1 Introduction MCNPX SIMULATIONS OF THE ENERGY PLUS TRANSMUTATION SYSTEM: NUCLEAR TRACK DETECTORS

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1 MCNPX SIMULATIONS OF THE ENERGY PLUS TRANSMUTATION SYSTEM: NUCLEAR TRACK DETECTORS M. Majerle 1,2, V. Wagner 1,2, A. Krása 1,2, J. Adam 1,3, S.R. Hashemi-Nezhad 4, M.I. Krivopustov 3, A. Kugler 1, V.M. Tsoupko-Sitnikov 3 and I.V. Zhuk 5 (1) Nuclear Physics Institute of ASCR PRI, Řež near Prague, The Czech Republic (2) FNSPE of CTU, Prague, The Czech Republic (3) Joint Institute for Nuclear Research Dubna, , Dubna, Russia (4) School of Physics, A28, University of Sydney, NSW 2006, Australia (5) Joint Institute of Power and Nuclear Research, Sosny, Minsk, Belarus majerle@ujf.cas.cz Abstract Energy plus Transmutation setup consists of a thick, lead target surrounded by natural uranium blanket and placed in a polyethylene box. In series of experiments, relativistic protons (deuterons) were directed to the target and neutron/proton flux was studied at different places in the setup by means of the activation analysis and solid state nuclear track detectors. Monte Carlo simulations of the Energy plus Transmutation setup were performed in order to study the influence of experimental deviations and individual setup parts on obtained data. Calculated neutron and proton fluxes were convoluted with appropriate cross-sections and subsequently compared to experimental results. 1 Introduction The Energy plus Transmutation (EPT) international collaboration studies neutron production and transport inside a thick, lead target and surrounding uranium blanket [1]. The neutronics of the system is investigated using mostly activation radiochemical detectors and solid state nuclear track (SSNT) detectors. The EPT setup is composed of three parts (target, blanket, shielding), of which each has its own function, but also its influence on the produced neutron field. Without understanding their contributions to the neutron field, it seems impossible to study the neutron production in the setup. Moreover, experiments with the EPT setup showed that systematic deviations are present in measured data, probably due to the deviations of experimental parameters (beam position and profile, misplacement of detectors). Therefore, any interpretation of experimental data is suspicious without good knowledge of the systematic deviation due to these parameters. Extensive Monte Carlo simulations of the setup were already performed for radiochemical detectors [2]. In this paper, similar calculations are presented for solid state nuclear track detectors, the functioning of these detectors is discussed, and the comparison between experimental and simulated data is done. Also, few new aspects of the EPT setup are discussed. 1

2 2 The setup The target-blanket part of the EPT setup is composed of four identical sections. Each of them contains a cylindrical lead target (diameter 8.4 cm, length 11.4 cm) and 30 natural uranium rods (diameter 3.6 cm, length 10.4 cm, weight 1.72 kg) distributed in a hexagonal lattice around the lead target. The four target-blanket sections mounted on the wooden plate are placed in a wooden container filled with granulated polyethylene. The inner walls of the box are covered with 1 mm thick cadmium layer (Fig. 1). The detailed geometrical arrangements and dimensions of the EPT setup can be found in [2]. a) mm of Cd beam steel+wood 38 textolite 164 polyethylene shielding wooden walls Figure 1: The layout of the EPT setup: the front cross-section and the side cross-section. Several experiments were carried out using the EPT setup and its target was irradiated with relativistic protons with energies in the range from 0.7 to 2 GeV. In these experiments the neutron flux was measured using activation foils (radiochemical detectors) and solid state nuclear track (SSNT) detectors that were placed between the blanket sections. The radiochemical detectors with dimensions of 2 cm 2 cm 0.1 mm were used. During the exposition to neutron flux, various nuclear reactions, e.g. (n,γ), (n,xn), (n,α), occurred in the radiochemical detectors, and the yields of the products were determined from their characteristic peaks in gamma spectra [3]. SSNT detectors with dimensions 1 cm 1 cm 1 mm were placed at the same places. Lead or uranium were used as irradiators, fission fragments left tracks in mica foils. These tracks were counted with the help of optical microscopy [4], and relative production rates were determined from the number of tracks in mica foils. 3 Polyethylene box - biological shielding MCNPX v2.6.c [5] Monte Carlo code was used to simulate the production and transport of neutrons and other particles in the EPT setup. CEM03 intranuclear cascade model (included in MCNPX code package) was employed to describe high energy nuclear reactions. Fig. 2 illustrates the EPT setup as described by the MCNPX code. The setup was studied by comparing the calculated spectra of produced particles (in MCNPX they were obtained with the F4 tally card, the energy bins were set with E4 card) under different experimental conditions. The energy of the beam was set to 1.5 GeV, because one of the first experiments with the EPT setup was performed at that energy. 2

