An Evaluation of Radioactivity Determination in a Volume Source with the Sum-peak Method Using Geant4

Abstract

In the determination of radioactivity in samples such as soil and water, a simple method with acceptable accuracy is desirable. A simulation is used for verifying the method under various conditions. The conventional sum-peak method and the modified sum-peak method were developed as simple absolute measurement techniques. However, it is known that radioactivity is underestimated in volume source samples, which concerns to the angular correlation of two gamma rays, the geometric effect, and the self-absorption in the sample. These effects depend on the Source-to-Detector-Distance (SDD) and the influences are difficult to measure separately. In this paper, we investigated the dependence of calculated radioactivity on SDD by using the Geant4 Monte Carlo simulation. The simulation modeled a high purity Germanium detector system and a volume source. The results showed that the effect of angular correlation and the geometric effect is significant when the SDD is less than 10 cm, whilst the self-absorption is the main factor of the underestimation and the constant when the SDD is larger than 10 cm.