加速器中子源大厅内散射中子分布的模拟

Simulation of Scattered Neutron Distribution in Accelerator Neutron Source Hall

本文针对加速器中子源可在较宽能量区间产生单能中子的特点,采用MCNP5对0.2~20 MeV的源中子在加速器中子源大厅内的散射情况进行模拟计算和分析。结果表明,直射中子通量随离源距离的增大呈平方反比衰减,散射中子通量则随离源距离的增大而几乎保持不变;大厅内的散射中子主要来自墙壁的贡献,离墙壁越近散射率越高。能量为0.4 MeV和1 MeV的源中子散射率最高,10 MeV和15 MeV的源中子散射率最低。用中子的宏观散射截面可较好解释散射率模拟结果,中子的弹性散射截面远大于非弹性散射截面,因此弹性散射起主导作用。中子能量大于1 MeV后,散射截面随中子能量增加而减小直至进入一段坪区,散射率也随之降低并进入坪区。结合待测位置处直射、散射中子通量和不同能量的散射中子份额的计算,能解释能量较高的源中子散射率较低的现象。通过在墙壁表面附上一层中子慢化吸收材料的方法可有效减弱中子散射,如5 cm的含硼聚乙烯(10%B4C)可降低散射率约40%。

The accelerator neutron source can provide monoenergetic neutrons in a wide energy range,which can be widely used in many fields,such as the neutron reaction cross-section measurement,neutron detector calibration,etc.Scattered neutrons might introduce errors or even cause troubles to these experiments,therefore it is necessary to precisely analyze and qualify the neutron scattering ratio in the accelerator neutron source hall.In this paper,MCNP5 was used to simulate the scattering ratio for neutron source energy from 0.2 to 20 MeV,and the distribution of the scattering ratio as well as the contribution from the air and the wall were analyzed.It is found that the direct neutron components decrease with the distance following the inverse square law,while the scattered neutron components are approximately constant.The scattered neutrons are mainly generated by the wall in the hall,while the contribution of the air could be ignored.The scattering ratio is much smaller for the range much closer to the neutron source,which increases quickly with the distance between the neutron source and the detector.When the energy of neutron source is greater than 1 MeV,the scattering ratio begins to decline until it enters a plateau in which the energy is higher than 7 MeV.The scattering ratio is the highest for the neutron source with the energy of 0.4 MeV and 1 MeV,while that is the lowest for 10 MeV and 15 MeV.Total macroscopic scattering cross-section of neutrons to materials is found to be able to explain simulated results of the scattering ratio properly,because the curve of total scattering cross-section versus energy is very similar to that of the scattering ratio versus the neutron source energy.Moreover,the elastic cross-section is much larger than the inelastic scattering cross-section,and the elastic scattering plays a leading role in neutron scattering.When the energy of the neutron source is higher,the total scattering cross-section is much lower which renders the scattering ratio much lower.However,due to the higher inelastic scattering cross-section,neutrons with higher energy are more likely to lose a lot of energy during each collision,which would make a higher proportion of the slow neutron components within scattered neutrons.The scattering ratio could be effectively reduced by attaching a layer of the neutron moderating and absorbing material on the wall.For example,5 cm boracic polyethylene(10%B 4C)could reduce the scattering ratio by about 40%.These results are instructive for applying accelerator neutron source as well as for the accurate analysis and attenuation of the scattering effect in neutron experiments.