3 Figure 2: Plot of the target placed in the polyethylene box (SABRINA plot, provided by Jaroslav Šolc). High energy neutrons which are produced in spallation reactions present a risk for the environment. Therefore, a box filled with granulated polyethylene was designed around the target-blanket assembly having function of biological shielding with little influence on high energy neutron spectrum inside the box. To achieve that, the inner walls of the box were covered with 1 mm cadmium layer, low energy neutron absorber. A set of simulations (without box, with box but no cadmium, and with both - box and cadmium) of neutron spectra at the place of the target blanket were compared, showed that only neutrons with energies less than 1 ev are stopped by the cadmium layer, and that field of neutrons ranging from 1 ev to 0.1 MeV inside the polyethylene box is produced by the combined effect of the polyethylene and cadmium [2]. Figure 3 shows calculated neutron spectra emitted to the environment for the targetblanket alone and for the target-blanket placed in the biological shielding. As can be seen, the polyethylene box essentially decreases the flux of emitted high energy neutrons by moderating them to lower energies. Calculations suggest that from 50 neutrons that are produced per one proton at 1.5 GeV, 42 would escape to the environment in the hypothetical case without the shielding, but with the shielding only 10 neutrons escape, 8 from these through front and back openings in the polyethylene box. N neutrons [cm -2 proton -1 ] 1E E E E E E without shielding with shielding 1E-5 1E E E-3 1E+0 1 1E Energy [MeV] Figure 3: Calculated neutron spectra emitted to the environment from the target-blanket only and from the target-blanket surrounded by the polyethylene box. 3

4 4 Influence of experimental conditions on SSNT detectors Reaction rates calculated under different experimental conditions were compared. In order to obtain the reaction rates, the simulated spectra of neutrons, protons, and pions (F4 tally card, the energy bins are set with E4 card) were convoluted with the cross-section for the specific reaction. Because the cross-sections for induced fission - (n,f), (p,f), (π,f) - are either not included in MCNPX cross-section libraries or are limited in the energy range, the missing cross-sections were previously simulated (calculated with MCNPX, extracted with FT8 RES card). The cross-sections for induced fission in lead by neutrons, protons, and pions are seen in Figure 4. Simulations showed that at the experiments with the EPT setup, SSNT detectors with lead irradiator mostly detect protons from the beam (contribution from 30%-90%, depending on the position) and neutrons with energies MeV. The contribution from pion induced fission is 5%-15%. Cross-section [barns] 1E+0 1 1E E-2 1E E E protons pions neutrons 1E Energy [MeV] Figure 4: Simulated cross-section for neutron, proton, and pions induced fission in lead calculated with MCNPX. Due to the higher cross-sections for reactions with protons, SSNT detectors are more sensitive to beam adjustment than radiochemical detectors. The beam used at experiments was assumed to have Gaussian profile in X- and Y-axis with extending tails. The intensity of the tails is some orders of magnitude lower than the intensity in the center of the beam. Calculations showed that SSNT detectors placed in gaps between the targetblanket sections at radial distances 5-15 cm from the target axis are sensitive to small changes of beam intensity in the tails - the differences between the simulations with the Gaussian beam profile and simulation with the experimentally measured beam can reach up to 40%. The displacement of the beam center for 3 mm affects the results for up to 40%. The change of the angle under which the beam enters the target of 3 changes the results in SSNT detectors placed near the target axis for up to 200%. SSNT detectors are also sensible to inaccurate positioning - misplacement by 5 mm results in 40% different simulated results. Sometimes, it is useful to approximate the hexagonal lattice of uranium rods in the setup with the same mass of uranium homogeneously distributed inside a cylinder or a hexagonal block of similar dimensions as the real blanket, see Fig. 5. It was found out that calculations with these simplifications give up to 10% different results in SSNT or radiochemical (e.g., 197 Au(n,2n) 196 Au) detectors. 4

