The 33S(n,α)30Si cross section measurement at n TOF-EAR2 (CERN): From 0.01 eV to the resonance region

EPJ Web of Conferences 146, 08004 (2017) DOI: 10.1051/epjconf/201714608004 ND2016 The 33S(n,α)30Si cross section measurement at n TOF-EAR2 (CERN): From 0.01 eV to the resonance region M. Sabat´ e-Gilarte1,2, J. Praena3, I. Porras3, J.M. Quesada2,O.Aberle 1, J. Andrzejewski4, L. Audouin5,V.B ´ ecares6, M. Bacak7,J.Balibrea-Correa 6, M. Barbagallo8,S.Barros 9,F.Be ˇ cv´ aˇ r10, C. Beinrucker11, E. Berthoumieux12, J. Billowes13, D. Bosnar14, M. Brugger1,M.Cama ˜ no15, F. Calvi˜ no16, M. Calviani1, D. Cano-Ott6,R.Cardella 1, A. Casanovas16, D.M. Castelluccio17,18, F. Cerutti1, Y.H. Chen5, E. Chiaveri1, N. Colonna8,G.Cort ´ es16, M.A. Cort´ es-Giraldo2,L.Cosentino 19, L.A. Damone8,20, M. Diakaki12,21, C. Domingo-Pardo22,R.Dressler 23, E. Dupont12,I.Dur ´ an15, B. Fern´ andez-Dom´ ınguez15, A. Ferrari1,P.Ferreira 9, P. Finocchiaro19, V. Furman24,K.G ¨ odel10, A. Garc´ ıa-Rios6, A. Gawlik4, T. Glodariu25,I.F.Gonc¸alves9,E.Gonz ´ alez6, A. Goverdovski26, E. Griesmayer7, C. Guerrero2, F. Gunsing12,1, H. Harada27,T.Heftrich 11,S.Heinitz 23,J.Heyse 28, D.G. Jenkins29, E. Jericha7, F. K¨ appeler30, Y. Kadi1, T. Katabuchi31,P.Kavrigin 7,V.Ketlerov 26, V. Khryachkov26,A.Kimura 27,N.Kivel 23, M. Kokkoris21,M.Krti ˇ cka10, E. Leal-Cidoncha15, C. Lederer32,11,H.Leeb 7, J. Lerendegui-Marco2,S.LoMeo 17,18, S.J. Lonsdale32,R.Losito 1, D. Macina1, J. Marganiec4,T.Mart ´ ınez6,C.Massimi 18,33, P. Mastinu34, M. Mastromarco8, F. Matteucci35,36, E.A. Maugeri23, E. Mendoza6, A. Mengoni17, P.M. Milazzo35, F. Mingrone18,M.Mirea 25, S. Montesano1, A. Musumarra19,37,R.Nolte 38, A. Oprea25, N. Patronis39, A. Pavlik40, J. Perkowski4,K.Rajeev 41, T. Rauscher43,44, R. Reifarth11, A. Riego-Perez16, P. Rout41, C. Rubbia1, J.A. Ryan13, A. Saxena41, P. Schillebeeckx28, S. Schmidt11, D. Schumann23, P. Sedyshev24, A.G. Smith13, A. Stamatopoulos21, G. Tagliente8,J.L.Tain 22,A.Tarife ˜ no-Saldivia16,22, L. Tassan-Got5, A. Tsinganis21, S. Valenta10, G. Vannini18,33, V. Variale8,P.Vaz 9, A. Ventura18, V. Vlachoudis1, R. Vlastou21, A. Wallner44,S.Warren 13, M. Weigand11,C.Weiss 1,7,C.Wolf 11, P.J. Woods32, T. Wright13,P. ˇ Zugec1,14, and the n TOF Collaboration (www.cern.ch/ntof) 1European Organization for Nuclear Research (CERN), Switzerland 2Universidad de Sevilla, Spain 3University of Granada, Spain 4University of Lodz, Poland 5Institut de Physique Nucl´ eaire, CNRS-IN2P3, Univ. Paris-Sud, Universit´ e Paris-Saclay, 91406 Orsay Cedex, France 6Centro de Investigaciones Energeticas Medioambientales y Tecnol´ ogicas (CIEMAT), Spain 7Technische Universit¨ at Wien, Austria 8Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 9Instituto Superior T´ ecnico, Lisbon, Portugal 10 Charles University, Prague, Czech Republic 11 Johann-Wolfgang-Goethe Universit¨ at, Frankfurt, Germany 12 CEA Saclay, Irfu, Gif-sur-Yvette, France 13 University of Manchester, UK 14 University of Zagreb, Croatia 15 University of Santiago de Compostela, Spain 16 Universitat Polit` ecnica de Catalunya, Spain 17 Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy 18 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 19 INFN Laboratori Nazionali del Sud, Catania, Italy 20 Dipartimento di Fisica, Universit` a degli Studi di Bari, Italy 21 National Technical University of Athens, Greece 22 Instituto de F´ ısica Corpuscular, Universidad de Valencia, Spain 23 Paul Scherrer Institut (PSI), Villingen, Switzerland 24 Joint Institute for Nuclear Research (JINR), Dubna, Russia 25 Horia Hulubei National Institute of Physics and Nuclear Engineering, Romania 26 Institute of Physics and Power Engineering (IPPE), Obninsk, Russia 27 Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan 28 European Commission JRC, Institute for Reference Materials and Measurements, Retieseweg 111, 2440 Geel, Belgium 29 University of York, UK 30 Karlsruhe Institute of Technology, Germany 