Simulation Study Of Soil Heavy Metal Measurement Technique Based On PGNAA

With the development of China’s social and economic progress,the problem of soil heavy metal pollution has become increasingly prominent,posing a serious threat to people’s living environment and health.Soil heavy metal pollution measurement is an important part of soil heavy metal pollution prevention and control.Therefore,the development and improvement of soil heavy metal pollution measurement technology are of great significance.Traditional laboratory measurement methods are time-consuming and inefficient.With the increasing demand for on-site rapid investigation,emergency measurement,and preliminary investigation of large areas,the development of soil heavy metal pollution on-site measurement technology has gradually attracted attention.The Prompt Gamma Neutron Activation Analysis(PGNAA)technology has unique advantages such as non-destructive,multi-element,continuous online measurement,and large volume measurement.In this study,the PGNAA-based in-situ measurement technology for soil heavy metals was investigated using the Geant4 simulation program.Am-Be isotope neutron source and D-D neutron generator were used as the neutron source for in-situ soil heavy metal measurement based on PGNAA.The measurement system structure design based on these two neutron sources was proposed,and the performance of the measurement system was studied and compared.A model of the in-situ measurement system for heavy metals in soil based on these two neutron sources was constructed using the Geant4 program.To improve neutron utilization efficiency,the materials and thicknesses of the moderation layer,reflection layer,and shielding layer of the measurement system were optimized.The ambient dose equivalent rate of the measurement system during the operation was calculated and evaluated.Under the same conditions of neutron yield and measurement point distance,the dose equivalent rate of the Am-Be neutron source measurement system was higher than that of the D-D neutron generator measurement system.By setting and changing the soil range and soil layer depth of the interaction between neutrons and soil during the operation of the measurement system,the variation trend of the characteristic γ-ray intensity of the measured heavy metal elements was analyzed,and the measurement range and depth of the two measurement systems for PGNAA in-situ measurement of heavy metals in soil were obtained.The full-spectrum analysis of the characteristic γ energy spectrum of soil was carried out using the spectral library least squares method to calculate the content of heavy metal elements in the soil.In order to improve the accuracy of spectral deconvolution,a weighted optimization method was introduced.The results of before and after optimization were compared for the peak height ratio of the 2.2 Me V energy peak of the characteristic γ energy spectrum and the 7.2 Me V and 7.7 Me V energy peaks,the inverse results of the singleelement energy spectrum,and the measured results of the heavy metal element content.The comparison results showed that the weighted optimization method can improve the accuracy of spectral deconvolution of the characteristic γ energy spectrum.A small standard soil sample spectrum library was constructed,and five sets of soil samples were set up for testing.The process of in-situ measurement of heavy metal elements such as iron,manganese,copper,nickel,and chromium with different contents in the soil was simulated,and the measurement relative errors of each heavy metal element content at different levels were obtained,which proved the feasibility of PGNAA for in-situ measurement of heavy metal pollution in soil.