line corresponding to the data reported by Meadows [6] for a 5,8 MeV deuteron beam impinging on a thick Be target, referred to the forward direction; and also the data from Weaver [7] with a deuteron beam of 5,0 MeV impacting on a thick Li target and measured at 3,5º. There is indeed an excellent agreement between the HiSPANoS and Meadows data in terms of both energy distribution and neutron flux and, in the case of Weaver, the neutron flux from HiSPANoS is slightly higher in the energy region from 3 to 10 MeV, which is expected due to the difference in the deuteron energy 56 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source Neutron energy (MeV) 12 3 4 5 6 7 8 910 20 30 C/MeVµn/sr/ 7 10 8 10 9 10 Li(d,n) Be(d,n) Meadows 1993 Weaver 1973 Figure 2.24: Neutron yields at Ed= 5,75 MeV and a time-of-flight distance of 150 cm in the forward direction for the Li and Be thick targets. The blue line corresponds to the data from Meadows [6] for 5,8 MeV deuteron beams at 0ºand the yellow line corresponds to the data from Weaver [7] at 5,0 MeV and 3,5º(Retrieved from EXFOR [8] F0218.015). and the measuring angle. The comparison at low energy evidences the limitations when measuring neutrons below 1-2 MeV with liquid scintillators, as in this work (see Fig.2.6), compared to measurements with another kind of detectors as the fission chamber used in Meadows. Then, the better energy resolution of Meadows and Weaver, related to the longer flight path used (3,75 and 11 m respectively against 1,5 m), is only and barely visible at the fall-off around 10 MeV in the case of Be, and 15 MeV in the case of Li. As discussed above, the energy resolution is also responsible for showing neutron energies beyond 20 MeV, which is the physical limit according to the Q-value of the corresponding reaction (see Tab. 2.1) and the beam energy. The broad energy neutron beams available at HiSPANoS for the maximum Ed= 5,75 MeV are summarized in Table 2.3 (lower energies are also available). Integral neutron production refers to neutrons above 1 MeV, the lower energy limit in the detection efficiency of the liquid scintillators used for neutron detection. As in the case of the quasi-monoenergetic neutron beams, the neutron fluxes for the continuous and pulsed beam configuration at a distance of 150 cm are given in units of n/cm2/s, and the flux at the target is given in n/sr/µC. 2| Neutrons at the CNA accelerator-based HiSPANoS neutron source 57 Table 2.3: Summary of the broad energy neutron beams available at HiSPANoS (intermediate energy configurations are also available). The neutron flux at the target is given in n/sr/µC, as well as in n/cm2/s at a distance of 150 cm for both continuous and pulsed neutron beams. Reaction EdMax. EnΦnΦn(n/cm2/s) (MeV) (MeV) (n/sr/µC)Continuous Pulsed Li(d,n) 5,75 ∼20 2,2(0,2)·1010 5,0(0,5)·1062,0(0,2)·105 Be(d,n) 5,75 ∼10 2,5(0,3)·1010 5,4(0,5)·1062,2(0,2)·105 To summarize, in this chapter, the neutron energy, resolution, background, and neutron flux of the different beams available at HiSPANoS (for different targets, beam energies, angles, and distances) have been characterized, and shown to be in agreement with the expectations from existing reference data. The corresponding neutron yields at the target are in the order of 106and 1010 n/sr/µC for quasi-monoenergetic and broad energy neutron beams respectively (see details in Tables 2.2 and 2.3). The reported 10% accuracy in the neutron fluxes described in this chapter is dominated by a 7% contribution from detector efficiency (which could be reduced by using an absolutely calibrated neutron detector) and another 7% from the reproducibility of the results. There is ongoing work at CNA aimed to reduce the latter. 59 3| Neutrons at the HZDR PW-laser facility DRACO The development of increasingly intense but compact neutron sources is interesting for many research fields and applications. As discussed in the previous chapter, a conventional accelerator producing ions up to just a few MeV can make a very versatile source of fast neutrons, providing even pulsed beams. However, the ion beam intensity and hence the corresponding neutron production per pulse for performing time-of-flight nuclear physics measurements is very far from that achieved at very large GeV-based facilities, such as n_TOF at CERN or Back-n at CSNS, among others. In this context, laser-driven neutron sources could become very intense compact neutron sources with capabilities comparable to those of the very large GeV-based facilities mentioned: low-frequency but very intense pulsed neutron beams; which is precisely what is needed for performing some of the most interesting nuclear physics experiments. This chapter describes the experiment designed and carried out at the DRACO laser facility of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in the context of the EC FP7 NeutAndalus project and funded through the H2020 ARIEL and LASERLAB-EUROPE projects. In order to assess the feasibility of nuclear physics experiments at DRACO and similar laser facilities, the ion beam and the neutrons produced were first measured with conventional diagnostics, and then a very particular semiconductor diamond detector was used to measure single neutron signals, requiring an involved signal analysis algorithm. The results presented herein are discussed briefly at the end of this chapter, and more in depth in the final chapter. 66 3| Neutrons at the HZDR PW-laser facility DRACO Proton profiler: Aligned with the pitcher, it consists of a lanex screen usually attached to an absorber plate that blocks the protons up to a certain threshold energy. Higher energy protons reach the lanex, which is imaged by two different CCD cameras with different filters. The plate has a hole to allow the central fraction of the beam to go through unaffected and reach further diagnostics. The images of the lanex show the spatial distribution and alignment of the proton beam. During the experimental campaign, the catcher (neutron converter) itself was used as absorber plate, attaching a lanex screen at its rear surface (Fig. 3.6). This way, the spatial distribution of the protons over the catchers can be observed shot-to-shot. Proton frontside spectrometer: Placed in the opposite target-normal direction, it is composed of a lanex screen and a filter that can suppress carbon ions. The protons are magnetically deflected to characterize their energy distribution. During the experimental campaign, these protons produce a secondary parasitic neutron source that goes backwards. Figure 3.6: Images from the proton profiler lanex screen in the rear surface of a Cu catcher for aligned (left) and misaligned (right) shots. The screen attached to the rear surface of the catchers is visible in Fig. 3.7. 3.3. Set-up for neutron production and detection at the DRACO laser The experiments at DRACO were done using a pitcher-catcher configuration setup for the production of neutrons: the ion beam produced in a first target (“Pitcher”) via TNSA 1.3.2 collides with a second target (“Catcher”), producing neutrons by means of nuclear reactions, as in HiSPANoS and other accelerator-based neutron facilities. In order to study and optimize the neutron production and the performance of the detectors, different pitcher-catcher configurations were compared. 3| Neutrons at the HZDR PW-laser facility DRACO 67 Regarding the detection of neutrons, the conventional systems employed in laser-driven experiments consist of bubble detectors and organic scintillators for time-of-flight operated in current-mode. However, the eventual nuclear physics experiments to be carried out require detectors that are sensitive to each single neutron interaction. Hence, the previously mentioned detection systems have been complemented with another, more adequate for nuclear physics experiments. This consists of two diamond detectors, which have been tested for the first time in this work in the very particular environment of a laser experiment in terms of γ-flash sensitivity, electromagnetic noise, signal pile-up, and dead-time. The results shall shed light on the feasibility and eventual limitations of carrying out these kinds of experiments at laser-driven neutron sources. 3.3.1. Pitcher-catcher setup The pitcher holder used during the experiment consisted of a matrix with a capacity of six-hundred thirty pitcher targets, divided into nine sectors with seventy targets each distributed, in turn, in fourteen rows and five columns. Each of the nine sectors could be loaded with different target materials. During the campaign at DRACO, three different materials for ion production were tested as pitchers: •200 nm thick formvar for protons, made on-site at HZDR. •180 nm thick polystyrene for protons, provided by TU Darmstadt. •180 nm thick deuterated polystyrene for a mix of protons and deuterons, provided by TU Darmstadt. There were also three different catcher targets for the production of neutrons: •A 3 mm thick copper plate (10x10 cm2), placed at 74 mm from the pitcher. •Three 10 mm thick LiF discs (3,8 cm in diameter) in an aluminum holder placed at 74 mm from the pitcher. •A 60 mm thick polyethylene block placed at 2 cm from the previous catchers (∼10 cm from the pitchers) used as an ion beam dump. The catchers include a centered 4 mm hole to allow a fraction of the ion beam to reach the TPS, enabling its shot-to-shot characterization. All the targets were mounted on motorized posts and rails, allowing any combination of pitcher and catcher without breaking the vacuum in the interaction chamber. Fig.3.7 shows a sketch of the pitcher-catcher configuration where all the relative distances are indicated, along with real pictures of the setup. 