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Submitted to ‘Chinese Physics C PGNAA neutron source moderation setup optimization Zhang Jinzhao1（张金钊）Tuo Xianguo1（庹先国） (1.Chengdu University of Technology Applied Nuclear Techniques in Geoscience Key Laboratory of Sichuan Province，Chengdu 610059，China) Abstract: Monte Carlo simulations were carried out to design a prompt γ-ray neutron activation analysis (PGNAA) thermal neutron output setup using MCNP5 computer code. In these simulations the moderator materials, reflective materials and structure of the PGNAA 252Cf neutrons of thermal neutron output setup were optimized. Results of the calcuations revealed that the thin layer paraffin and the thick layer of heavy water moderated effect is best for 252Cf neutrons spectrum. The new design compared with the conventional neutron source design, the thermal neutron flux and rate were increased by 3.02 times and 3.27 times. Results indicate that the use of this design should increase the neutron flux of prompt gamma-ray neutron activation analysis significantly. Key word: PGNAA; neutron source; thermal neutron; moderation; reflection 1. Introduction study, Monte Carlo calculation was carried out for the Prompt gamma ray neutron activation analysis design of a 252Cf neutron source moderation setup for the (PGNAA) is a rapid, nondestructive, powerful analysis cement samples[7]. The model of Monte Carlo multielemental analysis technique, large samples of some simulation was verified by experiment[8, 9].We improve minor, trace light elements and is used in industrial the thermal neutron source yield rate of 252Cf neutron by control[1-5]. In a PGNAA analysis, the sample nuclear the PGNAA neutron source structure to the design. The composition is determined from prompt gamma rays calculation results for the new design were compared which produced through neutron inelastic scattering and with the previous, example: themal neutron flux rate, fast thermal neutron capture. Since the inelastic scattering neutron flux rate, gamma rays yeild, results of these cross section is small, the PGNAA design based on studies are presented. thermal neutron capture. Low-energy neutrons were 2. The model of Monte Carlo simulations absorbed by the target nucleus forming compound Geometry of the neutrons moderation setup, used in nucleus, which has energy equal to the kinetic energy the study, has two forms to compare. Both of the two and the neutron capture neutron binding energy. forms geometry consist of a NaI detector of gamma rays, Compound nucleus emits γ photon to go background a cylindrical polyethylene sample compartment and a state in the 10-14s of time. The gamma yield was neutron. Inside diameter of the cylindrical polyethylene determined by the flux of themal neutron, and sample compartment is 20cm, external diameter is 30cm. determined the analysis results. So performing a PGNAA Height is 20cm. a neutron source of the previous is only setup depends on thermal neutron flux available at the wrapped by a moderator. The new one is different. It has sample, how to improve the thermal neutron produced a moderation and a reflector. Reflector places below the ratio of neutron source has been the research focus of neutron source and the a hemispheric moderator are PGNAA. taken on the upper of the neutron source. Our goal: the Accelerator neutron, source reactor neutron source, neutron is reflected back toward the upper portion of the Am-Be neutron source and 252Cf neutron source can be lower part and the utilization ratio of neutrons is used as PGNAA neutron source[6]. 252Cf neutron source improved, because of the different materials for different which has a high neutron fluxs density and low cost used neutron reflectivity. Fig.1 is a schematic of the previous in PGNAA setup commonly. However, 252Cf source is PGNAA setup source-moderator from paper[7]. Fig.2 is a isotropic neutron source, to sample less thermal neutron. schematic of new design of this study. We determined the Neutron source utilization rate is low. In the present best geometry by calculating. Supported by National Natural Science Foundation of China(41274109, 41025015) E-mail:[email protected] Submitted to ‘Chinese Physics C 3.5x10-6 -6 3.0x10 Counts 2.5x10-6 2.0x10-6 1.5x10-6 Counts 1.0x10-6 5.0x10-7 0.0 -5.0x10-7 0 2 4 6 8 10 12 Energy (MeV) Fig. 1 Schematic of the previous PGNAA setup Fig. 3 252Cf Fission neutron spectrum source-moderator This study refers to the literature[7, 10] of Portland cement composition as a sample for simulation calculation. 