31 Tokyo Institute of Technology, Japan 32 School of Physics and Astronomy, University of Edinburgh, UK 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, 08004 (2017) DOI: 10.1051/epjconf/201714608004 ND2016 33 Dipartimento di Fisica e Astronomia, Universit` a di Bologna, Italy 34 Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy 35 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy 36 Dipartimento di Astronomia, Universit` a di Trieste, Italy 37 Dipartimento di Fisica e Astronomia, Universit` a di Catania, Italy 38 Physikalisch Technische Bundesanstalt, Braunschweig, Germany 39 University of Ioannina, Greece 40 University of Vienna, Faculty of Physics, Vienna, Austria 41 Bhabha Atomic Research Centre (BARC), India 42 Centre for Astrophysics Research, University of Hertfordshire, UK 43 Department of Physics and Astronomy, University of Basel, Switzerland 44 Australian National University, Canberra, Australia Abstract.The 33S(n,α)30Si cross section measurement, using 10B(n,α) as reference, at the n TOF Experimental Area 2 (EAR2) facility at CERN is presented. Data from 0.01 eV to 100 keV are provided and, for the first time, the cross section is measured in the range from 0.01 eV to 10 keV. These data may be used for a future evaluation of the cross section because present evaluations exhibit large discrepancies. The 33S(n,α)30Si reaction is of interest in medical physics because of its possible use as a cooperative target to boron in Neutron Capture Therapy (NCT). 1. Introduction The neutron time-of-flight, n TOF, facility [1] is a neutron spallation beam facility at the European Organization for Nuclear Research (CERN), mainly dedicated to measure neutron-induced cross sections for nuclear technology [2,3], astrophysics [4,5] and medical physics [6,7]. A pulsed proton beam, with momentum of 20 GeV/c, is produced in the Proton Synchrotron accelerator (PS) at CERN. The proton beam impinges on a cylindrical lead target surrounded by water for cooling and neutron moderation purposes, and generates a large number of neutrons by spallation reactions. The neutron beam characteristics, the state-of-the-art of detectors and data acquisition systems make n TOF a unique facility for measuring neutron induced reaction cross section of very radioctive isotopes as for identifying and studying resonances in neutron cross sections. During the first 13 years of operation, the only experimental room was located underground at 185 m from the spallation target along the horizontal direction, n TOF-EAR1 [8], characterized by a high instantaneous flux (106neutrons/bunch), a wide neutron energy range (from 25 meV to over 1 GeV) and good energy resolution for most of the energy range, E/E=10−4[9]. In 2014, a new experimental hall, n TOFEAR2 [10,11], was built at 20 m from the target above the ground in the vertical direction. The main advantage of n TOF-EAR2 with respect to the existing one is the 30 to 40 times higher neutron flux, offering the possibility of measuring thin targets of radioactive material with short half-lives as well as reactions with low cross sections [12]. In this work it is presented the preliminary result of the 33S(n,α)30Si cross section in the energy range from 0.01 eV to 100 keV. The high flux at low neutron energies at n TOF-EAR2 and the experimental set-up [13], which consisted of an ionization chamber with micro-megas detectors in combination with optimized low level noise electronics, has allowed providing for the first time data below 10 keV. 10B(n,α)7Li has been used as reference because it is considered a standard cross section [14]in the energy range under study. The 33S(n,α) reaction is of interest in astrophysics due to the still open question about the origin of 36S[15] but also in Neutron Capture Therapy (NTC). 