EXPERIMENTAL AND CALCULATED NEUTRON PRODUCTION BY DEUTERONS IMPINGING ON BE, C AND U THICK TARGETS (17, 20, 28, 80, 160 AND 200 MEV)

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1 EXPERIMENTAL AND CALCULATED NEUTRON PRODUCTION BY DEUTERONS IMPINGING ON BE, C AND U THICK TARGETS (17, 20, 28, 80, 160 AND 200 MEV) N. Pauwels, F. Clapier, J. Proust Institut de Physique Nucléaire d Orsay F Orsay Cedex, France M. Mirea Institute of Physics and Nuclear Engineering Bucarest P.O. Box MG-6, Romania Abstract Experimental angular and energy distributions of neutrons secondary to the interaction between deuterons and thick targets of Be, C and U are presented. The incident deuteron beam energies were 17, 20, 28, 80, 160 and 200 MeV and the data were obtained using the time of flight or activation detectors methods. These data are compared with a model based on the stripping formalism extended to thick targets. 1

2 Introduction The production of neutron rich radioactive beams can be obtained through the fission of 238 U induced by fast neutrons [1]. The technique (Figure 1) can be summarised as follows: fast neutrons are produced which then irradiate a thick fissionable target; the resulting fission products are extracted, ionised, mass-selected and accelerated. The main objective of the PARRNe and SPIRAL-II R&D projects is the investigation of the optimum conditions for a neutron rich isotope source [2,3]. One way to produce neutron beams is to break deuterons on a thick target (called a converter). The energy and the angular distributions of the neutrons obtained with this method and used to irradiate the fissionable target are some of the important parameters to study. For this study, one needs experimental data and a theoretical model to predict the characteristics of neutron beams for a large range of incident deuteron energy and for different converters. Figure 1. Schematic set-up of the technique Neutrons Deuterons Converter Fissonable target The experimental data were obtained with deuteron beams of 17, 20, 28, 80, 160 and 200 MeV stopped in Be, C or U [4,5]. The model developed is based on Serber s theory [6]. Experimental set-up The measurement of angular and energy distributions of neutrons were performed with the time of flight method for deuteron energies ranging from 17 to 160 MeV and with the activation detectors method at 200 MeV. For the time of flight technique a set of DEMON and EDEN detectors were displayed around the target. These two kinds of detectors are filled with an organic scintillator liquid and are described in Refs. [7,8]. The activation detectors used for the experiment at 200 MeV were made of Al, Ni, Co and Bi. The characteristics of the neutron cross-sections of Bi were recently determined experimentally and compared with calculated ones [9]. The different experimental configurations (beam energy, converter) and the angles of detection are reported in Table 1. In all cases the deuterons were completely stopped in the converter. 2

3 Deuteron energy (MeV) Beryllium (deg.) Table 1. Experimental configurations Carbon (deg.) Uranium (deg.) 17* 0, 5, 10, 20 20* 0, 5, 10, 20 0, 5, 10, 20 0, 5, 10, 20 28* 0, 5, 10, 20 0, 5, 10, 20 0, 5, 10, 20 80** 2.4, 3.4, 6, 8.7, 11.1, 15.2, 20.6, 2.4, 3.4, 6, 8.7, 11.1, 15.2, 20.6, 24.5, 30.1, 33.4, 41.4, 50, 70, , 30.1, 33.4, 41.4, 50, 70, ** 2.4, 3.4, 6, 8.7, 11.1, 15.2, 20.6, 2.4, 3.4, 6, 8.7, 11.1, 15.2, 20.6, 24.5, 30.1, 33.4, 41.4, 50, 70, , 30.1, 33.4, 41.4, 50, 70, *** 0, 11, 36, 60, 84 0, 11, 36, 60, 84 * Time of flight (TANDEM, Orsay). ** Time of flight (KVI, Gröningen). *** Activation detectors (LNS, Saclay). Model The model is derived from the semi-classical Serber theory [6]. This theory provides an approximation for the relative neutron energy and angular distribution obtained by the bombardment of a thin target by high-energy deuterons. The relative distributions can be calculated for thick targets and intermediate energies by making averages of the target thickness and selected angles, and by extending the formalism towards low deuteron energies. The target thickness will always be considered as equal to the range of the deuteron as a function of incident energy in the given material. The theoretical relative distribution is normalised using an experimental systematic of neutron yields at 0 for Be targets. These yields are extracted from experimental data available in the literature for deuteron energies between 5 and 50 MeV. The yields between 50 and 200 MeV are taken from our experimental results. Afterwards, a simple formula is used to generalise the results for other kinds of targets. Results and discussion The neutron energy and angular distributions are presented in Figures 2-5 for 20, 80, 160 and 200 MeV deuterons and different converters. In all cases the neutron yields delivered by the Be thick targets are about 1.4 greater than those obtained with the C. The neutron yields produced with the U converter are always small in comparison. The energy distribution clearly shows two well-known regions: the fist region corresponding to the low-energy neutrons which are mainly produced by the evaporation of the target and the second region corresponding to high-energy neutrons generated by the break-up of the deuterons. The angular distribution of neutrons are slightly narrower with the Be than with the C. As expected, for both Be and C targets, the full width at half maximum of the angular distributions decreases when the deuteron energy increases. The experimental values are well reproduced by the model up to about 15, whereas large discrepancies appear above that parameter. These differences are due to the neutrons produced by the evaporation and which are not included in our simulation. In the forward direction, the break-up dominates the production, this being why the simulation is in good agreement with the experiment. At higher angles the main process becomes the evaporation. 3