2.1 Moderator material Monte Carlo simulation PGNAA analysis accuracy depends on irradiation neutron fluence rate and neutron capture cross section of to be analyzed nuclides . Radiative capture cross section is the main section for the heavy nuclei and low energy neutron action, the entire cross-section is almost equal with the radiative capture cross section, showing 1/ v . Fig. 2 Schematic of the new PGNAA setup So try to increase the sample thermal neutron fluence source-moderator of this study rate and reduce the fast neutron fluence rate in the 252 The simulation uses Cf spontaneous fission PGNAA analysis. Therefore, we must moderat neutrons, neutron source which neutron energy spectrum is a to improve the thermal neutron source neutrons in the fission spectrum and can be described watt distribution. proportion, but also pay attention to the material used in Formula: moderation to decrease γ background. We chose five E materials H O, D O, C H , C, Paraffin to calculation. f( E ) cea sinh bE a 2 2 2 2 The resluts will be analysised, and the best material will E is the neutron energy, a 1.025 , b 1.25, be found out. Fig.4, Fig.5, Fig.6, Fig.7 is a Monte Carlo c 0.365 . As shown of Figure1, it is a 252Cf fission calculation reslut of H2O, D2O, C2H2, C, Paraffin. neutron energy spectrum by MCNP5 simulation. 1.0 1.0 H O 2 D O 0.8 2 nC H 0.8 2 2 C 0.6 H O Paraffin 2 0.6 D O 2 0.4 Counts nC H 2 2 0.4 C Counts C H 0.2 30 60 0.2 0.0 0.0 0 20 40 60 80 Thickness/cm 0 20 40 60 80 Fig. 4 Thermal neutron flux of five moderator materials increase Thickness/cm with the thickness Fig. 5 Fast neutron flux of five moderate materials increase with Submitted to ‘Chinese Physics C stronger the reflection is. Fig.8, Fig.9 is a Monte Carlo 1.0 calculation result of H2O, D2O, C2H2, C, Be, paraffin 0.8 0.9 0.6 H O 2 D O 2 0.8 0.4 nC H Counts 2 2 C 0.7 nC H C H 2 2 0.2 30 60 0.6 C H 30 64 H O 0.0 0.5 2 Be Counts 0.4 0 20 40 60 80 C 2 Thickness/cm D O the thickness 0.3 Fig. 6 Neutron flux of five moderator materials increase with the 0.2 0.1 thickness 0 10 20 30 40 50 60 1.0 H O Reflector Thickness/cm 2 D O 2 nC H Fig. 8 The reflective neutron flux in different thickness of five 0.8 2 2 C C H 30 60 kinds of reflective material 0.6 0.14 nC H 0.4 2 2 Counts C H 0.12 30 64 H O 0.2 2 0.10 Be 0.0 C 0.08 D O 2 0 20 40 60 80 0.06 Thickness/cm Counts 0.04 Fig. 7 Gamma ray flux of five moderator materials increase with 0.02 the thickness 0.00 As shown of Figure2, Figure3, Figure4, Figure5: -0.02 0 10 20 30 40 50 60 Thermal neutrons: Thermal neutrons increased Reflector Thickness/cm rapidly with increasing light water, paraffin, Fig. 9 The γ ray flux in different material and thickness polyethylene thickness in 9cm-12cm growth to the Fig.8, Fig.9 shows the reflective neutron flux, γ ray maximum. But it with the moderator increasing the in different thickness of five kinds of reflective material. thickness starts to decrease, because a large number of Beryllium is the reflective effect best material on a thin the thermal neutron are absorbed. When heavy water and layer, followed by paraffin. Thick layer of heavy water graphite 48cm, the thermal neutron flux is maximum. and graphite is reflective of the best materials.The result Thermal neutrons loss is less with thickness increase in showed that Thick layer of heavy water and graphite of the moderator. reflection effect was the best. Because it contains Fast neutrons: The five substances fast neutron flux hydrogen nuclides, light water, polyethylene, paraffin is reduced with the increase in thickness of the will absorb neutrons and γ-rays be generated. Beryllium, moderator substance, sequentially: Paraffin, light water, heavy water and graphite which do not contain neutron polyethylene, heavy water, graphite. absorbing nuclides produces γ-ray flux is extremely low γ-rays: In a neutron moderator processing, paraffin, as the reflective materials. light water, polyethylene will produce gamma rays. 3. Neutron source setup optimization Yields get maximums and reduce subsequent with the Considering various substances above reflection and thickness increase in at a thickness of 22cm. Graphite moderating effect, design a new neutron source. As thin and heavy water is not substantially generated γ-rays. beryllium reflector best, we use a hemispherical wrapped 252 2.2 Reflective material Monte Carlo simulation Cf fission neutron source from the lower part. Thick The reflection is determined by the scattering cross layer of heavy water and graphite having a similar section of nuclide, the absorption cross section, surface reflectivity, and they γ-ray yield lower.
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