Simultaneous measurement of neutron-induced capture and fission reactions at CERN

DOI 10.1140/epja/i2012-12029-2 Tools for Experiment and Theory Eur. Phys. J. A (2012) 48:29 THE EUROPEAN PHYSICAL JOURNAL A Simultaneous measurement of neutron-induced capture and fission reactions at CERN C. Guerrero1,2a, E. Berthoumieux1,7, D. Cano-Ott2, E. Mendoza2, S. Andriamonje1, J. Andrzejewski4, L. Audouin3, M. Barbagallo10,V.B´ecares2,F.Beˇcv´aˇr5, F. Belloni6, J. Billowes26, M. Brugger1, M. Calviani1, F. Calvi˜no8, C. Carrapi¸co9,F.Cerutti 1, E. Chiaveri1,M.Chin 1, N. Colonna10,G.Cort´es8, M.A. Cort´es-Giraldo11, M. Diakaki25, I. Dillmann12, C. Domingo-Pardo13,I.Duran 14, C. Eleftheriadis28, M. Fern´andez-Ord´o˜nez2, A. Ferrari1,S.Ganesan 15, G. Giubrone16,M.B.G´omez-Hornillos8,I.F.Gon¸calves9, E. Gonz´alez-Romero2, F. Gramegna17,E.Griesmayer 18, F. Gunsing7,D.Jenkins 27, E. Jericha18,Y.Kadi 1,F.K¨appeler18, D. Karadimos25,J.Kroll 5,M.Krtiˇcka5, E. Lebbos1, C. Lederer20,H.Leeb 18, R. Losito1, M. Lozano11, A. Manousos28, J. Marganiec2,S.Marrone 10,T.Martinez 2, C. Massimi21, P.F. Mastinu17,M.Meaze 10, A. Mengoni22, P.M. Milazzo6, C. Paradela14,A.Pavlik 20,J.Perkowski 4, R. Plag13,J.Praena 11, J.M. Quesada11, T. Rauscher23, R. Reifarth13, F. Roman1,24, C. Rubbia1,R.Sarmento 9, G. Tagliente10, J.L. Tain16,D.Tarr´ıo14,L.Tassan-Got 3, A. Tsinganis251, G. Vannini21, V. Variale10,P.Vaz 9, A. Ventura22, M. Vermeullen27, V. Vlachoudis1, R. Vlastou25, A. Wallner20,T.Ware 26,C.Weiß 18, and T. Wright26 1European Organization for Nuclear Research (CERN), Geneva, Switzerland 2Centro de Investigaciones Energ´eticas Medioambientales y Technol´ogicas (CIEMAT), Madrid, Spain 3Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France 4Uniwersytet L´odzki, Lodz, Poland 5Charles University, Prague, Czech Republic 6Istituto Nazionale di Fisica Nucleare, Trieste, Italy 7Commissariat `al’ ´ Energie Atomique (CEA) Saclay - Irfu, Gif-sur-Yvette, France 8Universitat Polit`ecnica de Catalunya, Barcelona, Spain 9Instituto Tecnol´ogico e Nuclear (ITN), Lisbon, Portugal 10 Istituto Nazionale di Fisica Nucleare, Bari, Italy 11 Universidad de Sevilla, Sevilla, Spain 12 Physik Department E12 and Excellence Cluster Universe, Technische Universit¨at M¨unchen, Munich, Germany 13 GSI Helmholtzzentrum f¨ur Schwerionenforschung GmbH, Darmstadt, Germany 14 Universidade de Santiago de Compostela, Spain 15 Bhabha Atomic Research Centre (BARC), Mumbai, India 16 Instituto de F´ısica Corpuscular, CSIC-Universidad de Valencia, Spain 17 Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy 18 Atominstitut, Technische Universit¨at Wien, Austria 19 Karlsruhe Institute of Technology, Campus Nord, Institut f¨ur Kernphysik, Karlsruhe, Germany 20 University of Vienna, Faculty of Physics, Austria 21 Dipartimento di Fisica, Universit`a di Bologna and INFN, Sezione di Bologna, Bologna, Italy 22 Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile (ENEA), Bologna, Italy 23 Department of Physics and Astronomy - University of Basel, Basel, Switzerland 24 Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN HH, Bucharest - Magurele, Romania 25 National Technical University of Athens (NTUA), Greece 26 University of Manchester, UK 27 University of York, UK 28 Aristotle University of Thessaloniki, Thessaloniki, Greece Received: 21 December 2011 / Revised: 7 February 2012 Published online: 12 March 2012 – c The Author(s) 2012. This article is published with open access at Springerlink.com Communicated by R. Kr¨ucken Abstract. The measurement of the capture cross-section of fissile elements, of utmost importance for the design of innovative nuclear reactors and the management of nuclear waste, faces particular difficulties related to the γ-ray background generated in the competing fission reactions. At the CERN neutron time-of-flight facility n TOF we have combined the Total Absorption Calorimeter (TAC) capture detector with a set of three 235U loaded MicroMegas (MGAS) fission detectors for measuring simultaneously two reactions: capture and fission. The results presented here include the determination