Experimental studies and simulations of spallation neutron production on a thick lead target

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1 Institute of Physics Publishing Journal of Physics: Conference Series 41 (2006) doi: / /41/1/036 EPS Euroconference XIX Nuclear Physics Divisional Conference Experimental studies and simulations of spallation neutron production on a thick lead target M Majerle 1,JAdam 1,PČaloun 1,SAGustov 2, V Henzl 1, D Henzlová 1, V G Kalinnikov 2,MIKrivopustov 2,AKrása 1, FKřížek 1,AKugler 1, I V Mirokhin 2, A A Solnyshkin 2, V M Tsoupko-Sitnikov 2,VWagner 1 1 Nuclear Physics Institute of CAS, Řež, The Czech Republic 2 Joint Institute for Nuclear Research Dubna, , Dubna, Moscow Region, Russia majerle@ujf.cas.cz Abstract. An intensive beam of 660 MeV protons from the Dubna Phasotron was directed towards a thick, lead target (not surrounded by shielding or neutron reflector) for 10 minutes. Detectors and iodine samples were placed around the target. The neutron field and the transmutation of 129 I were studied by the Activation Analysis Method. MCNPX v was used to simulate the experimental setup. The results of the simulations were compared to the experimental values, and the influence of the setup parts to the neutron field was explored. 1. Introduction Accelerator Driven Systems are the combination of a classical reactor with an accelerator. The idea of the ADS was born in Los Alamos in the 50 s, then taken up by Tolstov in the 80 s [1], later by Bowman in 1992 [2], and Rubbia in 1993 [3]. ADS produce a large number of neutrons in the spallation process, and introduce them into a sub-critical reactor assembly, where they produce energy by fission of fissionable material, extra neutrons are used to produce fuel from 232 Th, and/or to transmute long-lived nuclear waste to stable isotopes, or to short-lived isotopes which decay to stable ones. The experiments with simplified ADS setups were performed on accelerators at the JINR (Joint Institute for Nuclear Research, Dubna, Russia) [4]. The targets were lead, tungsten, and uran cylinders (in some experiments surrounded with paraffin moderator or natural U blanket). The physical aim of the experiments was to study nuclear processes in the target, to inquire into the process of transmutation in radioactive samples, to measure the heat production, etc. Mostly, the Activation Analysis Method was used to measure the neutron field and the production rates of the produced isotopes. The Phasotron experiment is an example of this work. The experiment consisted of a bare, lead target irradiated with relativistic protons. The proton beam and the produced neutron field were measured with the set of activation detectors in the form of thin foils. Samples of iodine were put in the neutron field in order to study transmutation possibilities in iodine. Experimental yields of produced elements were compared with the yields calculated with MCNPX, and causes of discrepancies were explored IOP Publishing Ltd 331

2 332 Figure 1. The layout of the Phasotron experimental setup. Longitudinal and transverse crosssections. 2. The Phasotron experiment The Phasotron setup consisted of a lead cylinder (radius 4.8 cm, length cm) at which an intensive (10 13 protons/s), high energy (660 MeV) proton beam was directed for 10 minutes. The setup was placed in a long, narrow corridor, bounded by concrete walls (figure 1). The corridor was 20 m long, 2 m wide and high, and the target was placed 5 m before the back corridor wall in the middle of the corridor, 1.2 m from the floor. Other components of the setup were the iron table, where was placed the setup, a metal beam tube and a metal beam stopper (full, 10 cm thick, iron cylinder, placed 1 m behind the target, on the same axis as the beam tube). To measure the proton beam, neutron field, and transmutation rates for iodine isotopes we used the Activation Analysis method. We placed the detectors/samples in the neutron/proton field. During the irradiation, neutrons and protons induced different nuclear reactions in them and produced new radioactive nuclei. After the irradiation the quantities of produced isotopes were determined from the gamma spectra of activated detectors The beam The beam intensity and size were measured with two independent methods: with a wire chamber, and with a set of Al and Cu foils. The results from both methods were in good agreement. The wire chamber measured the flux of protons to be protons/s with the horizontal and vertical diameters of the beam 3.2 cm and 3.8 cm. The beam profile was approximated with a Gaussian profile, so the word diameter shall be considered as the border inside which were most protons. A set of 8 8 cm 2 Al and Cu foils, and another segmented set of five 2 2 cm 2 foils (in the shape of the cross) were put in the beam, ca. 10 cm in front of the target. After the irradiation we measured the yields of produced elements in the foils by means of the γ-spectroscopy. We interpolated experimental cross-sections [10] for reactions with protons for 660 MeV protons, and calculated the integral flux of protons that passed through the foils. Total of protons (systematic and statistic errors are 6%) passed 8 8 cm 2 foils. Five 2 2 cm 2 foils, with one foil on the central axis and four others around, showed that the beam diameter was roughly 4 cm, and shifted upwards for ca. 1 cm The neutrons Three types of detector foils of the square shape were used to study neutron field along the target: Al, Au (2 2 cm 2,50μm), and Bi ( cm 2,1 mm) foils placed on the top of the