68 3| Neutrons at the HZDR PW-laser facility DRACO Figure 3.7: Top: sketch of the pitcher-catcher configuration. Bottom: pictures of the pitcher-catcher arrangement (left), the pitcher holder with some pitchers (center), and the Cu and LiF catchers (right). 3| Neutrons at the HZDR PW-laser facility DRACO 69 3.3.2. Neutron detectors During the experiment, different kinds of detectors were employed, all sensitive to fast neutrons but with very different characteristics otherwise. They are summarized in Table 3.1 and discussed in detail in the following sections. Table 3.1: Overview of all the neutron detectors used during the experimental campaign and relevant information about the setup. * The shielding dimensions are given as front/sides/top. Detector Type Dimensions Position Shielding* InstitutionThickness Surface LT OF Ang. Bubbles (x13) Passive 145 mm 19 mm Variable None TUD/USC EJ200 (x2) Scint. 25,4 mm 51 mm 360 cm 15º15/10/6 cm Pb + 0/8/8 cm PE USE EJ232Q-USC Scint. 25 mm 150 mm 360 cm 15º15/10/6 cm Pb + 0/8/8 cm PE USC EJ232-Gated Scint. 20 mm 20 mm 360 cm 15º15/6/6 cm Pb + 0/8/8 cm PE HZDR CVDs (x2) Semicond. 140 µm 4x4 mm 144 cm 15º5 cm Pb HZDR 500 µm 148 cm CIVIDEC Bubble detectors Bubble detectors are small super-heated droplet detectors mainly designed as personal dosimeters in medical facilities, nuclear power plants, and nuclear applications. These passive detectors are based on an elastic polymer medium with microscopic superheated droplets of a liquid (typically a type of freon) dispersed throughout. When the neutron interacts with the medium, the energy deposited vaporizes the droplets, forming a visible gas bubble that remains trapped in the matrix [164]. By contrast, low-linear energy transfer radiation, such as γ-rays, does not deposit enough energy locally to be able to trigger the bubble formation, making these detectors insensitive to photons [165]. After the irradiation with neutrons, the number of bubbles is proportional to the received dose for a wide neutron energy range [166]. In Fig. 3.8 there is an example of a bubble detector before and after the irradiation. After the irradiation, the detector can be recompressed again and reused for several cycles. 70 3| Neutrons at the HZDR PW-laser facility DRACO Figure 3.8: Bubble detectors before (bottom) and after (top) being irradiated in a neutron source. For these devices, each bubble corresponds to ∼0,4 µSv. In the context of laser-driven neutrons, bubble detectors are widely used for characterizing the neutron field produced, since they are insensitive to γ-rays, do not rely on electronic systems that can be affected by the EMP produced in the laser-plasma interaction, cover a large energy range (from thermal to fast neutrons) with a relatively flat response and, most importantly, are straightforward to analyze (although the background contribution is normally neglected but can be quite significant, as it is demonstrated later on in this chapter). During the experimental campaign in DRACO there were thirteen 145 mm length x 19 mm diameter BD-PND bubble detectors from BTI Bubble Technology Industries available, each one with an integrated re-compression system; and a corresponding BDR III (Bubble detector reader, TUD). Twelve of the thirteen detectors were placed, always in pairs, in different positions around the interaction chamber and next to the main detectors for the different configurations. The remaining detector was kept in the control room as a monitor (Fig. 3.10). In the chamber wall, the measuring points were established at 0º,±23º,±45º,±68º,±90º, ±113º, 135º, 158º(from the proton line), coinciding with the vertexes of the geometrical plan of the chamber, in order to create an angular map of the neutron field produced by the p+LiF and p+Cu pitcher-catcher configurations. For some pitcher-catcher configurations, measures were taken at only 0º, 60ºand 120º, with the objective of characterizing and 3| Neutrons at the HZDR PW-laser facility DRACO 71 comparing the differences in the total neutron production and in the angular distribution of the generated neutron fields. For the bubbles that were placed next to the detectors, the measuring points were outside and inside the scintillators castles and on top of the diamond detectors. For each configuration, the bubble detectors remained in the experimental area for at least ten shots, to reduce the effect of the shot-to-shot fluctuations and accumulate enough statistics. Organic scintillators Organic scintillators, as introduced in section 2.3.1, are a conventional choice for neutron detection in TOF experiments and in many nuclear physics applications due to their easy use, sizable efficiency for fast neutrons and fast timing properties. As it was previously mentioned, the sensitivity to fast neutrons arises from the neutron elastic scattering from the H and C nuclei, as well as some reactions in C. They are also sensitive to γ-rays, mainly through Compton scattering, given the light elements that make up the scintillator molecules. Due to the nature of these interactions, for both neutrons and γ-rays, organic scintillators do not feature spectroscopic properties. They are well-known and reliable devices, as demonstrated for HiSPANoS in the previous chapter. However, at HiSPANoS they are used, as generally, to observe individual neutron signals, distinguishing them from γ-ray background using the Pulse Shape Discrimination (PSD) capability, while at laser-driven neutron sources they are used in current-mode. In Fig. 3.9 there is an example of a single-shot scintillator signal. The trace shows two structures corresponding with the prompt gamma radiation from the laser-plasma interaction (γ−flash) and a delayed component corresponding to all the radiation coming from the proton interaction in the catcher, including the direct component of neutrons, but also background caused by neutrons and secondary γ-rays. This current-mode allows measuring the neutron fluence emitted in a single laser shot (the output voltage is proportional to the number of neutrons striking the detector and its distribution is related to the neutron energies through the time-of-flight technique [144]), but prevents Pulse Shape Discrimination (PSD). Furthermore, to reduce the very large γ-flash and the EMP from the laser interaction, the detectors are usually fully surrounded by bulky lead and polyethylene shielding, which affects the neutron beam by eliminating, deviating or multiplying them. In addition, neutrons can produce γ-rays in the heavy shielding and vice-versa. It is thus clear that this configuration is by no means optimal for neither neutron measurements nor eventual nuclear physics experiments. 72 3| Neutrons at the HZDR PW-laser facility DRACO Time (ns) 100−0100 200 300 400 500 Amp (V) 1.4− 1.2− 1− 0.8− 0.6− 0.4− 0.2− 0 -flashγ -raysγNeutrons + Catcher components Figure 3.9: EJ232Q-USC scintillator signal for 200 nm formvar pitcher and LiF catcher. In this experiment, three types of quite different scintillators have been employed (described below). They were all embedded in a shielding of about 15 cm in all directions, with only lead in the front face and a combination of lead and polyethylene in the rest. As indicated in Fig. 3.10, they were located at 15ºfrom the proton acceleration line and at a distance of 360 cm, which is the maximum possible given the size of the experimental room. However, this meant that the detectors were very close to the walls. •EJ200 (Phobos and Deimos): coupled to 9214 PMTs from ET Enterprises, these are 51 cm3conventional fast-timing neutron detectors used at accelerator-driven neutron facilities for ToF experiments. •EJ232Q-USC: coupled to an R2083 PMT from Hamamatsu, this is a very large (442 cm3) and very fast detector for ultra-fast counting applications that has been used in previous laser-driven experiments (see for instance [167]). The large volume allows for sufficient efficiency to measure neutron spectra in a single laser shot when operated in current-mode. •EJ232Q-Gated: coupled to an H7680-01 "Gated" PMT from Hamamatsu, it features a volume of just 6 cm3. This small volume, nearly two orders of magnitude smaller than the latter, results in a reduced efficiency that allows measuring single neutron interactions instead of using the current-mode. This operational mode requires a significant repetition of the laser to accumulate meaningful information; hence reproducing the conditions required for nuclear physics experiments. 