Measurement of the fission fragment angular distribution for Th- 232(n,f) at the CERN n_tof facility

Transcription

1 Measurement of the fission fragment angular distribution for Th- 232(n,f) at the CERN n_tof facility D. Tarrío 1)*, L. Tassan-Got 2), L. Audouin 2), I. Duran 1), L.S. Leong 2), C. Paradela 1), S. Altstadt 3), J. Andrzejewski 4), M. Barbagallo 5), V. Bécares 6), F. Bečvář 7), F. Belloni 8), E. Berthoumieux 8,9), J. Billowes 10), V. Boccone 9), D. Bosnar 11), M. Brugger 9), M. Calviani 9), F. Calviño 12), D. Cano-Ott 6), C. Carrapiço 13), F. Cerutti 9), E. Chiaveri 8,9), M. Chin 9), N. Colonna 5), G. Cortés 12), M.A. Cortés- Giraldo 14), M. Diakaki 15), C. Domingo-Pardo 16), N. Dzysiuk 17), C. Eleftheriadis 18), A. Ferrari 9), K. Fraval 8), S. Ganesan 19), A.R. Garcìa 6), G. Giubrone 16), M.B. Gómez-Hornillos 12), I.F. Gonçalves 13), E. González-Romero 6), E. Griesmayer 20), C. Guerrero 9), F. Gunsing 8), P. Gurusamy 19), D.G. Jenkins 21), E. Jericha 20), Y. Kadi 9), F. Käppeler 22), D. Karadimos 15), P. Koehler 23), M. Kokkoris 15), M. Krtička 7), J. Kroll 7), C. Langer 3), C. Lederer 3,24), H. Leeb 20), R. Losito 9), A. Manousos 18), J. Marganiec 4), T. Martìnez 6), C. Massimi 25), P.F. Mastinu 17), M. Mastromarco 5), M. Meaze 5), E. Mendoza 6), A. Mengoni 26), P.M. Milazzo 27), F. Mingrone 25), M. Mirea 28), W. Mondalaers 29), A. Pavlik 24), J. Perkowski 4), A. Plompen 29), J. Praena 14), J.M. Quesada 14), T. Rauscher 30), R. Reifarth 3), A. Riego 12), F. Roman 9,28), C. Rubbia 9,31), R. Sarmento 13), P. Schillebeeckx 29), S. Schmidt 3), G. Tagliente 5), J.L. Tain 16), A. Tsinganis 9), S. Valenta 7), G. Vannini 25), V. Variale 5), P. Vaz 13), A. Ventura 26), R. Versaci 9), M.J. Vermeulen 21), V. Vlachoudis 9), R. Vlastou 15), A. Wallner 24), T. Ware 10), M. Weigand 3), C. Weiß 20), T.J. Wright 10), P. Žugec 11) 1) Universidade de Santiago de Compostela, Spain 2) Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France 3) Johann-Wolfgang-Goethe Universität, Frankfurt, Germany 4) Uniwersytet Łódzki, Lodz, Poland 5) Istituto Nazionale di Fisica Nucleare, Bari, Italy 6) Centro de Investigaciones Energeticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 7) Charles University, Prague, Czech Republic 8) Commissariat à l Énergie Atomique (CEA) Saclay - Irfu, Gif-sur-Yvette, France 9) European Organization for Nuclear Research (CERN), Geneva, Switzerland 10) University of Manchester, Oxford Road, Manchester, UK 11) Department of Physics, Faculty of Science, University of Zagreb, Croatia 12) Universitat Politecnica de Catalunya, Barcelona, Spain 13) Instituto Tecnológico e Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, Portugal 14) Universidad de Sevilla, Spain 15) National Technical University of Athens (NTUA), Greece 16) Instituto de Fìsica Corpuscular, CSIC-Universidad de Valencia, Spain 17) Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy 18) Aristotle University of Thessaloniki, Thessaloniki, Greece 19) Bhabha Atomic Research Centre (BARC), Mumbai, India 20) Atominstitut, Technische Universität Wien, Austria 21) University of York, Heslington, York, UK 22) Karlsruhe Institute of Technology, Campus Nord, Institut für Kernphysik, Karlsruhe, Germany 23) Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA 24) University of Vienna, Faculty of Physics, Austria 25) Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy 26) Agenzia nazionale per le nuove tecnologie, l energia e lo sviluppo economico sostenibile (ENEA), Bologna, Italy 27) Istituto Nazionale di Fisica Nucleare, Trieste, Italy 253

