EXPERIMENTAL STUDY OF NEUTRON FIELDS PRODUCED IN PROTON REACTIONS WITH HEAVY TARGETS A. Kugler, V. Wagner Nuclear Physics Institute AS CR, 25068 Rez Czech Republic I. Introduction One of important aspects of intensive neutron source design for ADS is thoroughly knowledge of neutron field around the production target. To get experimental data for benchmarking of corresponding simulation codes we lunched series of experiments in 1997. Bellow we report first results obtained in experiment with thick targets of tungsten and lead carried out by us at 1998 in Laboratory of High Energy (LHE), Joint Institute for Nuclear Research (JINR), Dubna 1, as well as results of analysis of experimental data obtained during TAPS 2 campaign at Kernphysisch Versneller Institute (KVI), Groningen, in 1997 with respect to azimuthal and energetic spectra of neutrons emitted from thin tungsten target II. Model of ADS neutron-production target Model of ADS neutron-production target was constructed at Nuclear Physics Institute (NPI). Cylinder with diameter of 2 cm and length of 60 cm is made from high purity 74 W. Target was fixed at back using special aluminum holder. Let us mention, that the activation of aluminum by relativistic protons is rather low, hence the manipulation with the whole setup was much easier. Intensity of produced neutron field was measured by threshold activation method. The target was surrounded by 14 thin foils from 197 Au and 28 Al which were fixed at surface of the special holder, thin-wall (4 mm) aluminum tube with outer diameter of 36 mm. The holder can be removed using distance manipulator without touching the target and hence it was possible to install new set of foils shortly after irradiation. Dimension of Au foils were 1.6 2.0 cm, their thickness was 100 µm and they were fixed with step of 4 cm in direction along the target. Al foils have thickness of 50 µm, their width was 1.6 cm and they surround whole circumference of the holder. We also irradiated cylindrical lead target with diameter of 9.6 cm and length of 50 cm placed into corresponding hole in box with dimension about 1 1 x 1 m made from boron plastic. The Au and Al foils were placed on the surface of lead target, for their position along the length of target see the Table II. 1 collaboration with M. I. Krivopustov, C. A. Novikov, P. A. Rukoyatkin, Ts. Tumendelger, D. Chultem (LHE JINR) and J. Adam (LNP JINR). 2 TAPS is a collaboration of GSI Darmstadt, GANIL Caen, University of Giessen, University of Mainz, KVI Groningen, NPI Rez, and University of Valencia. Page 1 (of 9)
III. Threshold activation method Interaction of neutrons within Au and Al foils leads to the reaction with main characteristics given bellow: Reaction Halftime (hours) Gamma lines (KeV) 27 Al (n,α) 24 Na 14.959 1368.633 197 Au (n,γ) 198 Au 64.68408 411.8 197 Au (n,2n) 196 Au 148.392 333.0, 355.72, 426.0 197 Au(n,4n) 194 Au 39.5 328.45 Corresponding cross sections as function of neutron energy were deduced from literature and Figure 1: Cross section for production of indicated isotopes in Au an Al induced by neutrons with indicated energy. are demonstrated in Fig.1. The isotope 198 Au is produced mainly by low energy neutrons, cross section is very high (1.3-2.1 barn) between 5-10 kev, it fells down approximately linearly in log-log scale from value of 1 barn at 15 kev to value of 0.01 barn at 4 MeV. Production of 24 Na starts from the neutron energy of 6.5 MeV, while production of 196 Au starts from the neutron energy of 8 MeV and production of 194 Au starts from the neutron energy as high as 28 MeV. Hence, foils of 197 Au a 27 Al can be used as energy threshold detectors of neutrons. Page 2 (of 9)