3| Neutrons at the HZDR PW-laser facility DRACO 73 Figure 3.10: A sketch of the reaction chamber and the detectors placed around in the experimental room with pictures of the experimental setup. The detectors are distributed along a TOF line at 15º from the proton acceleration direction. Diamond detectors Chemical Vapour Deposition (CVD) Diamond detectors are a recent but already commercially available type of carbon-based semiconductor detector. It features higher mobility of the electron-holes than other kinds of semiconductors, such as Si or Ge [168], which provides a faster response, making them the most suitable choice for time-of-flight and other applications that require a high temporal resolution. They can be used, among other things, to detect neutrons. In the case of detecting 74 3| Neutrons at the HZDR PW-laser facility DRACO thermal or epithermal neutrons, a converter material is needed, usually in the form of a thin foil or a coating of 6Li or 10B [169]. In the case of fast neutrons, the electron-hole pairs are produced in the active body of the detector by the C recoils from neutron elastic scattering or by the products of nuclear reactions such as 12C(n,α). Figure 3.11: Simplified layout of a diamond detector. The ionizing radiation (direct or indirect) produces electron-hole pairs that are driven to the electrodes thanks to the HV, generating an electrical pulse proportional to the deposited energy. The ionization density and spatial distribution characteristics of each kind of interaction determine the shape of the output signal. This is illustrated in Fig. 3.11: a diamond layer between two metallic electrodes. An HV is applied between the electrodes to collect the electrons and holes [170]. Nuclear reactions induced by the neutron interaction with the C nuclei of the medium produce point-like ionization inside the active body (Fig. 3.11, bottom); meanwhile, the reactions induced by γ-rays produce electrons, and subsequently a homogeneous ionization in the detector [171] (Fig. 3.11, top). The current distribution produced in the sensor is determined by the combination of the ionization’s placement and the drifting time of the electrons and holes to the respective electrodes. Therefore, diamond detectors allow for Pulse Shape Discrimination (PSD) in mixed radiation fields, since the current signals from ions (such as the neutron-induced recoils) and electrons (such as those produced by γ-ray interactions) are very different, as illustrated in Fig. 3.11: a point-like ionization produces a rectangular or step-like signal, while a homogeneous ionization produces a triangular shape. A detailed 3| Neutrons at the HZDR PW-laser facility DRACO 75 analysis of PSD in diamond detectors can be found in [171,172]. Figure 3.12: Diamond detectors before (left) and after (right) electromagnetic shielding. The noise reduction includes also Al wrapping around cable connectors, noise suppressors in cables, and electrical isolation between electronic modules with plastic foils and foams. At DRACO, two 4x4 mm2diamond detectors with different thicknesses (150 and 500 µm) were installed at a 150 cm flight-path and operated with a 2 GHz / 40 dB C2-HV Broadband Amplifier (low-noise) by Cividec. Due to their very small thicknesses, they feature much lower efficiency than the scintillators, but also a much lower sensitivity to the γ-flash since the associated high-energy electrons would go straight through them without depositing all their energy. Therefore, they require much lighter shielding compared to the scintillators: 5 cm of lead in the front and a similar amount on the left side to prevent the arrival of secondary γ-rays from neutron interactions with the beam pipe and the diagnostics distributed along the proton acceleration beamline (including the TPS). On the other hand, this kind of detector is quite sensitive to the EMP (electromagnetic pulse), and thus electromagnetic shielding had to be implemented. It consisted of a watertight wrapping with aluminum foil around the detectors and the amplifiers, with the addition of a plastic cover to prevent metallic contact between the detectors and surrounding materials (Fig. 3.12). More details about the noise suppression are found in section 3.5. Therefore, due to the fact that diamond detectors are commonly used in nuclear physics experiments, and that they have been operated at DRACO to observe single neutron interaction without the need for heavy shielding and far from the walls, they become a very good proxy for an eventual nuclear physics experiment with neutrons from a laserdriven source. 82 3| Neutrons at the HZDR PW-laser facility DRACO The data and discussion above demonstrate that the information of the TPS-lanex can be used as a quality check for shot-to-shot monitoring, discriminating good and bad shots in terms of proton production. Table 3.2 summarizes the average production and dispersion for different days in which the laser-plasma interaction was kept stable, once the outliers are discarded using a two standard deviation criteria. The reproducibility of the proton source is always better than 30%, with no more than ten shots discarded each day, showing the robustness of the laser system and the acceleration setup at DRACO. Table 3.2: Summary of the proton production according to the TPS (integrated spectrum between 10-25 MeV) for days in which the laser-plasma configuration was kept constant. The dispersion of the values is always between a 20-30%. Date Good/Total Proton production shots p/shot/sr σ 18.10.21 59/65 2.8·1012 24% 22.10.21 217/221 5·1012 27% 26.10.21 101/108 5·1012 23% 29.10.21 217/222 6·1012 20% 3.4.2. Integral neutron production from bubble detectors In the literature (see [118,144,174,175] for instance), bubble detectors are the most widespread diagnostic used to probe the neutron production in a laser facility, providing valuable information about the production and angular distribution of the neutron source. At DRACO, bubbles were used to monitor the neutron production for each different pitcher-catcher configuration (protons impacting on either Cu or LiF catchers) in, at least, three different angles (0º, 60º, and 120º) and on different days. A detailed angular scanning was also made at 0º,±22,5º,±45º,±68º,±90º,±113º, 135ºand 158º, as was detailed in section 3.3.2. The 0ºreference was always the proton emission line, i.e. normal to the pitcher target. After each irradiation of the Bubble detector with a given number of shots, the number of bubbles (nb) produced by the neutrons was determined by the BDR III (Bubble detector reader). The BDR takes two pictures of the detector from opposite sides, and then an identification software counts the bubbles in the pictures. For each irradiation, each detector was read twice, rotated inside the reader to obtain the information from different angles, in order to minimize the effect of bubbles hidden by others in a particular point of view. 3| Neutrons at the HZDR PW-laser facility DRACO 83 Then, following the instructions from the manufacturers, the bubbles are transformed into dose by applying a conversion factor Cin bubbles per rem (which depends on the calibration of each device and is given by the manufacturer). The neutron flux Φnin n/sr is then estimated according to eq. 3.3 [176] from: •the dose in rem, •a correction factor S= 3,5 ·105bubbles/(n·cm2) given for a detector with 1 bubble/mrem calibration [177], •L2(in cm), which represents the distance between the bubble detector and the neutron source. Φn(n/sr) = nb·L2 C·S(3.3) The systematic uncertainty in this neutron flux measurement is very difficult to assess, and generally neglected in the literature. The upper panels of Fig. 3.17 represent the neutron production (as n/sr/shot) measured in the bubbles for the positions 0º, 60ºand 120ºin the proton on Cu (left) and LiF (right) configurations. The error bars represent the statistical uncertainty in the measurement, calculated as the square root of the number of bubbles. Different data sets are represented with different colors and markers. The solid and dashed lines represent the average and standard deviation of all measurements, assuming an isotropic field. For both catchers, the bubbles: •do not hint to an angular dependence in the neutron flux measured, •evidence day-to-day variations of about 30-40%, •do not indicate a significant (considering the variations mentioned) difference in the neutrons produced by each catcher. The average values for each individual data set are collected in Table 3.3. The dispersion of the data on the same day fluctuates between 5% on the best day (19.10.21) and 46% on the worst (27.10.21). 84 3| Neutrons at the HZDR PW-laser facility DRACO )°Angle ( 0 20 40 60 80 100 120 n/sr/shot 0 20 40 60 80 100 6 10× p + Cu Run #1 Run #2 Run #3 Run #5 Run #6 Run #8 Run #9 n/sr/shot 7 10⋅7(2) )°Angle ( 0 20 40 60 80 100 120 n/sr/shot 0 20 40 60 80 100 6 10× p + LiF Run #4 Run #7 Run #10 n/sr/shot 7 10⋅6(1) Figure 3.17: Top: Measurements at 0º, 60º, and 120ºin different runs for p+Cu (left) and p+LiF (right) with DRACO formvar standard pitcher targets. The average production (solid line) and standard deviation (dashed lines) are included in the figures. Bottom: Angular mapping for p+Cu (red circles and blue squares) and p+LiF (yellow stars) pitcher-catcher configurations also with standard pitcher targets. Each measurement was made on a different day. The uncertainties for each run (dashed area) were established in a 30% 3| Neutrons at the HZDR PW-laser facility DRACO 85 Run Date Pitcher-Catcher Angles Shots n/shot/sr n/shot Dispersion #1 18.10.21 p-Cu 0º, 60º, 120º10 7,0(0,7) ·1078,9(0,9) ·1089,8% #2 18.10.21 p-Cu 0º, 60º, 120º10 5,4(0,7) ·1077(1) ·10815,5% #3 19.10.21 p-Cu 0º, 60º, 120º10 7,9(0,4) ·10710,0(0,5) ·1085,1% #4 21.10.21 p-LiF 0º, 60º, 120º10 5(1) ·1076(1) ·10823,3% #5 22.10.21 p-Cu 0º, 60º, 120º10 7(2) ·1078(3) ·10829,9% #6 22.10.21 p-Cu 0º, 60º, 120º10 8(2) ·10710(3) ·10826,3% #7 26.10.21 p+LiF 0º, 60º, 120º10 7(1) ·1078(2) ·10819,4% #8 26.10.21 p+Cu 0º, 60º, 120º10 6,5(0,8) ·1078(1) ·10812,5% #9 27.10.21 p+Cu 0º, 60º, 120º15 5(1) ·1076(2) ·10823,9% #10 27.10.21 p+LiF 0º, 60º, 120º15 5,9(0,6) ·1077,4(0,8) ·10810,4% #11 26.10.21 p+Cu Angular scanning 10 5(2) ·1077(2) ·10834,4% #12 27.10.21 p+Cu Angular scanning 15 4(2) ·1075(2) ·10845,8% #13 28.10.21 p+LiF Angular scanning 15 6(2) ·1077(2) ·10833,14% Table 3.3: Configurations measured with bubble detectors and their corresponding neutron fluxes. The bottom panel in 3.8 represents the neutron flux measured for three different configurations with detailed angular distributions in a polar plot. As was mentioned in section 3.3.2, the bubbles were placed around the source, attached to the interaction chamber, which was not possible to do in the backward direction. Furthermore, the chamber is not symmetrical around the interaction point, and hence the distances between the bubbles and the source are not constant, varying between 73 cm (0º) and 222 cm (-113º). As shown in the figures above, the data do not show significant differences between the different pitcher-catcher configurations. The angular distribution seems to be slightly isotropic, showing a higher flux for positive angles. The asymmetry in the