7Be(n,alpha) and 7Be(n,p) cross-section measurement for the cosmological lithium problem at the n TOF facility at CERN

EPJ Web of Conferences 146, 01012 (2017) DOI: 10.1051/epjconf/201714601012 ND2016 7Be(n,α)and7Be(n,p) cross-section measurement for the cosmological lithium problem at the n TOF facility at CERN M. Barbagallo1,a, N. Colonna1, O. Aberle2, J. Andrzejewski3, L. Audouin4,V.B ´ ecares5, M. Bacak6,J.Balibrea 5, S. Barros7,F.Be ˇ cv´ aˇ r8, C. Beinrucker9, E. Berthoumieux10, J. Billowes11, D. Bosnar12, M. Brugger2, M. Caama˜ no13, F. Calvi ˜ no14, M. Calviani2, D. Cano-Ott5, R. Cardella2, A. Casanovas14, D.M. Castelluccio15,16, F. Cerutti2, Y.H. Chen4, E. Chiaveri2,G.Cort ´ es14,M.A.Cort ´ es-Giraldo17, L. Cosentino18, L.A. Damone1,19, M. Diakaki10, C. Domingo-Pardo20, R. Dressler21, E. Dupont10,I.Dur ´ an13,B.Fern ´ andez-Dom´ ınguez13,A.Ferrari 2,P.Ferreira 7, P. Finocchiaro18,V.Furman 22, K. G¨ obel9,A.R.Garc ´ ıa5,A.Gawlik 3, T. Glodariu23,I.F.Gonc¸alves7,E.Gonz ´ alez5, A. Goverdovski24, E. Griesmayer6, C. Guerrero17, F. Gunsing10,2, H. Harada25, T. Heftrich9,S.Heinitz 21,J.Heyse 26, D.G. Jenkins27, E. Jericha6, F. K¨ appeler28, Y. Kadi2, T. Katabuchi29,P.Kavrigin 6,V.Ketlerov 24, V. Khryachkov24,A.Kimura 25,N.Kivel 21, M. Kokkoris30,M.Krti ˇ cka8, E. Leal-Cidoncha13, C. Lederer31,H.Leeb 6, J. Lerendegui-Marco17,S.LoMeo 15,16, S.J. Lonsdale31,R.Losito 2, D. Macina2, J. Marganiec3,T.Mart ´ ınez5,C.Massimi 16,32, P. Mastinu33, M. Mastromarco1, F. Matteucci34,35, E.A. Maugeri21, E. Mendoza5, A. Mengoni15, P.M. Milazzo34, F. Mingrone16,M.Mirea 23, S. Montesano2, A. Musumarra18,36,R.Nolte 37, A. Oprea23, N. Patronis38,A.Pavlik 39, J. Perkowski3, J.I. Porras2,40, J. Praena17,40, J.M. Quesada17,K.Rajeev 41, T. Rauscher42,43,R.Reifarth 9, A. Riego-Perez14, P.C. Rout41, C. Rubbia2, J.A. Ryan11, M. Sabat´ e-Gilarte2,17, A. Saxena41, P. Schillebeeckx26, S. Schmidt9, D. Schumann21, P. Sedyshev22, A.G. Smith11, A. Stamatopoulos30, G. Tagliente1,J.L.Tain 20,A.Tarife ˜ no-Saldivia20, L. Tassan-Got4, A. Tsinganis30, S. Valenta8, G. Vannini16,32,V.Variale 1,P.Vaz 7, A. Ventura16, V. Vlachoudis2,R.Vlastou 30,A.Wallner 44,S.Warren 11, M. Weigand9,C.Weiss 2,6,C.Wolf 9, P.J. Woods31,T.Wright 11,P. ˇ Zugec12, and the n TOF Collaboration 1Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 2European Organization for Nuclear Research (CERN), Switzerland 3University of Lodz, Poland 4Institut de Physique Nucl´ eaire, CNRS-IN2P3, Univ. Paris-Sud, Universit´ e Paris-Saclay, 91406 Orsay Cedex, France 5Centro de Investigaciones Energ´ eticas Medioambientales y Tecnol´ ogicas (CIEMAT), Spain 6Technische Universit¨ at Wien, Austria 7Instituto Superior T´ ecnico, Lisbon, Portugal 8Charles University, Prague, Czech Republic 9Goethe University Frankfurt, Germany 10 CEA Saclay, Irfu, Gif-sur-Yvette, France 11 University of Manchester, UK 12 University of Zagreb, Croatia 13 University of Santiago de Compostela, Spain 14 Universitat Polit` ecnica de Catalunya, Spain 15 Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy 16 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 17 Universidad de Sevilla, Spain 18 INFN Laboratori Nazionali del Sud, Catania, Italy 19 Dipartimento di Fisica, Universit` a degli Studi di Bari, Italy 20 Instituto de F´ ısica Corpuscular, Universidad de Valencia, Spain 21 Paul Scherrer Institut (PSI), Villingen, Switzerland 22 Joint Institute for Nuclear Research (JINR), Dubna, Russia 23 Horia Hulubei National Institute of Physics and Nuclear Engineering, Romania 24 Institute of Physics and Power Engineering (IPPE), Obninsk, Russia 25 Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan 26 European Commission, Joint Research Centre, Geel, Retieseweg 111, 2440 Geel, Belgium 27 University of York, UK 28 Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany 29 Tokyo Institute of Technology, Japan 30 National Technical University of Athens, Greece 31 School of Physics and Astronomy, University of Edinburgh, UK ae-mail: [email protected] c The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 146, 01012 (2017) DOI: 10.1051/epjconf/201714601012 ND2016 32 Dipartimento di Fisica e Astronomia, Universit` a di Bologna, Italy 33 Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy 34 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy 35 Dipartimento di Astronomia, Universit` a