of the three detection efficiencies involved in the process: εTAC(n, f), εTAC(n, γ)andεMGAS(n, f). In the test measurement we have succeeded in measuring simultaneously with a high total efficiency the 235U capture and fission cross-sections, disentangling accurately the two types of reactions. The work presented here proves that accurate capture cross-section measurements of fissile isotopes are feasible at n TOF. ae-mail: [email protected] Page 2 of 9 Eur. Phys. J. A (2012) 48:29 1 Introduction The measurement of accurate neutron-induced capture and fission cross-sections is essential for the design of innovative nuclear systems such as ADS and Gen-IV reactors [1,2]. Out of all the neutron-induced cross-sections that play a role in nuclear technology applications, those for which improvement is considered of utmost importance are included in the High Priority Request List [3] of the International Atomic Energy Agency IAEA. This list is continuously updated by the Nuclear Energy Agency (NEA). Half of the capture cross-section measurements included in this list (natHf, 233,235,238U, 239,241,242Pu, 241Am) are of fissile isotopes, and these are very difficult to measure because in such cases the γ-rays generated in fission reactions may constitute a very large background when measuring γ-rays emitted in neutron capture reactions. The measurement of neutron-induced cross-sections of actinides is one of the scientific goals of the n TOF Collaboration. Thanks to the unique characteristics of the nTOF facility [4] at CERN, valuable capture and fission cross-section data have been measured since 2002. Figure 1 shows the neutron capture measurements of actinides performed at n TOF-Ph1 (233,234U, 237Np, 240Pu and 243Am [5]) and those intended for n TOF-Ph2 (236,238U and 241Am [6]), some of which are being or have been measured since 2010. Among all those already measured, only 233U is fissile; meaning that below the characteristic fission threshold the fission cross-section is very small and does not compete with the neutron capture or scattering reactions. The results from capture measurements of non-fissile isotopes at n TOF have reached an accuracy better than 4%. However, in the case of 233U and due to the background from fission reactions the accuracy was limited to only 10% [7]. Such an accuracy is remarkable with respect to previous measurements, but still not sufficient for some actinides taking into account the nuclear data requirements in the field of nuclear technologies. Measured at n_TOF-Ph 1 Intended for n_TOF-Ph 2 (not fissile) Intended for n_TOF-Ph 2 (fissile ) 245 Cm 235 U 236 U 237 U 237 Np 238 Np 239 Np 236 Np 240 Pu 241 Pu 242 Pu 239 Pu 241 Am 242 Am 243 Am 240 Am 243 Cm 244 Cm 242 Cm 238 U 233 Th 234 Th 232 Th 233 U 234 U Fig. 1. Minor actinide part of the table of isotopes. Details on the isotopes measured already at n TOF-Ph1 and those intended in the present n TOF-Ph2, differentiating between fissile and non-fissile isotopes. In consideration of the importance of measuring the capture cross-section of fissile isotopes with high accuracy we have designed, built and tested a new set-up that would allow us to achieve such goal. The new set-up consists of the combination of the Total Absorption Calorimeter (TAC) [8] with a MicroMegas (MGAS) [9] fission detector. This new set-up has been used to measure simultaneously the capture and fission cross-sections of 235U using a stack of three thin 235U samples (see [10] for a set-up at LANL tested for a similar purpose). 