3 333 target along its length. In the foils neutrons produced radioactive isotopes via reactions (n, γ) or (n, xn) (see table 1). After the irradiation, we measured the γ-spectra of the foils and determined the yields of produced isotopes. Table 1. Observed reactions, their thresholds, and half-lifes of products in Al, Au, and Bi foils. Values in the table are determined by the mass difference calculated with QTOOL [12]. For the reaction 27 Al(n, α) 24 Na the threshold value should be modified for the contribution of the Coulomb barrier. The modified value is 5.5 MeV. Material Reaction Product Threshold (MeV) Half-life 27 Al (n, α) 24 Na h 197 Au 209 Bi (n, γ) 198 Au d (n, 2n) 196 Au d (n, 4n) 194 Au d (n, 5n) 193 Au h (n, 7n) 191 Au h (n, 4n) 206 Bi d (n, 5n) 205 Bi d (n, 6n) 204 Bi h (n, 6n) 203 Bi h (n, 7n) 202 Bi h (n, 8n) 201 Bi h From the yields of produced isotopes we calculated the numbers of produced atoms of isotope A per 1 g of the detector material and per 1 incident proton. This value is called the production rate B(A) [5, 6], and is defined as: B(A) = (number of A-atoms produced)/[(1 g sensor) (1 primary proton)]. (1) The experimental production rates against the position along the target are plotted in figures 2, 3, and 4 for all three sets of detector foils (errors at the graphs are only statistical errors; systematic errors, should contribute another 4% (main part is the error in the detector efficiency measurements). Systematic errors are the same for different foils and do not change the shape of the distribution, they are only replacing it upwards or downwards). All graphs show a specific shape: the maximum at around 10 th cm, and the point near 30 th cm, where the neutron field starts to decrease faster - spallations stop there. The exception is the graph for 198 Au, which is produced via (n, γ) reaction by low-energy neutrons (thermal, epithermal, and resonance neutrons), and shows a flat distribution. The results for Bi foils are preliminary. Near the end of the target the foils were weakly activated and the statistical errors reach 5%, anywhere else they are lower. The other possible error comes from the integral of the proton beam, which is determined with the 6% accuracy. Altogether, the worst results for neutron field are precise up to 10%, but in most cases the precision is ca. 5% Iodine samples High beam intensity and short irradiation time enabled us to observe and measure the production rates of higher threshold reactions - (n, 5n), (n, 6n),... in four iodine samples. Two samples