distribution can only be caused by the materials surrounding the neutron source, which can act both as shielding and as scatterers, reducing or increasing the neutron dose in each bubble detector depending on its position. The casuistic is quite complicated. For instance, negative angles correspond with the exit way corridor, which is a clearer area; while at positive angles, in addition to the detector castles and holders, there was a second massive vacuum chamber for the low-energy beamline. Inside the reaction chamber, at 90º, there was a lead block placed to shield a part from the γ-flash in that direction. In addition, there were two parasitic proton production beams: one backward, coming from the front surface of the catcher (towards -135º) and intrinsic to the TNSA process; and a secondary component toward the laser direction (45º), which is due to a hybrid acceleration mechanism between TNSA and RPA. 86 3| Neutrons at the HZDR PW-laser facility DRACO This discussion is meant to illustrate the limitations of the bubble detectors, which are the standard in laser-driven neutron experiments. Although they are easy to use and analyze and insensitive to γ-rays, their wide dynamic energy range (>200 keV) makes them highly sensitive to scattered neutrons, which is specially relevant in the case of measurements in small experimental rooms such as the one at DRACO. Furthermore, since the information provided is integrated in time, time-of-flight can not be used to estimate this background component. In addition, they are passive detectors capable of measuring during a few shots, but then need to be moved, analyzed and reset, which makes them not suitable for multi-shot experiments. Nevertheless, Bubbles can provide the order of magnitude of the upper limit of the neutron production. In this experimental campaign, a total production of neutrons larger than ∼108n/shot was achieved at DRACO assuming an isotropic production, with more than ∼107n/sr/shot in the forward direction. 3.5. Data analysis and results from the diamond detector Although the data retrieved by the bubbles provide valuable information about the neutron production with each different pitcher-catcher configuration, it is just an integral experiment that does not provide energy information, nor a way to estimate the contribution from the background caused by neutrons scattered in the room. The actual application of these laser-driven neutrons in nuclear physics experiments relies on the feasibility of measuring individual signals in time-of-flight experiments. This is also not possible to achieve with the organic scintillators set-ups commonly employed at laser facilities. As can be observed in the scintillators response shown in Fig. 3.9, in a laser facility the hard conditions of radiation background and the high instantaneous flux produce a strong saturation and pile-up effect that prevent the measurement of individual signals, a key factor in the viability of measuring nuclear reactions, which is the aim of this work. In the scintillator, the mixed field of neutrons and γ-rays produces a single structure that lasts hundreds of nanoseconds, collecting both the direct and background components. This signal has then to be deconvoluted relying on the detector response to single neutrons, neglecting in most cases the influence of the room background and the effect of the heavy shielding surrounding the detectors, illustrated in Fig. 3.10. This was done in the DRACO experiment in the context of a PhD thesis at the Technical University Darmstadt (TUD), and it is hence out of the scope of this work. 3| Neutrons at the HZDR PW-laser facility DRACO 87 Then, to achieve the objective of measuring a nuclear reaction via single events in the hard environment imposed by the laser-plasma acceleration, the particle detector used needs to have low-efficiency to prevent the pile-up, low γ-ray sensitivity to avoid background saturation and the need for heavy shielding and, hopefully, even PSD to distinguish neutrons from other particles. In the case of the DRACO campaign, the detectors chosen for this goal were two diamond detectors described above (section 3.3) and supplied by the neutron group from the HZDR (thin diamond) and the company CIVIDEC (thick diamond). Similar detectors operated in a similar configuration have been already demonstrated to be capable of measuring, for instance, neutron-induced reactions such as (n,α) [169] and (n,f) [178] reactions in time-of-flight experiments. At DRACO, Fig. 3.18 shows that the detector traces have an initial saturated part which corresponds to radiation traveling at the speed of light and coming from the laser-plasma interaction. This peak, called γ-flash, features a sharp beginning and a slow exponential tail, with a saturation lasting tens of ns and followed by an undershoot in the baseline. In order to improve the identification of neutron signals of high energy, 5 cm of lead were used as a filter to attenuate the γ-flash and minimize the saturation. The filter was able to reduce the diamond’s recovery time to tens of ns (see left graph in Fig.3.18) as well as the baseline undershooting. Time (ns) 0 50 100 150 200 250 300 350 Amp (mV) 200− 100− 0 100 200 300 400 500 Without Pb shielding 5 cm Pb shielding Time (ns) 0 50 100 150 200 Amp (mV) 40− 20− 0 20 40 60 80 100 120 140 Trace 00074 Trace 00079 Trace 00090 Trace 00067 Figure 3.18: Left: Diamond traces illustrating the beneficial effect of the 5 cm lead filter: it reduces the recovery time in half and also the undershooting in the baseline. Right: Diamond traces showing the difficulty of identifying detector signals on top of the γ-flash and the EMP ringing. Once the γ-flash ends, the detector shows a high-frequency oscillating structure that attenuates between 200 and 250 ns after the γ-flash and that is caused by the strong electromagnetic noise produced during the laser-plasma interaction (EMP, detailed below). 88 3| Neutrons at the HZDR PW-laser facility DRACO The potential neutron signals should be then observed as asynchronous isolated peaks, like those shown in the right graph of Fig. 3.18. The identification of the small neutron signals in such a fluctuating baseline and their analysis in terms of time, amplitude, area, and pulse shape is not straightforward, but it can be achieved, as it is discussed in this section. 3.5.1. Electromagnetic noise suppression A laser-acceleration laboratory is a hostile environment that produces a strong background of secondary radiation. Together with the γ-ray background due to the laser-plasma interaction and the Bremsstrahlung coming from the ejection of relativistic electrons from the plasma, the laser-plasma interaction also produces a strong electromagnetic pulse (EMP) in the form of high-frequency electromagnetic noise that lasts for hundreds of nanoseconds and is able to interfere with the electronic devices inside the laboratory. The nature and characteristics of this noise have been actively studied in different works, and depend on several factors, including the shape and materials of the pitchers and holding components [179]. Time (ns) 500−400−300−200−100−0100 200 300 400 Amp (mV) 150− 100− 50− 0 50 100 150 200 250 300 Without delay cables With delay cables Delay Figure 3.19: EMP and γ-flash in the diamond signal. Long cables connected to the oscilloscope delayed the signal long enough to allow the attenuation of the EMP. This noise is received by different components in the laboratory that act like antennae, including the cables themselves or the channel connections to the oscilloscopes. The EMP is recorded in the oscilloscope traces as a high-frequency ringing. To avoid some of these factors, security measures were adopted in the experiment. For instance, the oscilloscopes were placed outside the bunker and long cables were used so as to allow the EMP to 3| Neutrons at the HZDR PW-laser facility DRACO 89 faint before the detector signals arrive in the oscilloscope, as shown in Fig. 3.19. Still, a fraction of EMP, produced in the detector itself, traveled with the detector signal. Figure 3.20: Some of the control measures to reduce the EMP noise effect. Left: the watertight shielding for the diamond detectors and the amplifiers, and the clamp-on ferrite bead in cables. Center: the HV cables were picked up in such a way that the formation of coils is prevented. Right: some aluminum shielding around connections in the electronics acts as a Faraday cage. In the case of diamond detectors, this ringing is more critical than in the case of other diagnostics, since the signal-to-noise ratio is lower. If no other control measure is used, the amplitude of the noise rises up to hundreds of mV, making it difficult to identify isolated neutron signals with an expected amplitude around tens of mV. For this reason, additional EM shielding was required, consisting of a watertight Al wrapping around the detectors and the electronics as well as plastic foils to prevent metallic contact with other surrounding materials (Fig. 3.20, left panel). Also, diamond detectors were plugged into independent HV supplies to minimize electronic noise coming from the connection of other detectors. Furthermore, clamp-on ferrite beads (passive low-pass filters) were used in the HV cables to reduce the unwanted high-frequency noise. Also, the cables were picked up in such a way that coils were avoided (Fig. 3.20). With all these control measurements, the noise could be reduced by a factor of ten, but not totally avoided (see Fig. 3.21, left). Unfortunately, the remaining noise level changed from day to day as is shown in Fig. 3.21, right. 