2 254 D. Tarrìo et al. 28) Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN HH, Bucharest - Magurele, Romania 29) European Commission JRC, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium 30) Department of Physics and Astronomy - University of Basel, Basel, Switzerland 31) Laboratori Nazionali del Gran Sasso dell INFN, Assergi (AQ),Italy *Corresponding author: diego.tarrio@usc.es Abstract A fission reaction chamber was designed to measure the angular distribution of the fragments emitted in neutron-induced fission reactions at n_tof. Up to ten Parallel Plate Avalanche Counters can be included and kept at controlled low-pressure gas. Counters are tilted 45º with respect to the neutron beam direction and up to nine targets can be interleaved in between. A first measurement of the 232 Th(n,f) was recently done and preliminary experimental results demonstrating the suitability of the setup are presented here. 1 Introduction Accurate data on neutron-induced reactions at intermediate energies are crucial for different fields in physics. In particular, an accurate knowledge on the reactions involved in the so-called thorium cycle is of relevant interest for improving the existing nuclear energy-related technologies. With the aim of providing accurate values on the cross sections of neutron-induced reactions, an extensive experimental program is being carried out at the n_tof facility at CERN [1, 2]. One of the experimental setups used at n_tof for studying fission reactions is a reaction chamber containing Parallel Plate Avalanche Counters (PPACs). During the so-called Phase I (developed between 2002 and 2003), the fission cross section of several nuclei have been measured [3, 4] but, because of the limited angular acceptance exhibited by these detectors, the detection efficiency had to be corrected by the angular distribution of the emitted fragments, according to previous available data in the literature. In order to overcome this difficulty, and to get a simultaneous measurement of the fission cross section and of the angular distribution of the fragments emitted, a new geometrical configuration was developed and firstly used in the 2010 and 2011 campaigns to measure the 232 Th(n,f) reaction, demonstrating the suitability of the method described here. 2 Experimental setup The experiment was performed at the CERN Neutron Time-of-Flight (n_tof) facility [1, 2], where a very intense neutron flux is available covering a wide energy range, from thermal up to GeV neutrons. The long, 185-m flight path between the spallation target and the experimental area makes it possible to obtain high-resolution time-of-flight (TOF) measurements. 2.1 Parallel Plate Avalanche Counters and targets The PPACs used in this experiment have a central anode flanked by two cathodes. A low-pressure gas fills the 3-mm gaps between the 1.5- m aluminized Mylar foil electrodes. The cathodes of each PPAC are segmented in 2-mm wide strips separated 100 m to provide the spatial position of the hitting. PPAC anode signals are very fast (9 ns width at half maximum), reducing the pileup probabilities and makes it possible to reach energies as high as 1 GeV. 226