5 ... 11/28/05 14:56:26 11/28/05 14:19:14 11/28/05 14:20:11 c cell start c cell start c cell card for sample problem probid = 11/28/05 14:19:57 probid = 11/28/05 14:56:00 probid = 11/28/05 14:18:49 basis: XY basis: XY basis: XY ( , , ) ( , , ) ( , , ) ( , , ) ( , , ) ( , , ) origin: origin: origin: ( 0.00, ( 0.00, 0.00, 5.00) ( 0.00, 0.00, 5.00) 0.00, 5.00) extent = ( 15.00, 15.00) extent = ( 15.00, 15.00) extent = ( 15.00, 15.00) Figure 5: Front cross-sections of cylindrical and hexagonal target-blanket approximations and of the real target-blanket (MCNPX plot). 5 Comparison between experiment and simulation The reaction rates for 1.5 GeV experiment calculated with MCNPX code were compared to experimental results. The beam profile and displacement used in calculation were measured with the set of the SSNT detectors with a lead irradiator placed in the front of the target. In every gap and behind the target were placed additional SSNT detectors with lead irradiator of 1 cm 1 cm dimensions, 9 of them were placed along the positive Y-axis from 0 to 14 cm from the central target axis [4]. The ratios between the experimental and calculated values (κ = C B exp /B sim ) are seen in Figure 6. Constant C is determined by the ratio in the first foil in the first gap, which is normalized to 1, therefore, κ compares only the shape of experimental and calculated distribution of reaction rates and not absolute values. κ varies from 0.5 to 1.5 for almost all detectors, what implies that MCNPX satisfactory predicts reaction rates in SSNT detectors with lead irradiator. The disagreement between experiment and simulation for radiochemical detectors is much higher, up to few times [6]. exp/sim X along the target [cm] Y from target axis [cm] 4 Figure 6: Ratios between experimental and calculated reaction rates in SSNT detectors with a lead irradiator. On the X-axis there is longitudinal distance along the target (detectors were placed every 12.2 cm), and on the Y-axis there is the distance of the detector from the central target axis. 5

6 6 Conclusion Several experiments with the Energy plus Transmutation setup were performed. Radiochemical and solid state nuclear track detectors are used to study the neutronics of the system. Calculations for radiochemical detectors are being performed for some time, and their behavior in various experimental conditions is well known. They mainly interact with MeV neutrons through (n,xn) and (n,α) reaction channels and are less sensitive to deviations of experimental conditions than SSNT detectors [2]. Studies performed for solid state nuclear track detectors with lead irradiator showed that SSNT detectors detect mostly primary protons and MeV neutrons, and are essentially influenced by experimental conditions (mainly beam deviations). However, the comparison between experimental and calculated reaction rates for 1.5 GeV experiment shows that the shape of reaction rate distribution is satisfactory described by simulations with discrepancies up to 50%. In the case of radiochemical detectors sensitive to neutrons with energies MeV, these discrepancies are of the factor of few hundreds percent. These facts imply that the beam parameters are well measured and are not responsible for big discrepancies in the case of radiochemical detectors. Also, it is evident that neutrons with energies higher than 100 MeV are simulated reliably, and discrepancies are caused mainly by neutrons of energies MeV. The reasons are under investigation. The studies of the polyethylene box, which was primary designed as biological shielding showed that ca. 25% of neutrons produced in the target-blanket escape to the environment, mainly through front and back openings. To lower the number of escaped neutrons, it would be useful if two polyethylene walls were added in front and in the back of the target-blanket. Next improvement would be another cadmium layer at outer walls of the polyethylene that would prevent the emission of thermal neutrons to the environment. References [1] M.I. Krivopustov, et al., Kerntechnik 68 (2003) 48. [2] M. Majerle, et al., Monte Carlo studies of the Energy plus Transmutation system, paper accepted to NIM A, Conference series, ISRP-10 proceedings. [3] A. Krása, et al., JINR Preprint E [4] M.I. Krivopustov, et al., JINR Preprint E [5] John s. Hendricks et al., MCNPX, version 2.6.c, LA-UR [6] F. Křížek, et al, Czech. J. of Phys. 56 (2006)