4 334 1E-5 1E-6 1E-7 Na-24 1E-8 Figure 2. B-values for 24 Na in Al foils along the target. 1E-04 1E-05 1E-05 1E-06 1E-07 Au-198 Au-196 Au-194 Au-193 Au-191 1E-06 1E-07 Bi-206 Bi-205 Bi-204 Bi-203 Bi-202 Bi-201 1E-08 1E-08 Figure 3. B-values for different isotopes in Au foils along the target. Figure 4. B-values for different isotopes in Bi foils along the target (preliminary results). contained natural 127 I isotope, and other two the mixture of 127 Iand 129 I from the standard radioactive waste. The samples were placed at the 9 th cm and 21 st cm along the target. The isotopes produced in higher threshold reactions are far away from the line of stability and have short lifetimes, and we needed to measure them in series of short measurements immediately after the experiment. We could determine the yields of produced isotopes up to 118 I with the accuracy of 10%. The decay products of iodine isotopes up to 116 I were detected. The yields of produced isotopes for 129 I in the mixture of 127 Iand 129 I were calculated with the substraction of 127 I contribution, which was determined in 127 I samples. The graphs in figures 5 and 6 show the production rates of measured iodine isotopes at the 9 th cm and the 21 st cm for 127 Iand 129 I. The production rates for iodine isotopes are comparable with each other and with the production rates for other elements (10 6 g 1 proton 1 >B>10 7 g 1 proton 1 ). 3. MCNPX simulations Nowadays, there is a great motivation towards improving the precision of the simulation codes which might be once used to simulate ADS systems. Experiments like the Phasotron one can help in estimating the precision of simulations and finding the causes for discrepancies. We simulated our experiment with MCNPX, and compared the calculations with experimental values. Simulations offer a better insight into the experiment, a lot of things that cannot be seen experimentally can be calculated. We used a set of simulations to see how the experimental systematic error depends on setup uncertainties. Further, we compared the experimental results with simulated values to test the accuracy of the MCNPX simulation code.

5 335 1E-05 1E-05 1E-06 1E-07 1E-08 I-127 I-129 1E-06 1E-07 1E-08 I-127 I-129 1E-09 I-130 I-128 I-126 I-124 I-123 I-121 I-120 I-119 I-118 Isotope 1E-09 I-130 I-128 I-126 I-124 I-123 I-121 I-120 I-119 I-118 Isotope Figure 5. B-values for different isotopes in 127 Iand 129 I. Samples were placed at the 9 th cm. Figure 6. B-values for different isotopes in 127 Iand 129 I. Samples were placed at the 21 st cm. The latest production relase of MCNPX (version 2.4.0) was used to simulate the experimental setup. Nuclear reactions of incident protons with material, the transport, and further reactions of secondary particles are implemented in the code. The particles that cross a certain surface can be sampled to get the neutron spectrum. This spectrum can be further convoluted with crosssections for nuclear reactions to get B-values, which we compare with the experimental values. A satisfying agreement between experimental and simulated B-values shows that simulated neutron spectrum corresponds to the experiment Simulating the setup A simplified model of our setup - a bare, lead target with a narrow, homogenous, centrally impinging proton beam - was first used for calculations. Most calculated values were in good agreement with the experimental data, discrepancies were up to 20%. The simulated values for 198 Au do not agree with the experimental flat distribution along the target, but show a similar distribution as the neutron field for other isotopes, which is a few orders lower than the experimental values. Isotope 198 Au is produced by low energy neutrons, and the flat experimental distribution means that the field of low energy neutrons is homogenous along the target. That is possible if high energy neutrons from the target are moderated in concrete walls and partly reflected back, producing a low energy neutron field, which totally overcomes the neutron field from the target. Another calculation with walls confirmed that this assumption is true. The figure 7 shows the ratio between experimental and simulated values for some of produced isotopes, and the figure 8 shows the calculated neutron spectrum along the target. For these calculations, the Gaussian profile and the displacement of the beam were taken into account, as well as concrete walls and iron components in the corridor. Generally, for most similar experiments that we have analyzed, the MCNPX simulations underestimate the production rates near the end of the target. This is also seen in the case of this experiment (figure 7) The influence of different parameters All experimental conditions are not always precisely known (walls and metal components around the target, beam parameters,...). This determines the systematic error of our experimental results, and simulations help us to estimate its value. How accurate are simulations in describing the experiment depends on other additional factors (reactions with protons, choice of crosssection libraries, INC model,...). Series of simulations are used to estimate the influence of these parameters on the comparison between simulated and experimental values. Such series are done with two identical setups but one slightly changed parameter, the results are then