3.5.2. Redefinition of the baseline Once the electronic noise was studied and minimized, we proceeded to study the single events by analyzing the diamond output traces. Thanks to the fact that the remaining 90 3| Neutrons at the HZDR PW-laser facility DRACO Time (ns) 50−050 100 150 200 250 300 350 400 Amp (mV) 200− 100− 0 100 200 300 Without EM shielding With EM shielding Time (ns) 0 50 100 150 200 Amp (mV) 40− 20− 0 20 40 60 80 100 29-10-2021 (p-LiF) 26-10-2021 (p-LiF) 26-10-2021 (p-Cu) 22-10-2021 (p-Cu) Figure 3.21: Effect of the EM shielding in the detector signals, reducing the noise amplitude (left panel). The noise level changes on different days, but is independent of the catcher (Right panel). ringing in the detector was relatively constant between shots during each given day, the noise has been treated as an oscillating baseline on top of which neutron signals show up at different times. This way, an average baseline has been calculated and subtracted to all traces for the same data set (same day and same pitcher-catcher and laser parameter conditions, see Fig. 3.22, top). After the baseline subtraction, a much more stable trace was obtained where the signals can be identified more easily, as illustrated in Fig. 3.22 (bottom). This is especially relevant in the case of the signals marked with circles in the same figure: they were initially barely identified, but appear quite clearly after the baseline subtraction. In the region of the γ-flash tail, where the shot-to-shot fluctuations are higher, the subtraction worsenen and some artifacts started to appear. Thus, although some signals can be identified in this high neutron energy region, it is not possible to correctly determine the starting time, and therefore the corresponding neutron energy, of the signals. Hence, 20 ns was established as the temporal threshold in the analysis, rejecting all signals at shorter times. This limits our maximum detectable neutron energy to 30 MeV (for LT OF = 150 cm), although neutrons of up to 40 MeV could be expected. Once the average trace is subtracted from all the laser shots for each data set, the remaining noise level of each trace is determined as the standard deviation of the trace amplitude in the time interval between 50 and 250 ns (see Fig. 3.23, left panel). Previous times (20-50 ns) are avoided as they are still affected by the undershoot following the γ-flash. The noise level distribution of all the traces for the same data set is also shown in Fig. 3.23 (right panel). The mean value of the distribution, in this case 2,6 ±0,9 mV, is then considered the noise level representative of the same data set. 3| Neutrons at the HZDR PW-laser facility DRACO 91 Time (ns) 0 20 40 60 80 100 120 140 160 180 200 Amp (mV) 20− 10− 0 10 20 30 40 50 60 70 00033 00053 00086 00117 00169 Average Time (ns) 0 20 40 60 80 100 120 140 160 180 200 Amp (mV) 20− 0 20 40 60 80 Raw Average Clean Temporal threshold Figure 3.22: Top: individual traces for the same data set (solid lines) and the corresponding average trace (dashed line). Bottom: a selected raw trace (yellow) with the corresponding average baseline (blue) and the clean trace after the average baseline subtraction (red). The circles point to the signals which were barely visible before and clearly visible after the baseline subtraction. Times shorter than the temporal threshold (20 ns, shaded region) correspond with the γ-flash tail, where the subtraction is unreliable. Time (ns) 0 50 100 150 200 250 Amp (mV) 10− 5− 0 5 10 15 20 25 Entries 193 Mean 2.614 RMS 0.9289 Noise level (mV) 0 1 2 3 4 5 6 7 8 Counts 2 4 6 8 10 12 14 16 18 Entries 193 Mean 2.614 RMS 0.9289 Figure 3.23: Left: clean trace with the noise level indicated as dashed lines. Right: distribution of noise levels for all the shots in the same data set. The solid line indicates the average noise level and the dashed line corresponds to the threshold established for signal identification (∼two standard deviations). 98 3| Neutrons at the HZDR PW-laser facility DRACO which includes the high precision neutron model [182] and the model introduced by García et al. [183] in order to incorporate the breakup reactions physics models from the code NRESP developed at PTB in Germany [184]. The simulation was made in a simplified geometry, considering just the diamond detector’s active volume as a chip of diamond of 4x4x0,5 mm3. The source consisted of monoenergetic neutron pencil (1 mm) beams between 1 and 41 MeV, in 1 MeV steps. For each energy, 107neutrons were generated. Fig. 3.32 shows the simulated deposited energy spectrum for 5 and 15 MeV neutrons in the 500 µm diamond detector, with labels indicating the main reactions. Deposited energy (MeV) 0 1 2 3 4 5 6 7 8 9 10 11 12 Counts 1 10 2 10 3 10 C(n,el) 12 Deposited energy (MeV) 0 2 4 6 8 10 12 counts 1 10 2 10 3 10 C(n,el) 12 )αC(n,n+3 12 Be 9 )αC(n, 12 B 11 C(n,d) 12 Be 10 )αC(n, 13 B 12 C(n,p) 12 Figure 3.32: Deposited energy spectrum of 5 MeV (top) and 15 MeV (bottom) neutrons in a 500 µm diamond detector simulated by Geant4. 3| Neutrons at the HZDR PW-laser facility DRACO 99 The efficiency of the detector was calculated as the fraction of all the neutrons that produce reactions, depositing some energy in the detector. This is shown in Fig. 3.33 as a function of the neutron energy and for different detection thresholds. As it is shown in the figure, the efficiency of the detection strongly depends not only on the neutron energy but also on the detection threshold. The structures observed below ∼20 MeV are expected, and correspond to resonances in the reaction cross-sections. In order to establish the detection threshold that has been applied in the signal analysis, the detector had to be calibrated in energy. In the detector, the deposited energy is related to the area of the signals, not to their height. Therefore, the fixed Ampth threshold mentioned above implies a different energy threshold depending on the shape of the signal. (MeV) n E 0 5 10 15 20 25 30 35 40 Efficiency (%) 1 2 3 4 5 6 7 8 9 Total efficiency Th = 300 keVee Th = 400 keVee Th = 600 keVee Th = 800 keVee Th = 1000 keVee Th = 1200 keVee Figure 3.33: 500 µm diamond detector efficiency for different energy thresholds. The curves are obtained based on Geant4 simulations. According to the manufacturer, the energy deposited in the detector for a signal is given by Eq. 3.5: Edep =A·Eioniz Z·G·q(3.5) Where: •A is the signal area in units of V·s, •Z is the impedance (50 Ω), •G is the gain of the amplifier (150), 100 3| Neutrons at the HZDR PW-laser facility DRACO •q is the echarge (1,6·10−19 C), and •Eioniz is the C ionization energy (13 eV). This expression in fact depends on the area of the signal but, as it was previously stated (section 3.5.4), the area of the signals is distorted by the noise. There are two options to calculate the area: a) integrating the signal between tstart and tstop, which includes the effect of the noise; b) assuming the square shape of the signals, i.e. multiplying the signal’s width (FWHM) by its amplitude, being these variables less affected by the noise. Both methods have been applied to calculate the area and the corresponding deposited energy spectra of the thick diamond detector on data from 22.10.21. The result is displayed in Fig. 3.34, demonstrating that both methods are equivalent and thus that the energy calibration is not affected significantly by the sizable noise. It can also be appreciated that the 4 mV threshold corresponds to approximately ∼100 keV. Energy deposited (MeV) 0 1 2 3 4 5 Counts/shot 1 10 2 10 Area calculated as: Integral FWHM⋅Amp 100 keVee Figure 3.34: Energy deposited spectrum in the thick diamond detector using two different estimations for the area: its integral or the product of its amplitude and width. Hereunder, the energy calibration is based on the calculation of the signal area as the product of the signals’ width and amplitude. However, there is still the open question of how reliable the calibration from Eq. 3.5 is, since a systematic error in the energy calibration implies a corresponding one in the energy threshold applied, and subsequently in the detection efficiency (see Fig. 3.33). In order to quantify this effect, the neutron energy distribution of the diamond signals for thresholds of 400 keV and 600 keV has been calculated assuming the nominal calibration and assuming gain variations of ±30%. 3| Neutrons at the HZDR PW-laser facility DRACO 101 The effect is shown in Fig. 3.35 where, except for the first low energy bins in which the spectra include more noise, the fluctuations due to the 30% gain variation are within the statistical uncertainty, indicated by the error bars. Neutron energy (MeV) 0 5 10 15 20 25 30 Counts/shot 2− 10 1− 10 1Th = 400 keV (1,3x) Calibration (1x) Calibration (0,7x) Calibration Neutron energy (MeV) 0 5 10 15 20 25 30 Counts/shot 2− 10 1− 10 1Th = 600 keV (1,3x) Calibration (1x) Calibration (0,7x) Calibration Figure 3.35: Effect in the neutron energy distribution caused by a possible gain change in the calibration of a ±30% for two detection thresholds: 400 keV (top) and 600 keV (bottom). The measured neutron spectra can then be calculated for different deposited energy thresholds and scaled by the corresponding neutron energy-dependent detection efficiency (ε). Fig. 3.36 shows an example of this with the Cu catcher data set corresponding to 22.10.21. For all thresholds considered, the results are in good agreement for neutron 102 3| Neutrons at the HZDR PW-laser facility DRACO energies above 5 MeV; which becomes just 3 MeV if only thresholds up to 1000 keV are considered. At lower energies, the data for high thresholds are surely uncertain and not reliable because the efficiency drops to zero, as shown in Fig. 3.33). (MeV) n E 0 5 10 15 20 25 30 εn/shot/ 1− 10 1 10 2 10 3 10 4 10 5 10 Th = 300 keV Th = 400 keV Th = 600 keV Th = 800 keV Th = 1000 keV Th = 1200 keV Figure 3.36: Neutron distribution divided by the efficiency obtained for day 22.10.21 (p+Cu) data set