| Gamma spectrometer is usually used to measure the radioactivity level of materials in particle physics experiments,radionuclide activity detection in environmental samples,radionuclide analysis and other fields.At present,there is a big gap between the domestic low background spectrometer system and that of foreign countries.Many low background material measurements required for low background experiments need to be sent to foreign testing centers for testing.The neutrino experiment in Jiangmen is the first large-scale low background experiment in China,which has high requirements for the radioactive background of experimental materials.This paper relies on the needs of the neutrino experiment in Jiangmen,according to the existing experimental facilities and laboratory resources,a low background and high purity germanium gamma spectrometer system for radioactivity measurement was built and debugged.The main research contents of this article are as follows:(1)Passive detection efficiency calibration based on Monte Carlo simulation.Using Geant4 to simulate the three-dimensional structure of a high purity germanium detector,set the physical process of particle matter interaction and emission particle source terms,and simulate and calculate the detection efficiency.At the same time,using a known activity radioactive source to test the experimental detection efficiency,based on the comparison between the detection efficiency of the experimental test and the simulated detection efficiency,a modified detection efficiency curve is given,and using standard substance Th to verify the accuracy of the modified simulated detection efficiency,experiments have shown that using Geant4 to simulate passive detection efficiency can be better applied to the quantitative and qualitative analysis of natural radioactive nuclides in environmental samples.(2)Analyze the factors that affect the detection efficiency and optimize the detection efficiency.A detailed analysis was conducted on the efficient measurement of sample radioactivity,simulating the relationship between detection efficiency and sample box size,the optimal sample box size was designed to improve detection efficiency and thus increase detection limit.At the same time,the detection efficiency of different materials was simulated,and it was found that the detection efficiency of different materials was related to their density.In daily measurement of samples,the problem of unclear sample element composition was addressed,the detection efficiency can be simulated using materials of similar density instead.(3)Build and debug the entire low background gamma spectrometer system.After building the shielding main body and electronic system of the spectrometer system,and completing the debugging of the entire system,the background count rate was measured to be 0.22cps/100cm~3Ge,the detection limit of uranium and thorium in glass samples is 0.01Bq/kg in the order of magnitude,the detection limit for uranium and thorium in China is 0.1-1Bq/kg generally around,higher than the detection limit for uranium and thorium in general domestic gamma spectrometer systems.At present,the whole gamma spectrometer can operate normally,and it starts to measure the low background materials related to the Jiangmen neutrino experiment and the low background materials required for other experiments. |