2 Experimental set-up At the n TOF neutron time-of-flight (TOF) facility neutrons are generated in spallation reactions by a pulsed proton beam of 7·1012 protons per pulse with 20 GeV/c momentum impinging on a cylindrical lead block (60 cm in diameter and 40 cm in length) surrounded by water (1 cm). Neutrons are moderated by adding a 4 cm thick borated water (mixed with 1.28% of boric acid, 96% enriched in 10B) layer. The neutron beam reaching the measuring station at 185 m ranges from relativistic (TOF = 617 ns) to subthermal (TOF >84 ms) neutron energies. The measurement of the neutron-induced reactions, in this case for both the capture and fission reactions on 235U, as a function of their TOF allows one to determine the associated cross-sections as a function of neutron energy. At nTOF, the TOF is always measured with respect to the Current Monitor (PKUP) placed in the proton beam line just upstream for the spallation target. The measurement combines two detectors that have been used independently in previous n TOF campaigns: the Total Absorption Calorimeter (TAC) [8] and three MicroMegas (MGAS) detectors [9]. Fig. 2. Picture of one hemisphere of the 4πTotal Absorption Calorimeter (TAC) and the long gas chamber inserted along its axis. The picture includes labels indicating the direction of the neutron beam, the position of the three MGAS detectors, the individual crystals, and the connexions for cables. The spherical neutron absorber (see ref. [8]) was used but it is not shown in this picture. Eur. Phys. J. A (2012) 48:29 Page 3 of 9 Fig. 3. Sketch of one MicroMegas (MGAS) detector loaded with a 235U sample (not to scale). The TAC is a segmented 4πdetector made of 40 BaF2 crystals of 15 cm in thickness (see fig. 2). The detector was specifically designed and built for fully detecting with high efficiency the γ-ray cascades following neutron capture reactions, with typical total energies of 5–10 MeV and multiplicities that may range up to 6 or 8, depending on the investigated nucleus. The signals from the 40 BaF2crys- tals are directly connected to 8 bit flash-ADC digitizers (with 8 MB memory per channel) sampling at 250 MHz, which allow recording 32 ms long data buffers that cover the neutron energy range from relativistic energies down to 0.3 eV. The MGAS, based on the microbulk principle, is a very particular gas detector with an active volume divided in two regions by a thin micromesh (see fig. 3). The ionization produced by the fission fragments from 235U takes place in the first region (4 mm, ∼1kV/cm) and the electrons produced are multiplied by producing an avalanche in the second region (50 μm, ∼50 kV/cm), thus amplifying the corresponding signal. This low-noise detector is very transparent to neutrons and γ-rays, thus minimizing the background associated to neutron reactions in the structural materials and the absorption of the γ-rays emitted in the neutron-induced reactions. The MGAS signals are first preamplified and then connected to similar digitizers, in this case with a sampling rate of 100 MHz (because the signals are slower than those of BaF2) and thus allowing the record of 80 ms long data buffers that cover the neutron energy range down to 20 meV. A picture of the set-up combining both detectors is shown in fig. 2. The MGAS detectors were mounted in the center of the TAC, at 3 mm distance between each other and inside a sealed beam tube filled with gas: He+2% isobutane at atmospheric pressure. Each MGAS detector, with an active area of 3.5 cm in diameter, was loaded with a235U sample of 2 cm in diameter and 1.02(1) mg in mass, thus with a surface density of 324(3) μg/cm2. The uranium sample, with an isotopic purity of 99.94%, where prepared by evaporation onto a mylar foil in the Institute for Reference Materials and Measurements [11]. 3 Detector response and analysis in coincidence 3.1 Detector signals from TAC and MGAS The γ-ray signals from the BaF2crystals of the TAC are characterized by a very fast (0.7 ns) and a slow (630 ns) scintillation component (see top panel in fig. 4), carrying each approximately 15% and 85% of the total light output, respectively. The combination of the very fast scintillation component and the sampling rate of 250 MHz provide a time resolution better than 4 ns. However, due to the uncertainty in the time calibration of the digitizer’s internal clocks the overall time resolution of the TAC when all modules are analyzed in coincidence decreases down to 10 ns. The characteristics of the MGAS signals are determined by the geometry of the detector as well as by the Time (ns) 100 200 300 400 500 600 700 0 Normalized amplitude 0 0.2 0.4 0.6 0.8 1 