with a thick diamond detector for different energy thresholds. Based on this evidence and in order to maximize the already scarce statistics, a threshold in deposited energy of 400 keV was chosen for the analysis, and the resulting neutron energy distributions were considered reliable in the full neutron energy range above 3 MeV. 3.5.6. Background subtraction There is one last step to be considered before the neutron flux measured by the diamonds can be given. In the data discussed until now, there is no distinction between the neutrons coming directly from the neutron source or the background neutrons scattered in the walls, the floor, and the surrounding materials. In an ideal experimental setup, the walls should be far from the detector, the laboratory as empty as possible and the detector in a configuration as close to being floating in the room as possible. But that is by far not the case in DRACO. Although the closest wall was ∼2,5 m away from the diamond detector and there is not a massive shielding surrounding it as in the case of the scintillator detectors, the experimental room is quite compact (9,1x8,7x3,6 m3) and the detectors were placed on an optic table of stainless 3| Neutrons at the HZDR PW-laser facility DRACO 103 steel between two vacuum chambers, as illustrated in Fig. 3.37. For this reason, the background contribution to the measurement is relevant and needs to be studied. Figure 3.37: Diamond detectors surrounded by different materials in the laser acceleration laboratory. The big structures covered by black fabric are the two vacuum chambers. The red circle indicates the position of the diamond detectors. Figure 3.38: 10 cm width and 40 cm long borated polyethylene acting as shadow bar for the diamond detectors. A shadow bar measurement [157], such as the one performed in HiSPANoS and discussed in the previous chapter (section 2.4.2), was realized on 26.10.21 in the Cu catcher configuration. The shadow bar consisted of a 10 cm width and 40 cm long borated polyethylene block placed between the neutron source and the diamond detectors, as shown in Fig. 3.38. The extension of the shadow bar was limited by the available free space between the 104 3| Neutrons at the HZDR PW-laser facility DRACO chamber and the lead filter in front of the diamond detectors, but a Geant4 Monte Carlo simulation showed that it effectively removed or scattered around 99,9% of the incident neutrons up to an energy of 50 MeV, and 99,0% for energies above 10 MeV. Fig. 3.39 (left) shows the geometry simulated, considering the catcher, the shadow bar and the lead block. The incident neutrons (θ=1º) that reach the scorer 2 with and without the shadow bar are shown in the right graph, both for all the energies and only for energies above 10 MeV, demonstrating the reliability of the borated polyethylene block as shadow bar. )° (θ 0.0 0.2 0.4 0.6 0.8 1.0 Counts 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 Without shadow bar All energies > 10 MeV n E With shadow bar All energies > 10 MeV n E Figure 3.39: Left: Geometry simulated for the shadow bar, considering also the lead shielding. The neutrons are scored before and after the shadow bar. Right: Direct components of neutrons into detector direction (θ< 1º) with and without the shadow bar for all energies and for higher energy neutrons (> 10 MeV). Fig. 3.40 represents the neutron energy spectra measured for the Cu and LiF catchers (on 22.10.21 and 29.10.21, respectively) together with the corresponding shadow bar measurement and the result of the subtraction, which would be the clean neutron energy distributions representing the direct component coming from the catcher. Regarding the case of LiF, it must be taken into account that, although ideally the shadow bar measurement should be done in the same neutron production configuration, it was not possible. Therefore, the background measured for the Cu catcher has been used, just for illustrative purposes. This is valid as long as the neutron production from both catchers is quite similar, which is indeed the case as will be discussed below. Overall, the scattered neutron component in the detector is about 40-50% of the total measured. 3| Neutrons at the HZDR PW-laser facility DRACO 105 (MeV) n E 0 5 10 15 20 25 30 counts/shot 2− 10 1− 10 1Cu catcher Shadow bar Clean Cu(p,n) (MeV) n E 0 5 10 15 20 25 30 counts/shot 2− 10 1− 10 1LiF catcher Shadow bar Clean LiF(p,n) Figure 3.40: Neutron energy spectra for p-Cu (top) and p-LiF (bottom) pitcher-catcher configurations including the shadow bar measurement and the clean direct component coming from the catcher after the background subtraction. 3.5.7. Measured neutron flux Once the background has been subtracted it is finally possible to convert the spectra in Fig. 3.40 into a neutron flux Φ. The counts in the clean spectrum ciin each neutron energy bin ihave to be divided by the corresponding efficiency for that specific energy ε(Ei), the solid angle covered by the detector Ωdet, the number of laser shots and the width of the energy bin ∆Ei, to obtain the result in n/MeV/sr/shot according to Eq. 3.6: 106 3| Neutrons at the HZDR PW-laser facility DRACO Φi=ci ε(Ei)·Ω·∆Ei (3.6) The neutron flux corresponding to the laser-driven neutron production at DRACO for Cu and LiF catchers measured with the thick diamond detector is shown in Fig. 3.41. As anticipated in the discussion and figures above, the neutron production from both catchers is quite comparable. In both cases, the neutron spectrum features a nearly exponential behavior, which corresponds with the TNSA spectrum shape of the laserdriven proton spectrum (see Fig. 3.13). Fitting the spectra to Eq. 3.2 (dashed lines in Fig. 3.41) returns the integral and temperature (kT) of the two neutron fluxes: •LiF(p,n): 9(3)·106n/sr/shot with kT∼19 MeV, and •Cu(p,n): 7(2)·106n/sr/shot with kT∼17 MeV, with 90% of the neutrons in both cases featuring an energy larger than 1 MeV. The uncertainties of the flux measured in a specific data set have been estimated at around 30% considering the shot-to-shot fluctuations, but keeping in mind that the proton production per shot can change up to a factor of two from day to day, and so does the neutron flux. (MeV) n E 0 5 10 15 20 25 30 n/MeV/sr/shot 4 10 5 10 6 10 7 10 LiF(p,n) -E/kT (N/E)e (MeV) n E 0 5 10 15 20 25 30 n/MeV/sr/shot 4 10 5 10 6 10 7 10 Cu(p,n) -E/kT (N/E)e Figure 3.41: Neutron flux obtained for the Li(p,n) (left) and Cu(p,n) (right) reaction measured with a 500 µm diamond detector at the DRACO laser facility. These results are discussed further in the next section, where they are also compared with the expectation from Monte Carlo simulations and with the results of the integral neutron production from the bubble detectors. 3| Neutrons at the HZDR PW-laser facility DRACO 107 3.6. Discussion The main objective of the measurement at the DRACO laser facility is actually not just to characterize the neutron source, but also to assess the feasibility of carrying out nuclear physics experiments under the conditions and constraints imposed by the laser-plasma environment. The measurement of the neutron yield using a nuclear physics detector operated as it would in such experiments becomes then a milestone that has not been addressed prior to this work. In this sense, to claim the experiment as a success, the results from the diamond detector must be validated against the expectations considering the proton beam produced at DRACO, and also against the results of the integral neutron measurements with the bubble detectors. 3.6.1. Simulation of the neutron production at DRACO Contrary to the case of HiSPANoS at CNA, in the case of the neutron production at DRACO the measurement cannot be compared against previous data. Hence, dedicated simulations become the only way to estimate the neutron spectrum to be compared to the results from the diamond detector. The neutron sources for the two catchers considered in this work (Cu and LiF) have been simulated using the Geant4 toolkit [181]. The simulation consisted of a proton source with the energy distribution and number of particles given by the RCF, discussed in Section 3.4.1 and shown in the left panel of Fig. 3.42. In a first approximation, the beam divergence’s dependence on the energy was not taken into consideration. Nevertheless, it was eventually considered in a second set of simulations finding no relevant differences, as illustrated in the right panel of Fig. 3.42. Fig. 3.43 represents the different simulated geometries. For Cu catcher, only a realistic geometry was considered (3x100x100 mm2foil, left picture). In the case of the LiF catcher, three different geometries were simulated: First, a simplified geometry homogeneous foil of 1 cm thickness, big enough (10x10 cm2, the same surface that the Cu catcher) to collect all protons in the beam. Then, a realistic geometry was implemented, with the 10x10x1 cm3aluminum holder and the three 3,8 cm diameter LiF discs arranged as shown in Fig. 3.7. This geometry was simulated in the two positions that were used in the experiment: centered in the holder (which allows the protons traversing the hole to pass through and be detected by the TPS. It was the configuration used on the data set for day 29.10.21 measured with the diamond detector), and centered in one of the LiF disks. 