33S could be used in combination with 10B for treating superficial tumours [16] in accelerator based neutron sources where the energy of the neutron beam is in the epithermal energy region within which 33S(n,α) shows resonances with a cross section higher than the one of 10B(n,α) reaction. In a previous work [17], the enhancement of the physical dose in the first 2 cm of tissue due to the presence of 33Sinthe medium has been demostrated by means of Monte Carlo simulations for a realistic NCT set-up. 2. Nuclear data status The 33S(n,α)30Si cross section data are scarce with no experimental data below 10 keV and important discrepancies at 0.0253 eV. Only one measurement was able to resolve and analyze resonances [18]. However, it showed a factor of two discrepancy in the resonance parameters in comparison to the sole existing transmission measurement [19], although both experiments were analyzed in collaboration. Therefore, the main motivation for the present work was to provide a complete data set from 0.0253 eV to hundreds of keV. Up to now, the 1/v behaviour below resonances has not been experimentally confirmed and low energy resonances, below 10 keV, may exist in view of the work performed for the reverse reaction, 30Si(α,n)33S[20] from which data for 33S(n,α)30Si can be obtained using the principle of detailed balance. Moreover, evaluated nuclear data files, see Fig. 1, diverge not only in the value of the cross section in the resonance region but also above the thermal point with unrealistic behaviours from 0.0253 eV to 10 keV. Only EAF-2010 includes the observed resonances, but with an average cross section value ten times lower than reported in Ref. [18]. 3. 33S(n,α) measurement at n TOF-EAR2 In 2012, a measurement of the 33S(n,α) cross section was performed at n TOF-EAR1 [21] with the aim of solving the discrepancy in the resonance parameters. Nevertherless, no data below 10 keV were obtained. The present measurement at EAR2 will complete the data set 2 EPJ Web of Conferences 146, 08004 (2017) DOI: 10.1051/epjconf/201714608004 ND2016 Figure 1. Status of evaluated data for the 33S(n,α)30Si cross section. ENDF/B-VII, JENDL-4.0 and ROSFOND-2010 are superposed since they show the same tendency. Data taken from: https://www-nds.iaea.org/exfor/endf.htm. Figure 2. Experimental set-up showing the set of micromegas and samples (left) and the detector chamber at the measuring position in the neutron beam (right). taking advantage of the higher neutron flux specially at low neutron energy. 3.1. Experimental set-up A low-mass gaseous micromegas detector [22], based on micro-bulk technology [23] was used, which is characterized by a high signal to background ratio, radiation resistance, its high efficiency and minimal perturbation for the neutron beam. These features make it the most suitable detector for a successful in-beam measurement. A micromegas detector has two gas volumes separated by a 5 µm thick micro-mesh layer. The first volume is the conversion gap, with a thickness of few millimiters, where the ionization processes take place. The electrons produced are drifting through the electric field of ∼1 kV/cm to the mesh. The second volume, typically 50 µm in thickness is characterized by a high electric field of ≥10 kV/cm, where the signal is amplified by electron multiplication via avalanche processes. It works inside a chamber filled with a mixture of 88% Ar, 10% CF4and 2% iC4H10 at atmospheric pressure. In back-to-back configuration, four micromegas were loaded, as seen by the beam, with a 20 nm thick 10B4C sample, a 2.6·10−7at/barn 33S sample, a blank target made of the same material as the 33S backing for background determination, and another 3.7·10−7at/barn 33S deposit (Fig. 2). 3.2. Cross section determination The reaction yield of a specific reaction channel is the probability for a neutron to undergo that reaction. The theoretical reaction yield, Yth, includes the contribution of the primary and the multiple interaction events whitin the sample. In case of thin layers or small cross sections, n·σtot 1, the multiple scattering becomes negligible, and Yth≈n·σr, where σris the reaction cross section and n Figure 3. 