TAC(BaF2) signal Normalized amplitude 0 0.2 0.4 0.6 0.8 1 MGAS signal 1 MGAS signal 2 Time (ns) 100 200 300 400 500 600 700 0 Fig. 4. Typical signals from the TAC (top panel) and MGAS (bottom panel) detectors. See text for a detailed discussion on the differences between the two MGAS signals. Page 4 of 9 Eur. Phys. J. A (2012) 48:29 Signal amplitude (flash-ADC channels) 100 150 200 250 Counts (arb. units) 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 500 α Fission Fragments Saturated signals Threshold Fig. 5. Pulse height distribution of one of the MGAS detectors. The dashed vertical line corresponds to the threshold applied to distinguish between signals associated to αparticles and fission fragments. The sharp edge at channel 248 corresponds to saturated signals, that is with an amplitude larger than the range of the 8 bit flash-ADC. gas and electronics chosen. In this work, helium has been chosen instead of argon (or Ar+CF4) because of its lower neutron cross-section that helps to reduce the background induced in the TAC. The drawback is a slower drift velocity of the electrons with respect to argon [12]. Two MGAS signals are illustrated in the bottom panel in fig. 4, showing that the typical rise times are in the range 50 to 150 ns. It is observed in the figure that signal 1 is broader than signal 2 due to a much slower rise time, which happens when the fission fragments are emitted at very low angles with respect to the sample surface and thus travel up to a few cm through the gas, sometimes without reaching the mesh. In such cases, most electrons are produced a few mm away from the mesh and need a larger drift time to reach the amplification region of the detector (at 4 mm distance). As a consequence of the longer drift time, the signals are produced with a slower rise time and hence result in a worse time resolution. The signals from each detector are analyzed offline by dedicated pulse shape analysis routines that return the amplitude, integral and time of flight of each signal, the latter determined as in a Constant Fraction Discriminator (at 30% of the maximum amplitude). An example of the signal amplitude distribution for one of the MGAS detectors is shown in fig. 5, where the threshold that allows us to separate fission fragments (high amplitude) from α particles (low amplitude) is found around 30 flash-ADC channels. In the neutron energy region of interest, around a few eV, the counting rates recorded in the detectors remained below 0.1counts/μs (100 kHz) even at the peak of the resonances; meaning that problems related to pile-up and summing were negligible in both TAC and MGAS. 3.2 Analysis in coincidence of TAC and MGAS Prior to the analysis of signals from the TAC and MGAS all the internal clocks of all the digitizer modules are synchronized with respect to one module used as a reference. The individual signals of the BaF2crystals are then grouped into TAC events using a coincidence window of 20 ns, resulting in a list of events characterized by the total energy Esum deposited in the TAC and the number mcr of crystals hit, referred to as crystal multiplicity. The time of the first detected γ-rays is also used to extract the time of flight and, consequently, the neutron energy. For each signal detected in any of the MGAS detectors, a loop runs over the TAC events selecting those with a TOFTAC between TOFMGAS −300 ns and TOFMGAS + 50 ns (the asymmetry of this selection is explained later in the text). In 60% of the coincidences there is a single TAC event candidate inside such time interval, whereas two or three candidates are found in 30% and 7% of the cases, respectively. When there is more than one candidate, the fission events are discriminated efficiently from the background because of their higher multiplicity and energy (the background candidates have a multiplicity close to 1 and energies of a few hundred keV). The algorithm identifies the fission event as the one with the largest multiplicity and the largest energy deposition. In this way, a coincident event is found in the TAC for 97% of the MGAS fission events. This