4 Conclusion Given the data made available one can consider that a good basis for Monte Carlo codes benchmarking has been developed at energies above 50 MeV for deuterons impinging on beryllium and carbon thick targets. As for the project we pursue, specifically the choice of the best deuteron energy and neutron production target to produce neutron rich radioactive beams, the work presented here adds precious guidance. REFERENCES [1] J. Nolen, Third International Conference on Radioactive Nuclear Beam, D.J. Morrisey, ed., Edition Frontières, Gif-sur-Yvette (1993), 111. [2] F. Clapier, A.C. Mueller, J. Obert, O. Bajeat, M. Ducourtieux, A. Ferro, A. Horbowa, L. Kotfila, C. Lau, H. Lefort, S. Kandri-Rody, N. Pauwels, J.C. Potier, J. Proust, J.C. Puteaux, C.F. Liang, P. Paris, A.C.C. Villari, R. Lichtenthäler, L. Maunoury, J. Lettry. Phys. Rev. Special Topics Accelerators and Beams, Vol. 1 (1998), 1-3. [3] S. Kandri-Rody, J. Obert, E. Cottereau, O. Bajeat, M. Ducourtieux, C. Lau, H. Lefort, J.C. Potier, J.C. Puteaux, F. Clapier, J. Lettry, A.C. Mueller, N. Pauwels, J. Proust, C.F. Liang, P. Paris, H.L. Ravn, B. Roussière, J. Sauvage, J.A. Scarpaci, F. Leblanc, G. Lalu, I. Lhenry, T. Von Egidy, R. Antoni, Nucl. Instr. Methods, B 160 (2000), 1-6. [4] S. Menard, M. Mirea, F. Clapier, N. Pauwels, J. Proust, C. Donzaud, D. Guillemaud-Mueller, I. Lhenry, A.C. Mueller, J.A. Scarpaci, O. Sorlin, Phys. Rev. Special Topics Accelerators and Beams, Vol. 2 (1999), [5] N. Pauwels, F. Clapier, P. Gara, M. Mirea, J. Proust, Nucl. Instr. Methods, B 160 (2000), [6] R. Serber, Phys. Rev., 72 (1947), [7] I. Tilquin, Y. El Masri, M. Parlog, Ph. Collon, M. Hadri, Th. Keutgen, J. Lehmann, P. Leleux, P. Lipnik, A. Nimane, F. Hanappe, G. Bizard, D. Durand, P. Mosrin, J. Péter, R. Réggimbart, B. Tamain, NIM A 365 (1995), [8] H. Laurent, H. Lefort, D. Beaumel, Y. Blumenfield, S. Fortier, S. Galès, J. Guillot, J.C. Roynette, P. Volkov, S. Brandenburg, NIM A 326 (1993), [9] E. Kim, T. Nakamura, A. Konno, Y. Uwamino, N. Nakanishi, M. Imamura, N. Nakao, S. Shibata, S. Tanaka, Nuc. Sci. Eng., 129 (1998),

5 Figure 2. Energy and angular distribution of neutrons produced by 20 MeV deuterons on thick targets d (20 MeV) + Be d (20 MeV) + C 0,0014 0, ,0012 0,0012 Neutron flux (neutron.sr -1.MeV -1.deuton -1 ) 0, , Experiment Modelisation Energie (MeV) Energie (MeV) Netron yield (netron.sr -1.deuton -1 ) 0,011 0,009 0,007 0,005 0,003 Solid line and square : Be Dot line and circle : C Dash line and triangle: U Neutron Yield 0 Yield 0 energy (MeV) Be C 4 - Emax FWHM (exp) , Angle (deg) 5

6 Figure 3. Energy and angular distribution of neutrons produced by 80 MeV deuterons on thick targets d (80 MeV) + Be d (80 MeV) + C Neutron flux (neutrons.sr -1.MeV -1.deuton -1 ) 2.4 Experiment Modelisation Neutron yield (neutrons.sr -1.deuteron -1 ) 0,4 0,3 0,2 0,1 Solid line and square: Be Dash line and circle : C Neutron Yield 0 Yield 0 energy (MeV) Be C Emax FWHM , Angle (deg) 6

7 Figure 4. Energy and angular distribution of neutrons produced by 200 MeV deuterons on thick targets d (160 MeV) + Be d (160 MeV) + C 0,024 0,024 0, , Neutron flux (neutrons.sr -1.MeV -1.deuton -1 ) 0,020 0,018 0,020 0,018 Experiment Modelisation ,4 Neutron yield (neutrons.sr -1.deuteron -1 ) 1,2 1,0 0,8 0,6 0,4 0,2 Solid line and square : Be Dash line and circle : C Neutron Yield 0 Yield 0 energy (MeV) Be C Emax FWHM , Angle (deg) 7

8 Figure 5. Energy and angular distribution of neutrons produced by 200 MeV deuterons on thick targets d (200 MeV) + Be d (200 MeV) + U 0,020 0,020 0, ,018 0 Neutron flux (neutrons.sr -1.MeV -1.deuton -1 ) Experiment Modelisation ,6 Neutron yield (neutrons.sr -1.deuteron -1 ) 1,4 1,2 1,0 0,8 0,6 0,4 0,2 Points: Experiment Line : Modelisation Beryllium Uranium Neutron Yield 0 Yield 0 energy (MeV) Be U Emax FWHM , Angle (deg.) 8