6 336 2 Ratio sim/exp 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 Au-198 (f4) Au-196 (htape3x) Au-194 (htape3x) Na-24 (htape3x) Neutrons per incident proton e-04 1e Distance along the target e-08 1e-06 1e Neutron energy Figure 7. The comparison of simulated production rates with experimental data for the isotopes, produced in the Al and Au foils. F4 and HTAPE3X are two methods of calculating B-values. Figure 8. The neutron spectrum along the target. Homogenous low energy neutron field and high energy neutron field with the specific shape. compared to estimate the influence of the changed parameter The concrete walls The influence of concrete walls around the target is discussed in the previous section. Walls produce a homogenous field of low energy neutrons without changing the field of high energy neutrons, what was confirmed with a set of simulations with and without the walls. figure 9 shows that the walls have no influence on threshold reactions, and figure 10 shows the difference between the production rates for the non-threshold reaction 197 Au(n, γ) 198 Au when we take in account the concrete walls without them. The 2 cm thick iron table on which the target is placed, the beam stopper made of iron after the target, and the iron beam tube were added to the setup description, and showed to have a negligible influence on the neutron field E-4 Ratio no_walls/walls Au-196 B [g-1 proton-1] 1E-5 1E-6 1E-7 walls no walls Figure 9. The ratios between the calculated production rates for Au-196 for the cases with and without concrete walls. 1E-8 Figure 10. The calculated production rates for Au-198 for the cases with and without concrete walls Beam parameters The beam profile and its displacement at our experiments are never exactly known, on the other hand these parameters influence results significantly. Accelerator beams are usually approximated with a Gaussian profile with different FWHM in x and y

7 337 directions. We measured the beam profile and displacement (Section 2.1) with the wire chamber and five activation detectors, the accuracy of the results is ca. 0.5 cm. To know to which point such inaccuracy matters, we did simulations with different beam thicknesses and profiles, and with displaced beams. The simulations with 6 mm, 6 cm thick beams and a beam with the Gaussian profile (all beams were directed to the center of the target) showed that beam profile cannot influence the results for more than 5%. On the other hand, the simulation with a homogenous, for 1 cm upwards displaced beam (beam diameter was 6 cm) gave for the yields of produced isotopes for up to 40% higher values Reactions with protons Primary and secondary protons react with our detectors and produce small yields of the same isotopes as neutrons. MCNPX does not contain libraries for these reactions, so we needed to convolute simulated proton spectra with cross-section values found in other libraries. Near the 30 th cm the proton influence reaches its maximum : ca. 15% of 24 Na and 196 Au are produced with protons at this point. For 194 Au this number is 30% The choice of Intra-Nuclear Cascade model and cross-section libraries Three Intra- Nuclear Cascade models are implemented in our version of MCNPX: BERTINI, CEM, and ISABEL. The results from these three models differ up to 15%. In figures 11 and 12 are the ratios between the calculated production rates with different models along with the statistical errors. B-values can be calculated with built-in cross-section library (ENDF/B-VI [11]), or the calculated neutron spectra (SSW with HTAPE3X) can be convoluted with the cross-sections from other libraries (EXFOR [10]). The choice of the cross-section library influences B-values for up to 20%. We cannot say, which combination of INC model and cross-section library is the right one, together the differences from model/library choice are ca. 30%. 1,3 1,3 1,2 1,2 ratio 1,1 1,0 0,9 Au-196 Au-194 Na-24 ratio 1,1 1,0 0,9 Au-196 Au-194 Na-24 0,8 0,8 0,7 0,7 Figure 11. The ratios between the calculated production rates with the use of the CEM INC model and the BERTINI INC model. Figure 12. The ratios between the calculated production rates with the use of the ISABEL INC model and the BERTINI INC model. The cause for the experimental uncertainties are mainly not exactly known beam parameters (this is the systematic experimental error, which does not change the shape of the distribution, but is replacing it upwards or downwards) - ca. 20% for 0.5 cm inaccuracy. As the proton influence can be calculated and taken into account, the differences coming from the choice of the INC model/cross-section libraries are setting the limits for the accuracy of simulations - 30%. Experimental errors are smaller or in the range of the differences due to model/library choice, and our experimental data can be used to test the simulation code. For most produced isotopes,