value corresponds to the TAC efficiency for detecting fission events in coincidence with the MGAS and such events are marked as fission tagged. The time distribution between fission events in the MGAS and the TAC is displayed in fig. 6. The distribution shows two well-distinguished regions: a symmetric peak around 0 ns (FWHM∼20 ns) containing 80% of TOFMGAS-TOFTAC (ns) -50 100 150 200 250 300 350 Counts (arb. units) 10 2 10 3 10 1 050 80% 20% Fig. 6. Time difference between fission events detected in the MGAS and the TAC. The coincidence window for the analysis corresponds to the range TOFMGAS–TOFTAC ∈ [−50 ns,+300 ns]. Eur. Phys. J. A (2012) 48:29 Page 5 of 9 Counts (arb. units) 0 5 10 15 20 25 30 35 40 45 Deposited energy (MeV) Counts (arb. units) 0.1 1 10 Capture+Background Background Fission Capture+Background Background Fission 04 812 16 20 Counts (arb. units) 5 10 15 20 25 Crystal multiplicity 0 5 10 15 20 Counts (arb. units) 0.1 1 10 Capture Fission Capture Fission Fig. 7. (Color online) Left: Distributions of deposited energy corresponding to fission (red), capture+background (blue), and background (green) in linear (top) and logarithmic (bottom) scales. Right: Distributions of crystal multiplicity for events with Esum >3 MeV corresponding to fission (red) and capture (blue) in linear (top) and logarithmic (bottom) scales. The capture (blue) events are obtained by subtraction of the Background from the Capture+Background distribution shown in the left panel. the events, and a tail at longer times containing all other events. The cumulative number of events amounts to 80%, 87%, 97% and 99.8% for Δt = 50, 100, 200 and 300 ns, respectively. The symmetric contribution corresponds to fission fragments emitted at small angles with respect to direction of the neutron beam, which reach the mesh within a few ns so that the electrons are collected very rapidly. On the contrary, the tail is related to fission fragments emitted at large angles as discussed in the previous section. This tail could be minimized by using a geometry with a thinner drift region and also by adding a few percent of CF4to the gas mixture, which would increase the drift velocity of the electrons. 3.3 Response of the TAC to fission and capture reactions The response of the TAC to all events in terms of observed deposited energy and multiplicity distributions is shown in fig. 7. In order to minimize the background, the spectra in the figure have been constructed with a condition on neutron energy between 1 and 20 eV (i.e. 3.0<TOF (ms) <13.4), a region in which several strong resonances are present in the capture and fission crosssection of 235U. The background spectrum shown in the figure has been obtained by a dedicated measurement without the 235U samples. The left panel in fig. 7 shows the deposited energy distributions of capture, fission and background events. The plot illustrates significant differences between the TAC responses to different types of reactions. The spectrum for capture events is limited to energies below 7 MeV and is dominated by the structure at Sn(236U)∼6.6 MeV corresponding to the total absorption of γ-ray cascades from 235U(n,γ) reactions. On the contrary, the fission distribution shows a smooth behavior peaking around 5 MeV and ranging up to 15–20 MeV, well above the total absorption peak of capture. The structures observed in the background measurement are caused mainly by the activity of the crystals (Esum <4 MeV) and by neutron capture reactions in the BaF2crystals (see [8] for details). Page 6 of 9 Eur. Phys. J. A (2012) 48:29 The crystal multiplicity distributions for events with Esum >3 MeV are shown in the right panel in fig. 7. The capture events are characterized by relatively low multiplicities compared to fission events, whose multiplicity distribution peaks around 7 and ranges up to 20, indicating that as much as half of the TAC is firing. The differences in the TAC response to fission and capture events are a powerful tool to learn about the physics behind the processes, for instance, by comparing the experimental results with expectations from models of fission fragment distributions and electromagnetic decay, including information on Photon Strength Functions [13]. Such differences are also very useful for separating the capture and fission contribution when analyzing the fission and capture data, respectively, by imposing conditions on the deposited energy and crystal multiplicity. This is illustrated in fig. 7 where, for instance, all events observed at Esum >10 MeV or mcr >10 clearly correspond only to fission reactions and can therefore be neglected in the analysis of the capture events. 