IV. Thick target experiment Both targets were irradiated by collimated beam of protons (diameter of 18 mm) with an energy of 1.5 GeV accelerated by synchrophasotron at the Laboratory of High Energies, Joint Institute for Nuclear Research, Dubna, Russia. The range of protons in tungsten was 54.5 cm, hence they were completly stopped in our 60 cm long tungsten target in contrast with the case of lead target. Gamma decay of isotopes produced in foils were measured using two highpurity Ge-detectors to remove systematical errors due to possible error in detection efficiency. Dead time corrections were checked using measured yield of photons corresponding to. K- line (1460 KeV) as its intensity is constant in time as it belongs to natural background radiation. Corresponding gamma spectra were analyzed using PC-code DEIMOS, which carries out gaussian fit of gamma peaks. Obtained areas bellow peak were corrected to: Time interval between moment of the irradiation and of the measurment: C (time) = e t/τ., here 'τ ' is corresponding half-live Weight of the foil: C (weight) = 1 / weight The absolute efficiency of detector as well as correction due to accidentally summing of photon cascade leading to misinterpretation of two photon hit as one photon with summed energy was taken into account, too. Results obtained from both detector did not differ more than 1%. Number of protons hitting the tungsten target was 1.86 x 10 12. It was deduced from the activation of aluminum foil placed in front of target. The back-scattering of protons from the target was estimated using LAHET based simulation. The error of beam dose is estimated to be around 20%. Number of protons hitting the lead target was 42.2 x 10 12. This number was reported to us by operator staff as deduced from the beam current integrator. Its error is estimated to be around 30%. Resulting numbers of radioactive nuclei produced per one proton are given in tables I and II and are presented at Fig. 2-4. The errors in number of protons are not included! Let us mention, that values corresponding to detection of high energy neutrons coincide for both targets. In other words, amounts of neutrons produced at "equivalent" depth of both targets are almost the same despite of different diameters of targets. Here the "equivalent depth" means that primary protons passed by same number of target atoms. For low energy neutrons the situation is completely different The corresponding intensity of neutron field surrounding lead target is more than two orders of magnitude higher in comparison with tungsten target, see Fig.4. Detailed LAHET simulations, see bellow, revealed that the source of these low energy neutrons is rescattering of high energy neutrons produced in target in huge "moderator", which surrounded lead target. This finding points out possible misinterpretation of experimental data about transmutation cross section as well as other low energy (or even thermal) neutron induced reaction in vicinity of target surrounded by moderator. V. Thin target experiment Barium fluoride was shown to be an efficient detector material for neutrons in a wide range of energies [1,2]. Therefore the high acceptance BaF 2 photon detector system TAPS is an effective tool for exclusive neutron measurements. We analyze in this direction data obtained during irradiation of thin tungsten target by the beam of 190 MeV protons from superconducting cyclotron AGOR of KVI at 1997. Neutrons were detected by TAPS Page 3 (of 9)
Figure 2: Number of produced radioactive nuclei per one proton hitting tungsten target as function of thickness of tungsten to be passed by primary protons to achieve corresponding foil position. Symbols correspond to Figure 3: Number of produced radioactive nuclei per one proton hitting lead target as function of thickness of lead to be passed by primary protons to achieve corresponding foil position. Symbols correspond to experimental data, curves to results of simulation, see text. Page 4 (of 9)
Figure 4: Number of produced radioactive nuclei per one proton hitting lead (upper part) or tungsten (lower part) target as function of thickness of lead or tungsten, respectively, to be passed by primary protons to achieve corresponding foil position. Symbols correspond to experimental data, curves to results of simulation, see text. Figure 5: Experimental setup of thin target experiment carried out by TAPS at AGOR KVI proton beam. modules, which were configured into 6 blocks of 64 BaF 2 scintillators shielded in front by thin charge veto plastic detectors. Modules were positioned around the target to cover the polar angles from 60 up Page 5 (of 9)