3 Measurement of the fission fragment angular distribution for 232 Th(n,f) at the CERN n-tof facility 255 The fission reaction chamber includes 10 PPACs with 9 targets in between, so that the fission events were identified as coincidence signals in the anodes of two consecutive PPACs. The samples used in this experiment have 8 cm diameter and were produced by electrodeposition on an aluminium foil of 0.75 m thick (six 232 Th samples) and of 2.5 m (for 235 U, 238 U and 237 Np samples). A schematic view of the samples and detectors can be seen in Fig. 1. Fig. 1: Schematic view of the PPAC detectors and of the samples used in this experiment. 3 Simulation work The angular acceptance and the total detection efficiency of this tilted experimental setup were investigated by means of Geant4 simulations, and compared with the situation where the detectors and targets are perpendicular to the beam direction. A simplified version of the real setup was implemented in Geant4, including only two PPAC detectors and one target in between. In order to study the geometrical detection efficiency, we are only interested in the slowing down of the fission fragments in the detection setup, so that we ignore the signal generation in the electrodes. The event generator produced two complementary fission fragments that are emitted in opposite directions from a random point inside the target volume. The mass and charge of the fission fragments were randomly selected according to a probability distribution given by the fission yield provided by the ENDF/B-VII.1 evaluation [5]. The total kinetic energy released in the reaction was calculated using the Viola s systematic [6] and shared between both fission fragments in an inverse relation accordingly with their masses. 3.1 Detection efficiency In the case where the PPACs and the target are perpendicular to the neutron beam direction, the distance travelled by the fission fragment inside the material layers is proportional to cos, so that the energy loss is minimal for fission fragments crossing the detectors in the direction of the beam (perpendicular to the detector surface) and it increases until the maximum polar angle is reached for the emitted fragments. Since both fission fragments must be detected, the most restrictive case occurs when the heavy fission fragment passes through the backing. Geant4 simulations indicate the maximum angle to be around 65º, as shown in Fig. 2(a), where the detected events are histogrammed as a function of cos and. There is no dependence on the angle in this case. However, in the tilted setup used in the present experiment, the angular range covers all the possible values for cos between 0 and 1, although there is no axial symmetry in this case, being the efficiency dependent also on the angle. The detected events in this case are represented in Fig. 2(b). 227

4 256 D. Tarrìo et al. Fig. 2: Detected events as a function of cos and for both geometrical setups: In the perpendicular configuration (Fig. 2(a)), the cosine of the polar angle is limited to angles below 65º and does not depend on the azimuthal angle around the beam axis. However, in the tilted setup (Fig. 2(b)), the angular acceptance covers all the possible values of but the acceptance in the azimuthal angle decreases as increases. The detection efficiency, defined as the ratio between the number of detected and generated fission events, is shown in Fig. 3 as a function of cos for both cases. In the perpendicular setup, the efficiency is very close to 1 for cos ( 61 ) and drops quickly to zero for larger angles, while the tilted setup presents a non-zero efficiency for all values of, even though this is never constant. The fact that it is possible to detect fission fragments emitted at every angle converts this geometrical configuration into an excellent experimental setup for measuring fission fragment angular distributions. Despite the different behaviour of the angular acceptance in both cases, the overall detection efficiency is nearly the same (60% for both configurations). 228

5 Measurement of the fission fragment angular distribution for 232 Th(n,f) at the CERN n-tof facility 257 Fig.3: Comparison of the detection efficiency for perpendicular and tilted setups. 4 Data analysis The detection principle of fission events is based on the detection in coincidence of both fission fragments in two adjacent PPACs in a coincidence window of 10 ns, as it was done in previous experiments [3, 4], rejecting most of the background produced by the emission of the radioactive samples and by spallation reactions in the materials surrounding the samples. The stripped cathodes provide the spatial position and, therefore, the emission angle of the fragments can be calculated. The fission fragment angular distribution (FFAD) is defined with respect to the cosine of the angle between the beam axis and the direction of the fission fragments, assumed to be emitted back to back. The detection efficiency (that is angle-dependent) was calculated by 235 using the data from the U (n,f) reaction at low energies measured in the same experiment, since it is well-known to be isotropic. After the efficiency correction, the angular distributions P cos, according to the expression: for 232 Th were fit to a serie of Legendre polynomials L L max W A 0 1 A L P L cos (1) L 0 L even where A L are the fitting coefficients. Only even terms in cos are used because of the backward-forward symmetry of the emitted fragments. The maximum order L max used for 2 each energy bin was chosen by a test. A more detailed description of the analysis and of the results is given in Ref. [7]. 229

