Development of Efficiency Calibration Method for Radioactive Volume Sources Based on Virtual Calibration Principle

Using the Monte Carlo simulation method, the virtual point source position of the volume source for different HPGe detectors can be quickly obtained. In the calculation, it is necessary to simulate the detection efficiency of the point source and the volume source. The effects of the size and type of HPGe crystals on their virtual point source positions were investigated through ²⁴¹Am, ¹³⁷Cs, ⁶⁰Co point sources and volume sources. The simulation results show that the virtual point position of the ²⁴¹Am source presents obvious differences. In the low energy range, the VPS calibration method is not valid in the measurement of small-volume samples of the surface of the detector. Finally, the semi-empirical formula for the height of the volume source and the position of the virtual point source is obtained by using the ¹³⁷Cs and ⁶⁰Co sources. In the test measurement, the detection efficiency of two types of HPGe detectors was calibrated using the standard soil source of the size of 70 mm×65 mm. Compared with the simulated detection efficiency curve and the calculation results of virtual point source efficiency, the validity of the semi-empirical formula was verified. In the gamma energy range of 300~2 000 keV, the relationship between the position of the volume source and the virtual point source can be established through a suitable radioactive source. This study provides a new approach in the radiation measurement to avoid the repeated detection efficiency calibration in detectors.