33S(n,α) energy dependent cross section from 0.01 eV to 100 keV and a comparison of the result with Wagemans et al. [18] folded with the n TOF energy response function (inset). is the areal density of the target deposit. However, even more important for the thin target approximation to be valid is that the self-shielding becomes negligible. The experimental yield, Yexp, can be obtatined as: Yexp =C−B ε··Np (1) where C-B is the total number of background corrected counts in the detection system, is the neutron flux per proton pulse integrated over the full area of the beam, Np the total number of protons used for the measurement, εthe effective detection efficiency of a reaction product which is the combination of a set of energy- and non energydependent parameters such as the angular distribution of the nuclear reaction and the geometrical efficiency. Combining Yth and Yexp, the absolute value of the reaction cross section can be obtained as: σr=C−B ε··Np·n(2) In order to extract the neutron flux, a reference target with a standard cross section is installed in the detector system. Using the same principle exposed in Eq. (1), the flux can be extracted and replaced in Eq. (2): σr=C−B ε·Np·n·ε·Np·n C−B10 B(n,α)·σr(10 B(n,α)) (3) 3.3. Results Figure 3shows the preliminary energy dependent cross section of 33S(n,α), using 10B(n,α) as reference reaction in the energy range of interest. As mentioned before, the cross section has been measured in this energy region for the first time and was found to follow a 1/v behaviour below 100 eV. In addition, an enhacement of the cross section can be noticed from 100 eV to 3 keV deviating from the 1/v tendency. This region corresponds with the 2.3 and 0.3 keV resonances predicted from the reverse reaction [20]. The inset in Fig. 3shows a comparison of the first resonance (at 13.5 keV) between the present work and Ref. [18] convoluted with the n TOF energy response function [24]. 3 EPJ Web of Conferences 146, 08004 (2017) DOI: 10.1051/epjconf/201714608004 ND2016 The cross section at 0.0253 eV has been obtained to 142±25 mb, within uncertainties in agreement with the previous measurements of 180±80mb [25], 151±22 mb [26] and 140±30 mb [27]. The uncertainty of the present value is the quadratic sum of the uncertainties due to the sample mass, systematics and counting statistics effects. 4. Conclusions The preliminary data of the 33S(n,α)30Si cross section measured at the n TOF-EAR2 facility at CERN from 0.01 eV to 100 keV has been determined using the 10B(n,α) cross section as a reference. The obtained value at 0.0253 eV is within previous measurements. From 0.01 to 100 eV the cross section follows a 1/v shape, but deviates from this trend between 0.1 to 3 keV, where resonances were predicted by experiments from the reverse reaction. Furthermore, experimental data are provided below 10 keV for the first time. This result completes the measurement performed at nTOF-EAR1 in 2012 which was mainly focused on the resonance region, above 10 keV. The comparison of the resonance parameters of the first resonances between the two measurements is ongoing. In a future work, the new data will be included in Monte Carlo simulations of the kerma rate in depth in order to obtain a more accurate estimation of the effect of 33SonBNCT. This work has been partially supported by the projects of Junta de Andaluc´ ıa (P07-FQM-02894, FQM-220, P11-FQM-8229), Ministerio de Ciencia y Tecnolog´ ıa (FPA2011-28770-C03-02), CEI-Biotic Granada (P-BS-64), Mineco (FPA2013-47327-C2- 1-R, FPA2014-53290-C2-2-P, FIS2015-69941-C2-1-P) and the funding agencies of the participating institutes. References [1] www.cern.ch/ntof [2] N. Colonna et al., Energy Environ. Sci. 3, 1910 (2010) [3] A. Nuttin et al., Ann. Nucl. Energy 40, 171 (2012) [4] F. 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