di Trieste, Italy 36 Dipartimento di Fisica e Astronomia, Universit` a di Catania, Italy 37 Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany 38 University of Ioannina, Greece 39 University of Vienna, Faculty of Physics, Vienna, Austria 40 University of Granada, Spain 41 Bhabha Atomic Research Centre (BARC), India 42 Centre for Astrophysics Research, University of Hertfordshire, UK 43 Department of Physics, University of Basel, Switzerland 44 Australian National University, Canberra, Australia Abstract.The Cosmological Lithium Problem refers to the large discrepancy between the abundance of primordial 7Li predicted by the standard theory of Big Bang Nucleosynthesis and the value inferred from the so-called “Spite plateau” in halo stars. A possible explanation for this longstanding puzzle in Nuclear Astrophysics is related to the incorrect estimation of the destruction rate of 7Be, which is responsible for the production of 95% of primordial Lithium. While charged-particle induced reactions have mostly been ruled out, data on the 7Be(n,α)and7Be(n,p) reactions are scarce or completely missing, so that a large uncertainty still affects the abundance of 7Li predicted by the standard theory of Big Bang Nucleosynthesis. Both reactions have been measured at the n TOF facility at CERN, providing for the first time data in a wide neutron energy range. 1. Introduction The innovative neutron time-of-flight facility n TOF based on a neutron spallation source, was built at CERN with the aim of addressing the request of high accuracy nuclear data for Nuclear Astrophysics and for advanced nuclear energy systems [1]. In particular Nuclear Astrophysics presents many cases that require high precision neutron induced reaction data, for example to improve modelling of stellar Nucleosynthesis and Big Bang Nucleosynthesis. One of the most important unresolved problems in Nuclear Astrophysics is the so-called “Cosmological Lithium problem” (CLiP) [2]. It refers to the large discrepancy (factor 2–3) between the abundance of primordial 7Li predicted by the standard theory of Big Bang Nucleosynthesis (BBN) and the value inferred from the so-called “Spite plateau” in halo stars. In the framework of Standard Model, a possible explanation for this longstanding puzzle is related to the incorrect estimation of the destruction rate of 7Be. Indeed in the standard theory of BBN, 95% of primordial 7Li is produced by the decay of 7Be (t1/2=53.2 days), relatively late after the Big Bang, when the temperature of the Universe was low enough to allow for electrons and nuclei to combine into atoms. Therefore, the abundance of 7Li is essentially determined by the production and destruction of 7Be at temperature values between 0.2<T9<1.2, corresponding to neutron energy between about 20 keV and 120 keV. Charged-particle induced reactions responsible for the destruction of 7Be are known with satisfactory accuracy. On the other hand data on neutron induced reactions, namely 7Be(n,α)4He and 7Be(n,p)7Li reactions, have been so far scarce or completely missing, mainly due to unsurmountable experimental difficulties arising from 7Be specific activity (∼13 GBq/µg). Concerning 7Be(n,α) reaction cross-section, only one measurement exists, performed at thermal energy in the 60’s [3]. This cross section was then extrapolated to the energy range of interest for BBN in a compilation by Wagoner [4]. On the other hand 7Be(n,p) reaction, that represents the leading channel among 7Be destruction processes during BBN [5], has been measured in the 80’s and cross-section determined only up to 13.5 keV [6]. Taking advantage of state-of-art techniques for the production of high-purity radioactive samples, of high performance detection systems and, especially, of the innovative features of a new beam line and measuring station (EAR2) particularly suited for challenging measurements on short-lived radioisotopes, both (n,α) and (n,p) reaction cross-sections on 7Be have been recently measured at nTOF (CERN). The two measurements, performed with two different silicon detection systems, provide for the first time nuclear data on 7Be(n,α) and 7Be(n,p) cross-section in a wide neutron energy range, allowing to clarify the role of these reactions in Nuclear Astrophysics. 