114 3| Neutrons at the HZDR PW-laser facility DRACO detectors is about a factor two larger than the simulations (but in agreement within one standard deviation), and a factor of four larger than the results from the diamond detector (see 3.5.7). This difference could indicate that the background contribution in the bubbles is even larger than estimated (such as neutrons produced by the secondary proton sources that originate backwards or in the laser direction). Besides, the simulations have a sizable uncertainty, which is very difficult to assess, but that includes, for instance, the variations of up to a factor of 2 in the proton source –and hence also the neutron source–, from one day to another (see Table 3.2). Hence, the agreement for integral neutron production can be considered, overall, very satisfactory. )°Angle ( 0 20 40 60 80 100 120 140 160 180 n/sr/shot 10− 0 10 20 30 40 50 60 70 6 10×Bubbles (Average) p + LiF Simulations p + LiF (Geant4) p + LiF (PHITS) )°Angle ( 0 20 40 60 80 100 120 140 160 180 n/sr/shot 10− 0 10 20 30 40 50 60 70 6 10×Bubbles (Average) p + Cu Simulations p + Cu (Geant4) p + Cu (PHITS) Figure 3.49: Comparison between the neutron production retrieved by the bubble detectors and the simulations. The diamond detector operated as a time-of-flight neutron detector offers much richer information than the bubbles. In Fig. 3.50, the yield in the forward direction from the simulations is compared with the clean neutron yield obtained for the proton production in LiF and Cu catchers using the thick diamond detector. Simulations are displayed as a band between Geant4 (lower limit) and PHITS (upper limit). The differences between both codes provide a lower limit of the uncertainty associated with the simulations. In the case of the LiF catcher (left), the experimental data agree with the simulations in the full energy range up to ∼25 MeV. Looking at the experimental data, the less steep behavior of the neutron flux above ∼15 MeV could appear as an artifact, but in fact the simulations indicate that this is expected. The main differences are observed in the first and last bin, which is reasonable considering that the first is more affected by the detection threshold (as discussed in Section 3.5.5) and the last is very close to the undershoot in the tail of the γ-flash (see Fig. 3.26). In the case of the Cu catcher (right), the simulated and measured neutron fluxes also 3| Neutrons at the HZDR PW-laser facility DRACO 115 agree in shape very well, but the measurement is about a factor of 2 lower than the simulations. Again, as in the case of the bubble detector’s results discussed above, this is still at the level of agreement within the systematic uncertainty of both the simulation and the experiment considering the day-to-day variations of the laser source. (MeV) n E 0 5 10 15 20 25 30 n/MeV/sr/shot 4 10 5 10 6 10 7 10 p + LiF Simulations Exp. Data (MeV) n E 0 5 10 15 20 25 30 n/MeV/sr/shot 4 10 5 10 6 10 7 10 p + Cu Simulations Exp. Data (2x) Exp. Data Figure 3.50: Experimental and simulated neutron flux achieved at DRACO with the LiF (left) and Cu (right) catchers. On a general note, it must be emphasized that the excellent agreement observed for the LiF can be misleading. When comparing experiments to simulations, the uncertainties of both have to be taken into account. In the case of the simulations, the proton source is assumed to be that of a previous measurement with RCF; but the discussion in Sec. 3.4.1 illustrates that a laser-driven ion source is far from being an accelerator: the proton flux in terms of integral, shape and cut-off energy varies slightly from shot to shot and quite significantly from day to day. Furthermore, in the real environment of a laser facility there are secondary proton beams intrinsic to the acceleration process, and a fraction of ions that come along with these protons and are also a source of uncertainties. Lastly, the experiment herein was designed and carried out as a feasibility assessment, not as a high-precision nuclear data measurement. For that reason, given the fact that the diamond detector has been successfully operated in the very harsh environment of a laser ion acceleration facility, and that the results agree within uncertainties with the simulations, it can only be considered a truly successful result. 117 4| Conclusions and future perspectives This thesis work has been dedicated to exploring the viability of carrying out nuclear physics experiments, in particular studying nuclear reactions, in the context of laserdriven neutron sources. As discussed in the introduction, in the last decades high-power lasers with ultrashort pulses are emerging as a promising alternative to conventional accelerators, in particular for the production of neutron beams. This is happening while the neutron users community is searching for high-performance compact neutron sources as a complement to very large accelerator-based facilities. There is currently very active research devoted to studying and optimizing ion acceleration and neutron production mechanisms: target design, enhanced production, tailored beams, moderated sources...). Indeed, there are already successful applications to material analysis, irradiation studies, or imaging. Apart from these, one of the applications in which laser-driven neutrons are expected to have a major impact in nuclear physics, thanks to the excellent performance expected for time-of-flight measurements due to their short pulse and high instantaneous flux. However, there is a lack of studies in this direction, and this is precisely the aim of this thesis. The work presented herein does not pretend to be a high-precision measurement of a very interesting nuclear reaction, but it is the first step to make it possible. For the first time, a high-performance semiconductor detector used to study neutron cross-sections at world-class accelerator-driven facilities has been installed and operated at a high-power laser-driven neutron source. The non-trivial but finally successful experiment has provided the first characterization of a laser-driven fast neutron beam by means of the detection of single neutron events. In order to properly ponder this achievement at DRACO, the measurement was also carried out with common laser-driven neutron diagnostics (bubble detectors and scintillators in current-mode) and at a conventional compact neutron source, HiSPANoS. 118 4| Conclusions and future perspectives 4.1. Comissioning of the HiSPANoS neutron source at CNA Since 2013, the HiSPANoS facility at CNA operates in continuous mode as the first accelerator-based neutron source in Spain. In 2018, the setup was upgraded with a pulsing system producing ∼2 ns pulses that enable new fields of research and applications using time-of-flight techniques. While the capability of HiSPANoS to produce pulsed epithermal neutron beams had already been assessed [151], this work has rather focused on the study of the pulsed fast neutron beams that can be produced at this facility. Fast pulsed neutron beams are produced at HiSPANoS by making a deuteron beam of up to 6 MeV and ∼5µA interact with the different targets available. While a thin deuterated titanium (D/Ti) target produces quasi-monoenergetic neutrons between 2 and 6 MeV, the use of thick Be and Li targets produces neutrons with a broad energy distribution up to 10 MeV and over 15 MeV, respectively. These have been characterized in terms of neutron energy, resolution, background, and neutron flux for different targets, beam energies, angles, and distances, finding results compatible with the expectations from the known reactions’ yields. Figure 4.1: Neutron yields at a time-of-flight distance of 150 cm from the main fast neutron beams available at HiSPANoS. Intermediate quasi-monoenergetic neutron beams and error bars have been omitted for clarity. As a summary, Fig. 4.1 shows some of the neutron beams available in units of n/sr/µC/MeV in the forward direction (except for the lowest energy quasi-monoenergetic neutron beam of 2,2 MeV, which corresponds to 120º). The corresponding neutron yields at the target 4| Conclusions and future perspectives 119 are in the order of 106and 1010 n/sr/µC for quasi-monoenergetic and broad energy neutron beams respectively (see details in Tables 2.2 and 2.3). The reported 10% accuracy in the neutron fluxes resulting from this work is dominated by a 7% contribution from detector efficiency (which could be improved by using a neutron detector calibrated in efficiency) and another 7% from the reproducibility of the results. There is ongoing work at CNA aimed to reduce the latter. 4.2. Laser-driven neutrons at DRACO A high-power laser-driven neutron source implies a harsh radiation environment in which particle detectors struggle to function. In addition to the effects of the laser-plasma interaction, the high instantaneous neutron flux, one of the advantages of these sources, limits the measurements to the very short-flight lengths imposed by the size of the experimental halls in which they are normally installed. Furthermore, since high-power laser and related experiments have usually been carried out in single-shot experiments, the reproducibility of the laser performance in multi-shot experiments has always been questioned. This work successfully addresses these questions for the first time thanks to the following features of the experimental set-up: •The pitcher target holder has allowed a multi-shot experiment at a high (in the context of PW lasers) repetition rate of ∼0,02 Hz. During the two weeks campaign, more than 200 laser pulses were shot each day with an intensity of ∼5·1011 p/sr/shot. •The pitcher-catcher configuration has been designed and implemented in a way that makes possible to monitor the proton production shot-to-shot in terms of flux and energy, using the standard Thomson Parabola Spectrometer (TPS) of DRACO. •Conventional bubble neutron detectors have been used to assess integral neutron production by the same means commonly used in laser-driven facilities. After background correction based on Monte Carlo simulations, a neutron flux of ∼3·107 n/sr/shot has been estimated, in agreement with the expectations, within uncertainties considering the characteristics of the proton source. •A diamond semiconductor detector