地表附近宇宙射线中子测量土壤含水量刻度函数研究

Research on Calibration Function for Using Cosmic Ray Neutron Near the Surface of the Earth to Measure Soil Moisture

为解决地表附近宇宙射线中子测量土壤含水量刻度函数研究中基于MCNP和URANOS的计算模型存在的中子源位置和能谱设置不准确问题,提出利用Geant4中宇宙射线粒子输运程序PLANETOCOSMICS分别计算不同条件下宇宙射线中子能谱并分析差异,由此获得合理的中子源入射高度和能谱,从而建立新的计算模型。结果表明:①模型中宇宙射线中子源需要设置在海拔600 m以上;②通过对比分析背散射中子能谱和10^-8~10^-4 MeV能量区间的刻度函数,表明Geant4模型计算结果优于URANOS和MCNP结果;③地表2 m高度处,10^-6~10^-2 MeV能量区间的中子计数对土壤含水量的灵敏响应优于10^-8~10^-4 MeV能量区间。因此,利用Geant4计算模型计算获得的10^-6~10^-2 MeV能量区间的刻度函数适合用于宇宙射线中子实时测量土壤含水量。

In order to solve the problem of improper setting of neutron source location and energy spectrum in the calculation of soil moisture measurement calibration function based on MCNP or URANOS models,PLANETOCOSMICS based on Geant4 was proposed to calculate the neutron spectrum of cosmic ray under different conditions.The differences of these results were analyzed to obtain the reasonable neutron source incident height and energy spectrum.A new calibration function calculation model based on these rational neutron source settings was established.The results show that:①The cosmic ray neutron source used in model should be set at altitude above 600 m.②Through the comparative analysis of backscattered neutron spectrum and 10^-8~10^-4 MeV calibration function,the calculation results of Geant4 model are better than those of URANOS and MCNP.③Neutron in the 10^-6~10^-2 MeV energy range at altitude 2 m is more sensitive respond to the variation of soil moisture,which is better than 10^-8~10^-4 MeV energy range.Therefore,the calibration function in the energy range 10^-6~10^-2 MeV calculated by this model is suitable for real-time measurement of soil moisture by cosmic ray neutrons.