2. The n TOF facility The facility is based on the 7 ns wide, 20 GeV/c pulsed proton beam from CERN Proton Synchrotron (PS) with typically 7 ×1012 protons per pulse, impinging on a lead spallation target. The process yields about 300 neutrons per incident proton, which are subsequently collimated and guided trough two beam lines to different experimental areas (EAR1 and EAR2, located respectively at ∼180 m and ∼20 m from the spallation target) where the samples under investigation and the detection systems are located. A layer of selected liquid (i.e demineralized water or borated water) around the spallation target moderates the initially fast neutrons down to a white energy spectrum, which spans from meV to GeV neutron energy [7]. In order to avoid the overlap of consecutive neutron bunches, the time distance between two consecutive proton pulses is a multiple of 1.2 s, related to PS working cycle. Both 7Be(n,α) and 7Be(n,p) reaction cross-sections have been measured at the newly built second experimental area, n TOF-EAR2, that is characterized by an extremely 2 EPJ Web of Conferences 146, 01012 (2017) DOI: 10.1051/epjconf/201714601012 ND2016 Figure 1. n TOF neutron flux at EAR1 with normal (blue) and borated (red) water as moderator compared with the neutron flux at EAR2 (black) [8]. high intensity neutron flux (∼107neutrons/pulse), about 30 times more intense than EAR1 [8], as shown in Fig. 1. The availability of a pulsed and high luminosity neutron beam makes EAR2 ideal for measurements on isotopes available in very small amounts, with short half-lives, or both, as indeed is the case for 7Be. 3. Measurement and results of the 7Be(n,α) reaction cross-section 7Be(n,α)4He reaction is characterized by a Q-value of 18.99 MeV. At the neutron energies under investigation, the two alpha-particles are emitted back-to-back with a maximum energy of 9.5 MeV each, depending on the 8Be state populated in the reaction that proceeds indeed trough energy levels populated by γ-ray transitions. In the present measurement few of them were experimentally accessible, the partial cross-sections for the corresponding 7Be(n,γα)4He reactions were therefore measured and later on the total cross-section was determined as described in details in Ref. [9]. The detection system used in the measurement consisted of two detector sandwiches, each sandwich composed by two single pad silicon detectors 140 µm thick and 3 ×3cm 2wide. Each sandwich hosted a 1.4 µg 7Be sample, corresponding to about 20 GBq activity. Both samples were prepared at Paul Scherrer Institute (PSI) [10], although two different techniques were used to produce them. The first sample was electroplated on a 5µm thick Al foil and the second one was prepared by droplet deposition on a 0.6 µm thick polyethylene foil. More details about the experimental setup and samples can be found in Ref. [11,12]. The stack of detectors and 7Be samples was placed in air inside a scattering chamber shielded by 1 cm lead in order to reduce the external dose due to the 478 keV γ-rays arising from the 7Be decay and, consequently, to facilitate transportation and operations in proximity of the setup. The whole assembly was placed in the neutron beam. Prior to the 7Be(n,α) measurement, the performances of the detection system, particularly its radiation hardness and its capability of detecting high energy charged particles and rejecting background even if exposed to an extremely harsh environment, were fully tested, proving the feasibility of the measurement [13]. Such a system allowed to detect in coincidence the alpha particles emitted, providing a peculiar signature of the occurrence of 7Be(n,γα) reactions and a strong Figure 2. Scatter plot for signal amplitudes in all possible pairs of detectors of the stack. Top left and bottom right plot refer to pairs hosting the 7Be sample, they clearly shows the expected back-toback coincidences not present in uncorrelated pairs of silicons. rejection of any other source of background. Furthermore, the use of two different sandwich detectors provided redundancy and allowed on the one hand to compare the rate of true events, and on the other hand to estimate the background related to random coincidences evaluated by analyzing uncorrelated pairs of detectors. The real and random coincidences can be clearly distinguished in Fig. 2, which shows the amplitude correlation for all silicon pairs: top-left and bottom-right