Research On Passive Calibration Method Of In-situ Gamma Spectrometer Based On Monte Carlo Simulation

Compared with the traditional y-spectrometer measurement method of sample collection and laboratory analysis,the in-situ y-spectrum measurement method has great advantages in time and cost.This method can quickly identify radionuclide and detect environmental gamma radiation dose levels and their changes,so it is widely used to determine radioactive levels in the ground or air,measurement of radioactive pollution in nuclear facilities and their vicinity,and radiation monitoring in geological exploration.However,the scope of application of this technology is currently relatively limited in China,and there is no national standard for in-situ gamma spectrometer measurement technology.The in-situ gamma spectrum technology is based on the calculation formula proposed by Beck et al.The three scaling factors,such as detection efficiency factor,angular response factor and photon flux factor,are linked to evaluate the overall sensitivity of the in-situ gamma spectrometer.At present,in the research on the calibration of the in-situ gamma spectrometer,the research on the radionuclide in the soil to the gamma photon flux rate somewhere in the air is relatively small.Generally,refer to the recommended value in the Report No.ICRU-53.The composition,density,and relaxation depth of the soil in the measurement may be different from those in the soil sample in ICRU-53 report.In addition,the Beck formula is only for the case of infinitely large plane radioactive soil sources,and the actual measurement needs to deal with a variety of relatively complex terrains.At this time,the use of the Beck formula may bring large deviations.Because the Monte Carlo program Geant4 can realistically simulate the actual physical process of particles in uniform and non-uniform media,and can build various geometric models that conform to the actual measurement environment,it can deal with various complex problems that cannot be solved by analytical algorithms.The thesis focuses on the research based on Geant4.The content and results are as follows:(1)A reliable simulation calculation method for the detection efficiency factor,angular response factor,and primary gamma photon flux factor of the in-situ HPGe y spectrometer was developed.(2)Taking the hillsides where the radionuclides are distributed uniformly as an example,the primary gamma photon flux rates generated by them somewhere in the air are simulated and calculated.(3)Based on the latest nuclear evaluation database,an application program for passive calibration of in-situ gamma spectrometers has been developed.The research in this paper can provide guidance for the establishment of in-situ gamma spectrometer calibration specifications in China.