4 Determination of the detection efficiencies With two detectors and two different reactions to detect, it is important to determine with high accuracy the different efficiencies that play a role in the measurement and their interrelations. Because capture reactions are only detected in the TAC, the efficiency εTAC(n, γ) is independent of the use of the fission tagging technique. On the contrary, when a fission reaction occurs, there are four possibilities: i) the event is detected in both detectors, ii) in none of them, iii) only in the MGAS, and iv) only in the TAC. In the calculations that follow, it is assumed that the efficiencies of the TAC and MGAS are equal for reactions occurring in any of the three MGAS detectors, separated only 15 mm from each other. 4.1 Detection efficiency of the MGAS: εMGAS(n,f) The efficiency for detecting fission events from the 235U samples in the MGAS has been determined experimentally. Three reasonable assumptions are made regarding the high-multiplicity and/or high-energy events in the TAC: i) they are only due to fission (see fig. 7), ii) the efficiency of the TAC for detecting them is ∼100% (see the next section and details in ref. [14]), iii) regarding their detection in the MGAS, these fission events are representative for all the events seen by the MGAS. Based on these assumptions, the detection efficiency of the MGAS is calculated as the ratio of fission tagged to all TAC events for multiplicities larger than 10. The mentioned ratio is shown in fig. 8 for two different conditions in the variable deposited energy: Esum >3 MeV and Esum > 7 MeV. Under the first condition there could be both capture and fission events while only fission events fulfill the second condition (see left panel in fig. 7). However, in both cases the ratio above mcr = 10 is constant and yields the 0.0 0.2 0.4 0.6 0.8 1.0 Crystal multiplicity 10 12 14 16 2468 0 Ratio of events (Tagged/All) Esum>3 MeV Esum>7 MeV Fig. 8. Ratio of TAC events identified as fission versus all the detected events. The blue and red lines correspond to different analysis conditions, Esum >3 MeV and Esum >7 MeV, respectively. The constant value for mcr >10 is εMGAS(n, f)= 0.90(2). same value: εMGAS(n, f)=0.90(2), where the uncertainty corresponds to the level of the fluctuation of the two distributions shown in fig. 8 above multiplicity 10. 4.2 Detection efficiency of the TAC for fission events: εTAC (n,f) As mentioned in sect. 3.2, a coincident event in the TAC is found for 97% of the MGAS fission events, i.e. signals with amplitudes larger than 20 channels. This value, very close to 100%, corresponds to the TAC efficiency for detecting fission events εTAC(n, f). However, the analysis of the TAC is always carried out under certain conditions on deposited energy and multiplicity in order to reduce the overall background, therefore the detection efficiency εTAC(n, f)(i.e. the ratio of TAC coincidence to total MGAS events) has been determined for different conditions on deposited energy and multiplicity, yielding the values listed in table 1. The large reduction of εTAC(n, f) under the conditions mcr <9andEsum <7 MeV allows reducing the background from fission reactions in the analysis of capture events, which are characterized by lower multiplicities (see fig. 7). 