panels are related to pairs of silicon hosting the 7Be samples while all the others refer to uncorrelated pairs of detectors. Applying proper thresholds allowed to select coincidence events as a function of neutron energy and finally to determine the cross-section of the 7Be(n,α) reaction in the energy range 10 meV–10 keV [9]. The result of this measurement at thermal energy was found to be consistent with the only previous existing measurement. In the energy range of interest for BBN the present data indicate that Wagoner’s compilation overestimates by a factor of 10 the reaction rate. The results of the present measurement leave unresolved the Cosmological Lithium Problem, whose solution should then be searched in other physical scenarios. 4. Measurement of the 7Be(n,p) reaction cross-section In order to complete the n TOF program on the CLiP, the 7Be(n,p)7Li cross-section was also measured. The measurement was performed by detecting the emerging protons, emitted either with energy equal to 1.44 MeV (b.r.∼99%) or with 1.02 MeV (b.r.∼1%), depending on the state of 8Li populated in the process. In this case the detection system consisted in a telescope made of 20 µm and 300 µm silicon strips detector, 5 ×5cm 2active area. The system was previously characterized at n TOF-EAR2 using the well-known 6Li(n,t)4He reaction, and the results of this measurement showed that low energy charged particles could be easily discriminated from background in the telescope. Contrarily to outcoming α-particles emitted in 7Be(n,α)4He reaction, low energy protons emitted here do not represent a strong signature of the occurrence of the 7Be(n,p)7Li reaction, so in order to avoid a priori some background, an extremely high purity sample had to be used. It has been realized starting from 200 GBq of 3 EPJ Web of Conferences 146, 01012 (2017) DOI: 10.1051/epjconf/201714601012 ND2016 Figure 3. E-E coincidences matrix for events detected in the telescope. The red arrow points to the region of protons emitted in the 7Be(n,p)7Li reaction. 7Be collected from the neutron spallation source SINQ at PSI. After chemical purification it has been made available in the form of nitrate solution to the ISOLDE facility at CERN [14], where a 7Be beam has been produced, accelerated to 30 keV, isotopically separated at the GLM beam line and implanted on a 18 µm aluminum backing. A sample of ∼1.1 GBq total activity was prepared and successively installed at the EAR2 neutron beam line at nTOF. At n TOF the sample was tilted of 45◦with respect the incident direction of the neutron beam and protons from the reaction detected and identified in the telescope, as clearly shown in Fig. 3. A preliminary analysis indicates that it will be possible to determine the cross section of the 7Be(n,p) reaction from thermal neutron energy to a few hundred keV, thus covering for the first time the region of interest for BBN, from 20 to 200 keV, and sheding some light on the longstanding CLiP problem. 5. Conclusions In order to investigate on Nuclear Physics solutions to the long standing Cosmological Lithium Problem, the 7Be(n,α) and 7Be(n,p) reaction cross-sections have been measured in a wide energy range at the n TOF facility in the new, high flux, experimental area (EAR2). The 7Be(n,α)4He measurement has revealed that the reaction rate used in BBN calculation has been so far overestimated, leaving then the problem unsolved. On the other hand preliminary results on 7Be(n,p)7Li reaction look extremely promising, already proving that a final word on the role of this reaction in BBN can be provided by this measurement. References [1] n TOF Collab., Eur. Phys. J. Plus 131, 371 (2016) [2] R.H. Cyburt et al., Rev. Mod. Phys. 88, 015004 (2016) [3] P. Bassi et al., Il Nuovo Cimento XXVIII, 1049 (1963) [4] R.V. Wagoner, Astrophys J. 148, 3 (1967) [5] C. Broggini, Jour. Cosm. Astr. Phys 06, 030 (2012) [6] P. Koehler et al., Phys. Rev. C 37, 917 (1988) [7] M. Barbagallo et al., Eur. Phys. J. A 49, 156 (2013) [8] M. Sabat` e-Gilarte et al., in preparation [9] M. Barbagallo et al., Phys. Rev. Lett. 117, 152701 (2016) [10] https://www.psi.ch/ [11] M. Barbagallo et al., Il Nuovo Cimento 39C, 277 (2016) [12] E. Maugeri et al., Jour. of Instr., 12, P02016 (2017) [13] L. Cosentino et al., Nucl. Inst. Meth. Phys. Res. A 830, 197 (2016) [14] http://isolde.web.cern.ch/ 4