8 338 discrepancies between experiment and simulation have specific trends, which we are currently trying to explain Number of produced neutrons per one incident proton An important parameter of our setup is how many neutrons are in average produced by one incident proton. This parameter can be read from MCNPX output file. In average, per one proton 10 neutrons are produced in spallation reactions, and 5 more with (n,xn) reactions or are reflected from the walls. The sum is 15 neutrons per one incident proton. 4. Conclusion The Phasotron experiment is for its simplicity an ideal example for benchmark codes testing. The experimental results were determined by means of Activation Analysis Method with the accuracy better than 5% (systematic error for not accurately measured position of the beam is additional 20%). Therefore, we can use the experimental values to test different models/libraries, as the differences between simulations with different models/libraries are in this range. The production rates for every isotope with known cross-sections of production reaction can be calculated, either with F4 card (if cross-section libraries are included in MCNPX), or with the convolution of the neutron spectra (HTAPE3X) with our cross-section values. For some isotopes we have cross-section libraries for reactions with neutrons and protons, for some only for reactions with neutrons, and for some we have none. For isotopes, produced in iodine samples, cross-sections for reactions with neutrons or protons do not exist. The experiences gained at Phasotron experiment are used in analyzing data and simulating more complicated experiments (especially Energy plus Transmutation setup [7],[8]). Complicated experimental setups are not as well described by MCNPX as the simple Phasotron experiment, however, they show the deficiencies of the code better. Analysis of the influence of different parameters for other experiments confirms the results obtained at the Phasotron experiment. A very useful tool when simulating was the use of parallel processing. We found out that for all calculations of similar experiments we can profit from a small cluster of computers described in [9]. Concerning this experiment, we plan to evaluate all results, and to test some other calculation codes on them - FLUKA [13], GEANT4 [14], CASCADE-04 [15]. In general, we will carry on with experiments on various spallation targets to provide as much experimental data as possible to help improving the accuracy of MCNPX and similar codes. Acknowledgments The authors are grateful to the staff of the Dubna Phasotron accelerator for providing a good proton beam for our experiment. These experiments were supported by the Czech Committee for collaboration with JINR Dubna. This work was carried out partly under support of the Grant Agency of the Czech Republic (grant No. 202/03/H043) and ASCR K (the Czech Republic). [1] Tolstov K D 1989 Some aspects of accelerator breeding JINR preprint [2] Bowman C D et al 1992 Nuclear energy generation and waste transmutation using an accelerator-driven intense thermal neutron source Nucl. Inst. and Meth. in Phys. Res. A [3] Rubbia C et al Conceptual design of a fast neutron operated high power energy amplifier CERN preprint AT [4] Krása A et al 2005 JINR preprint E [5] Adam J et al 2003 First Nuclear Activation Experiments Using the New Accelerator Nuclotron in Dubna Kerntechnik

9 [6] Westmeier W et al 2005 Transmutation experiments on 129 I, 139 La, and 237 Np Using the Nuclotron Accelerator Radiochimica Acta [7] Krivopustov M I et al 2004 JINR preprint El [8] Wagner V et al 2004 Proc. Baldin ISHEPP XVII (Dubna, Russia) Experimental Studies of Spatial Distributions of Neutrons Produced by Set-ups with Thick Lead Target Irradiated by Relativistic Protons [9] Majerle M et al 2005 Proc. M&C 2005 (Avignon, France) MCNPX Benchmark Tests of Neutron Production in Massive Lead Target 328 [10] [11] [12] [13] [14] [15] Kumawat H, Barashenkov V S 2005 Development of the Monte Carlo model CASCADE-2004 of high-energy nuclear interactions Euro. Phys. Jour. A