4.3 Detection efficiency of the TAC for capture events: εTAC (n,γ) The detection efficiency εTAC(n, γ) can be calculated accurately by means of Monte Carlo simulations. We have demonstrated in a previous work [14] that using a GEANT4 [15] simulation it is possible to determine with good accuracy (2–3%) the detection efficiency εTAC(n, γ) Eur. Phys. J. A (2012) 48:29 Page 7 of 9 Table 1. Efficiency (in %) of the TAC for detecting fission events εTAC(n, f) under different conditions on deposited energy and crystal multiplicity. The systematic uncertainty is estimated to be a unit in the last digit. Energy Crystal multiplicity (MeV) >0>1>2>3 1–9 2–9 3–9 4–9 >0 97949084 71686458 >1 93939083 67676457 >2 88888682 62626056 >3 80807977 54545351 0–7 65 62 58 52 58 56 52 45 1–7 61 62 58 51 55 54 51 45 2–7 56 56 54 50 49 49 48 43 3–7 48 48 47 45 41 41 40 38 for any condition on Esum and mcr, provided that the details on the detector geometry and the information available on the nuclear levels and transition probabilities of the compound nucleus formed after neutron capture are included in the simulation. In this work we have done the same type of calculation, which returns the value εTAC(n, γ)=0.70(2) under the conditions mcr >2 and 3 <E sum(MeV) <7chosenfor the analysis. 5 Cross-sections and comparison to evaluations 5.1 Background subtraction The fission rate measured in the MGAS is free of any background when the condition in signal amplitude is applied (see fig. 5). In the TAC, the capture contribution to the total measured counting rate is estimated by subtracting the contributions from fission and other backgrounds. Regarding the subtraction of the fission contribution to the TAC, as shown previously the MGAS has an efficiency for detecting fission reactions of 0.90, and thus the fission counting rate is scaled up by 0.90−1before subtracting it from the TAC events. The remaining background component, which changes smoothly with increasing neutron energy, has been determined from a measurement in a similar configuration where the 235U samples have been dismounted from the chamber (see left panel in fig. 7). 5.2 Calculation of the cross-sections The capture and fission cross-sections σn,x(En) are calculated from the measured yield under the assumption of the thin target approximation in which Yn,x(En)= nat ·σn,x(En), where the equivalent thickness of the three 235U samples together is nat =8.32 ·10−7atoms/barn. Thus, the experimental cross-section is determined as σn,x(En)= 1 nat ·C(En)−B(En) εTAC(n, x)·Φ(En)·FBIF ,(1) where C(En)andB(En) are the total and background counts, εTAC(n, x) is the detection efficiency (different for fission and capture), Φ(En) is the incident neutron flux, and FBIF is a scaling factor that corresponds to the fraction of the incident beam (3.7 cm in diameter) intercepted by the samples (2 cm in diameter). The latter, which is similar for fission and capture, can be estimated from the position of the sample and the knowledge on the beam profile, but it is more accurately determined as the value required for the calculated σ(n, f) to match the ENDF/B- VII.0 evaluation, which is known within 1% in this energy range [16]. One can thus say that the measurement is normalized to the ENDF/B-VII.0 fission cross-section between 6 and 22 eV. In future measurements, the use of samples with larger diameter will eliminate the need of normalizing to an external reference (in this case the fission cross-section) because the full neutron beam will be incident on the samples. 5.3 Cross-sections and comparison to evaluations The measured capture and fission cross-sections are displayed together with the evaluations in the two upper panels of fig. 9, where the evaluated cross-sections from ENDF/B-VII.0 (similar to all other evaluations) are shown for comparison. The ratio between the resonance integral of the capture and fission cross-sections obtained in this work and those from the ENDF/B-VII.0 evaluation are shown in the third panel. Last, the bottom panel shows a comparison between the αvalues of this work and those of the evaluations, where αis the ratio betwee the capture and fission cross-sections (α=σγ/σf). The comparison between the measured and evaluated cross-sections (two upper panels) shows an overall good agreement between measurement and