to 170 degrees, see Fig.5. The energy of neutrons were derived from their Time-Of-Flight over the distance of about 60 cm, see Fig.6. Figure 6: The neutron production on the W target. The energy spectra for different angles (left side) and angular distributions for the low and high energy component (right side). VI. Simulations Simulations were carried out using LAHET (Bertini) + MCNP4B code combination. As an example, see energy spectra of neutrons passing through foils positioned at 3.6, 27.6 and 47.6 cm distance from the beginning of tungsten target presented in Fig.7. Let us mention enhancement of high energy part of these spectra with growing distance from the beginning of target due to strongly forward peaked preequilibrium high energy neutrons emitted in spallation reaction in first part of target, compare specta at Fig.6. These energetic spectra were at each foil's position convoluted with corresponding cross sections, see upper part of Fig.7. Page 6 (of 9)
Figure 7: Results of simulations: number of produced neutrons per one proton hitting tungsten target passing through three different foil's position as function of neutron energy (lower part). Cross sections corresponding to used actiavtion reactions in foils deduced from literature (upper part). Figure 8: Results of simulations: number of produced radioactive nuclei per one proton hitting tungsten target as function of thickness of tungsten to be passed by primary protons to achieve corresponding foil position. Contribution due to activation induced by scattered and secondary protons are indicated, see text. Page 7 (of 9)
Table I: Experimental yield of radioactive nuclei produced in Au and Al foils placed along the W target per one primary proton. Numbers have to multiplied by 10-3. Errors given are statistical only, see text. Position X[cm] X.ρ [g/cm 2 ] 198 Au 196 Au 194 Au 24 Na 1.6 31 5.92(13) 16.36(15) 4.64(11) 1.35(5) 5.6 108 6.09(13) 17.53(15) 5.34(11) 1.44(5) 9.6 185 4.06(9) 12.26(14) 3.83(12) 1.11(4) 13.6 262 2.40(15) 7.97(16) 2.75(12) 0.710(29) 17.6 340 1.45(8) 5.13(9) 2.02(10) 0.456(28) 21.6 417 0.98(9) 3.29(8) 1.27(10) 0.304(17) 25.6 494 0.58(8) 2.38(9) 0.85(9) 0.236(20) 29.6 571 0.48(8) 1.74(8) 0.50(8) 0.173(19) 33.6 648 0.25(8) 1.38(8) 0.58(7) 0.153(25) 37.6 726 0.10(6) 0.86(6) 0.35(7) 0.096(20) 41.6 803 0.10(7) 0.61(6) 0.31(7) 0.062(20) 45.6 880-0.40(8) 0.15(9) 0.051(19) 49.6 957-0.24(6) 0.11(7) 0.017(15) 53.6 1034-0.13(6) 0.06(7) 0.052(16) Table II: Experimental yield of radioactive nuclei produced in Au and Al foils placed along the Pb target per one primary proton. Numbers have to multiplied by 10-3. Errors given are statistical only, see text. Position X[cm] X.ρ [g/cm 2 ] 198 Au 196 Au 194 Au Position X[cm] X.ρ [g/cm 2 ] 1.0 11 1706(5) 10.79(8) 2.86(8) 1.0 11 1.14(9) 5.0 57 1673(5) 16.12(8) 4.73(8) 5.0 57 1.74(12) 7.0 79 1678(5) 16.32(8) 5.02(9) 7.0 79 1.56(10) 9.0 102 1682(5) 15.82(8) 4.78(9) 9.0 102 1.69(11) 13.0 148 1618(5) 14.01(9) 4.42(8) 13.0 148 1.56(10) 17.0 193 1580(5) 11.85(10) 3.67(7) 17.0 193 1.26(8) 21.0 238 1554(4) 9.93(9) 3.29(7) 21.0 238 0.99(9) 25.0 284 1504(4) 7.88(5) 2.67(7) 24.0 272 0.83(6) 29.0 329 1441(4) 6.22(7) 1.98(7) 28.0 317 0.72(5) 33.0 375 1359(4) 4.83(5) 1.59(7) 32.0 363 0.48(5) 37.0 420 1268(4) 3.68(5) 1.17(7) 35.0 397 0.46(4) 41.0 465 1140(4) 2.90(5) 1.04(6) 40.9 464 0.30(3) 45.0 511 1044(4) 2.14(5).65(7) 46.9 533 0.158(24) 49.0 556 992(4) 1.47(4).52(8) - - - 24 Na Page 8 (of 9)
Emission of secondary protons due to spallation reaction as well as scattering out of primary proton beam were taken into account, too. To judge these effects, see Fig. 8. Results are indicated in Fig.2.-4. by plotted curves. VI. Conclusions Presence of massive shielding close to the target has dominant role in spatial distribution of low energy neutrons Phenomenological model can describe shape of spatial distribution of high energy neutrons along the target However detailed simulations revealed significance of proper accounting of scattered and secondary protons and azimuthal distribution of neutrons produced in spallation reaction as well. Detailed simulation by LAHET(Bertini)+MCNP4B lead to absolute values compatible within 30% with experimental data References [1] T. Matulewitz et al., Nucl. Instr. and Meth. A 274(1989)501 [2] V. Wagner et al., Nucl. Instr. and Meth. A 394(1997)332 Page 9 (of 9)