Efficiency Calibration of Reactor Noble Gas Monitor Using Radioactive Simulated Gas Standard Source

In order to accurately calibrate the gamma ray full energy peak efficiency of the noble gas monitor to the gas source, a Marinellibeaker radioactive simulated gas standard source with expandable polystyrene (EPS) particles as the matrix material was prepared. The sample density of the simulated gas source is 4.1 kg/m³, which contains eight kinds of single energy γ-ray emission nuclides, including ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ⁵¹Cr and so on. Using the simulated gas standard source, the gamma ray full energy peak efficiency of HPGe detector in the reactor noble gas field monitor was calibrated. The calibration energy region is 60-1 836 keV, and the maximum standard uncertainty of calibration efficiency is 4.4%. At the same time, a point source at a representative point method was used to calibrate the efficiency, and the calibration results of the simulated gas source and the point source at the representative position were compared. It is found that the efficiency ratio between the simulated gas source and the point source at the representative position is not a constant in the calibration energy region, and the maximum efficiency deviation is up to 28%. The deviation can be reduced by efficiency transfer coefficient and the fitting curve of efficiency transfer coefficient can be obtained. Finally, at the 81 keV energy point, the efficiency ratio of 1.26 is obtained between the simulated gas standard source and the practice gas standard source, which can be used as a reference for the practical application of the simulated gas standard source.