evaluation for both types of reaction. Still at a qualitative level, one observes the absence of contamination from fission in the capture data and vice versa. The ratios displayed in the last two panels indicates that the tagging of fission events as well as the subtraction of the fission contribution, which includes the scaling by the detection efficiency εMGAS(n, f), are accurate. Considering that the data have been normalized to the fission cross-section in the region of interest, the good agreement in the absolute value of the capture cross-section provides good evidence that the calculation of the detection efficiency εTAC(n, γ) by Monte Carlo simulations is accurate. The result shows that the combination of two efficient fission and capture detectors, when taking care of the interdependence with each other in the data analysis, allows the extraction of fission and capture cross-sections of fissile isotopes without ambiguity. Page 8 of 9 Eur. Phys. J. A (2012) 48:29 σ(n,γ) (barns) 0 200 400 600 Measured ENDF/B-VII σ(n,f) (barns) 0 200 400 600 Measured ENDF/B-VII Measured / ENDF/B-VII 0.9 0.95 1 1.05 1.1 Neutron energy (eV) 10 12 14 16 18 20 22 Measured / ENDF/B-VII 0.9 0.95 1 1.05 1.1 86 Resonance Integral ratios α ratios (n,γ) (n,f) a) b) c) d) Fig. 9. (Color online) Overall comparison of the results from this work and the ENDF/B-VII.0 evaluated cross-sections. a,b) Experimental (red and blue histograms) and evaluated (black lines) capture and fission cross-sections. c,d) Ratio of measured and evaluated resonance integrals and α(= σγ/σf) ratios. The shaded areas in c) and d) indicate the region of agreement within ±3%. 6 Conclusions The measurement of capture cross-sections of fissile isotopes is of utmost importance in the field of nuclear technology but it poses a challenge from the experimental point of view: the γ-ray background emitted in fission reactions complicates the measurement of weaker γ-signals associated with (n,γ) reactions. We have built and tested a new set-up at the CERN n TOF facility that allowed us to measure simultaneously neutron-induced fission and capture reactions by combining a 4πTotal Absorption Calorimeter (TAC) with several MicroMegas (MGAS) detectors loaded with a total of 3 mg of 235U. A detailed analysis of the measured data including the optimization of the coincidence algorithm and the determination of the several detection efficiencies involved in the process has confirmed the successful and unambiguous identification of capture and fission events. The analysis of the extracted capture and fission cross-sections shows that the results are in good agreement with the ENDF/B-VII.0 evaluation in the energy region under study (6–22 eV). In view of the present results, a new experimental campaign is envisaged at n TOF for measuring capture crosssections of fissile isotopes such as 233,235U, 239,241Pu and 245Cm, always considering the very limited availability of suitable samples. The first measurement using ∼30 mg of 235U is scheduled for 2012 [17]. Eur. Phys. J. A (2012) 48:29 Page 9 of 9 The authors are indebted to Damien Grenier and Vincent Barozier, the CERN TE-MPE-EM group, and IRMM for their work on the manufacturing of the fission chamber, the MicroMegas detectors and the 235U samples, respectively. This work was supported in part by the Spanish national company for radioactive waste ENRESA, through the CIEMATENRESA agreements on “Transmutaci´on de residuos radiactivos de alta actividad”, the Spanish Plan Nacional de I+D+i de F´ısica de Part´ıculas (project FPA2008-04972-C03-01), the Spanish Ministerio de Ciencia e Innovaci´on through the CONSOLIDER CSD 2007-00042 project and the European Commission 7th Framework Programme project ANDES (PN- 249671). Open Access This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. References 1. A.J. Koning , J. Blomgren, R. Jacqmin, A.J.M. Plompen, R. Mills, G. Rimpault, E. Bauge, D. Cano-Ott, S. Czifrus, K. Dahlbacka, I. Goncalves, H. Henriksson, D. Lecarpentier, E. Malambu-Mbala, V. Stary, C. Trakas, C. 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