has been used to detect single neutron events and assess in this way the neutron production through the means that would be employed in an eventual nuclear physics experiment; as discussed below. The diamond detector response to the laser-plasma interaction when looking at single neutron interaction events suffered from several complications. Mainly, the γ-flash was 120 4| Conclusions and future perspectives very intense, saturating the detector for a long time; and the electromagnetic pulse (EMP) coupled to the cables and connectors, producing baseline oscillations comparable to the size and shape of the neutron signals. These and other issues have been solved by means of (light) lead shielding, EM shielding of the electronics, displacement of the data acquisition systems outside the bunkers and, most importantly, the development of a quite involved and high-performance signal analysis routine. (MeV) n E 0 5 10 15 20 25 30 n/MeV/sr/shot 4 10 5 10 6 10 7 10 LiF(p,n) -E/kT (N/E)e (MeV) n E 0 5 10 15 20 25 30 n/MeV/sr/shot 4 10 5 10 6 10 7 10 Cu(p,n) -E/kT (N/E)e Figure 4.2: Neutron flux obtained for the Li(p,n) (left) and Cu(p,n) (right) reaction measured with a 500 µm diamond detector at the DRACO laser facility. As a result, the fast neutron flux produced by a PW laser-driven (shown in Fig. 4.2) has been measured with a nuclear physics detector for the first time, providing results in agreement with the expectations from simulations. This definitely opens the door to future measurements of nuclear reactions in this configuration. 4.3. Future perspectives 4.3.1. Lessons from DRACO The main limiting factors faced during the experimental campaign at DRACO and the solutions that could be implemented can be summarized as follows: •Background and limited flight path: A compact room, not designed for the purpose pursued in this work, limits the maximum time-of-flight distance and produces a very sizable background contribution (∼50% for the diamond detector in this case) from neutron scattering in the walls and all the structural materials around. =⇒SOLUTION: An adjacent room dedicated to neutron time-of-flight experiments, separated from the laser interaction room by a wall with a hole acting as collimator. 4| Conclusions and future perspectives 121 This configuration is common at most large neutron facilities such as NFS, nELBE, GELINA, n_TOF, etc. If a separate room is not available, a bigger and empty one is desirable, as in the case of the HiSPANoS neutron source. •Limited pulse frequency: The limited statistics are not associated with the 1 Hz maximum frequency of the laser, but with the maximum of 222 shots that the plasma mirror could receive. Furthermore, the pitcher target positioning was carried out manually allowing a maximum of a one-shot per minute, which is already a very high performance in the field of laser-driven acceleration. =⇒SOLUTION: The limitation in the repetition rate due to pitcher target refreshing is a very hot topic in the laser acceleration community [188,189]. Innovative solutions such as synchronized rotatory targets [190], continuous tape-shape [191], or liquid targets [192] are already being tested and could be implemented soon. If the plasma mirror limitation was solved, these high-frequency targets would allow running at 1 Hz, producing, in just a few minutes, the statistics achievable during two weeks in this campaign, thus opening the door to real multi-shot experiments. •Parasitic ion/electron beams: their existence is intrinsic to the laser acceleration process and can be responsible for a relevant source of background, such as that caused by the backward proton emission. =⇒SOLUTION: The acceleration regime used can influence the number and direction of parasitic ion/electron beams but yet, their presence cannot be prevented. However, since the acceleration regime used can be somehow limited by the laser parameters, a complete understanding and characterization of the acceleration process involved is needed in order to allow a design of the neutron production setup with beam dumps of low background production (as the PE beam dump used in DRACO). •Stability and monitoring: although the shot-to-shot variation has been assessed with the TPS by looking at the protons passing through the hole in the catcher target, it is not clear if the 30% variability observed is real or related to small variations in the angular emission of the beam. Furthermore, sizable day-to-day variations of up to a factor of two were observed, which are mainly related to the positioning of all the system, the laser performance, etc. =⇒SOLUTION: Laser performance will definitely become more and more stable as the technology develops, and there is work in progress aimed to develop more reliable shot-to-shot diagnostics of the proton beam [193]. Also, transporting and focusing the ion beam in a similar way to conventional acceleration [194,195] would allow the installation of online monitors along the beamline. 122 4| Conclusions and future perspectives •The EMP noise: it is an unavoidable issue inherent to any laser facility, whose consequences for this work have been extensively addressed in Chapter 3. In addition, the EMP also affects other electronic systems, such as the controllers for motorized targets or ion beam diagnostics. =⇒SOLUTION: The issue of the noise produced by the EMP in the detectors can also be resolved by using a separate room. If it is not possible, the EM shielding has proven to be helpful. Faraday cages for all the electronic modules would be desirable. The HV supplies could be substituted for batteries and sockets independent of the laser chamber have to be used if possible. Still, the EMP is a state-of-the-art issue into which the laser acceleration and plasma community [196,197] is focusing strong efforts nowadays, and improvements in this direction should be expected in the near future. 4.3.2. HiSPANoS vs. DRACO: can neutron induced reactions be studied at a laser-driven neutron source? In order to compare it to a potential laser-driven neutron source, the neutron flux obtained in HiSPANoS can be expressed at n/sr/shot. Taking the neutron fields produced by Ed = 5,75 MeV on the thick Li and Be target as a reference, 2,2·1010 n/sr/µC and 2,5·1010 n/sr/µC correspond to a source strength of 4,4·1010 and 5,0·1010 n/sr/s, respectively, assuming 2µA over the target. Considering then the 500 kHz repetition rate, HiSPANoS delivers ∼105n/sr/shot. At DRACO, 9·106n/sr/shot and 7·106n/sr/shot were measured, respectively, for LiF and Cu catcher with the diamond detector. 90% of the flux has energies above 1 MeV, making it comparable to the production in HiSPANoS. Although other laser systems have produced neutron yields higher by one order of magnitude, these are based on long pulse single-shot systems, which would not allow for the multi-shot experiments that are the object of this work. Considering these values, the neutron fluxes featured by HiSPANoS and DRACO can eventually be compared. This is done in Fig. 4.3 assuming the nominal repetition rate of 1 Hz of DRACO. The data are represented in terms of flux per second and per shot in order to illustrate the differences between both facilities in terms of average (per second) and instantaneous (per shot) flux. The comparison illustrates the very superior performance of DRACO in terms of neutron production per shot, but also the lower average flux, due to its much lower frequency. It would be a similar result to compare, for instance, facilities like GELINA (200 kHz) 4| Conclusions and future perspectives 123 (MeV) n E 0 5 10 15 20 25 30 n/sr/MeV/s 5 10 6 10 7 10 8 10 9 10 10 10 HiSPANoS DRACO (MeV) n E 0 5 10 15 20 25 30 n/sr/MeV/shot 1 10 2 10 3 10 4 10 5 10 6 10 7 10 HiSPANoS DRACO Figure 4.3: Comparison between the average (left) and instantaneous (right) neutron yields of the main broad spectrum neutron beams available at HiSPANoS (Li(d,n), 2 µA, 500 kHz and Ed= 5,75 MeV) and DRACO (LiF(p,n) and 1 Hz.) and n_TOF (0,8 Hz) (see Ref. [198]): some very interesting reactions on radioactive targets, such as 7Be [199], 7Be(n,α) [200], 79Se(n,γ) [201] or 171Tm(n,γ) [202] can only be addressed in the latter due to its higher instantaneous neutron flux. Regarding the average flux, it will rapidly improve in the laser-driven case, because higher frequencies will soon be available. For instance, HALPS at ELI [203] will soon deliver 1 PW at 10 Hz and the EuPRAXIA project aims at increasing this frequency to 100 Hz [204]. Another important aspect to be considered in neutron time-of-flight experiments is energy resolution. At HiSPANoS, the 2 ns in pulse width results in a resolution of 8% for 5 MeV at a flight path of 150 cm; which at DRACO must be just about 0,1% for the same energy and distance due to the shorter length (∼50-10 ps) of the proton pulse, increasing only to 0,4% for 20 MeV neutrons. This means that the same experiments can be carried out at a much shorter flight path in a laser-driven neutron beam facility, hence increasing the neutron flux, which decreases with the square of the distance. This broadens the range of useful energies for time-of-flight measurements to tens of MeV, which is not easily achievable in CANS. Overall, it can be concluded that, given that this work successfully demonstrates the viability of measuring single neutron interaction events, and considering the characteristics of the neutron beam produced at DRACO, an actual neutron-induced nuclear reaction experiment should be feasible already, provided that: •it is carried out in a room adjacent to the laser-interaction room, in order to reduce the EMP, the gamma-flash saturation and the neutron scattering background; •it uses a pitcher target system, allowing to fully exploit the maximum frequency of at least 1 Hz of the current PW lasers.