6 258 D. Tarrìo et al. 5 Results Two examples of the angular distributions obtained for the neutron-induced fission of 232 Th are given. Fig. 4(a) represents an emission peaked in the beam direction, while Fig. 4(b) shows a side-peaked emission with a maximum at 45º and a minimum at 0º. In both cases, fits including up 2 nd, 4 th, and 6 th order polynomials are represented showing that, at least, the 4 th order must be included in the fits. A way to characterize the behaviour of the angular distribution with the neutron energy is the anisotropy parameter, defined as the ratio of the number of fragments emitted along the beam axis with respect to the perpendicular direction: A=W(0º)/W(90º). By inserting Eq. (1), the following analytical expression for the anisotropy parameter is obtained: 1 A2 A4 A6 A (2) A2 A4 A The anisotropy parameter itself only provides information at 0º and at 90º, hiding the behaviour at intermediate angles. The main advantage of this setup is that fragments emitted at every angle can be measured, even along the neutron beam direction, thanks to the insensitivity of the PPACs to the neutrons, so that the full angular distribution can be measured. The anisotropy parameter obtained for 232 Th(n,f) in the fission threshold region is represented in Fig. 5. The good agreement with previous data available in EXFOR [8] demonstrates the suitability of this setup to perform this kind of measurements. As it was shown in previous experiments at n_tof, results up to 1 GeV can be obtained thanks to the excellent time properties of the PPACs. These results for the angular distribution in the whole energy range will be part of a forthcoming publication. 6 Summary and conclusions A new fission chamber based on Parallel Plate Avalanche Counters tilted 45º with respect to the neutron beam direction was used to measure the angular distribution of the fragments emitted in neutron-induced fission. Preliminary results on the analysis of the first measurement of 232 Th(n,f) with this new reaction chamber at n_tof were shown here, demonstrating the suitability of this setup to perform this kind of measurements. New experiments to measure other isotopes are already scheduled to be done at the n_tof facility. 230

7 Measurement of the fission fragment angular distribution for 232 Th(n,f) at the CERN n-tof facility 259 Fig.4: Examples of angular distributions of fragments emitted in neutron-induced fission of 232 Th. In Fig. 4(a) most of fission fragments are emitted in the beam direction, while in Fig. 4(b) the maximum of emission is at 45º and the minimum is along the beam direction. In both cases, fits to the 2 nd, 4 th, and 6 th order are drawn. 231

8 260 D. Tarrìo et al. Fig.5: Dependence of the anisotropy parameter on the neutron energy for the 232 Th(n,f) reaction in the present experiment (black markers), compared with previous results from other authors. References [1] C. Rubbia, S. Andriamonje, D. Bouvet-Bensimon et al. Proposal for a Neutron Time of Flight Facility. CERN/LHC/98-02(EET) and CERN/LHC/98-02(EET)-Add. 1 (1998). [2] The n_tof Collaboration. European Collaboration for High-Resolution Measurements of Neutron Cross Sections between 1 ev and 250 MeV. CERN/SPSC 99-8 SPSC/P310 (1999). [3] C. Paradela et al. Phys. Rev.C 82, (2010). [4] D. Tarrío et al. Phys. Rev. C 83, (2011). [5] M. B. Chadwick, M. Herman, P. Oblozinsky et al. Nucl. Data Sheets 112, 2887 (2011). [6] V. E. Viola, K. Kwiatkowski, and M. Walker. Phys. Rev. C 31, 1550 (1985). [7] D. Tarrío. Neutron-induced fission fragment angular distribution at CERN n_tof: The Th-232 case. PhD thesis, Universidade de Santiago de Compostela, Spain (2012). Available at: [8] EXFOR (Experimental Nuclear Reaction Data). National Nuclear Data Center (NNDC) (Brookhaven),