Determination of Effective Detection Distance and Minimum Detectable Activity for Radiation Monitoring System in Water

수중 방사선모니터링 시스템의 성능평가를 위한 검출기의 수중 내 유효검측거리 및 최저검출농도 산출

The concerns about the potential contamination of drinking water with accidental nuclear matter have been escalated as more nuclear power plants are being constructed in many countries. Radiation monitoring systems in water environment are basically the same as the conventional ones in principle, but its structure needs to be adapted and modified so as to be suitable to the aquatic operation conditions. As a result, its specifications should be investigated in terms of the minimum detectable activity (MDA) as well as the effective detection distance (EDD) because they determine the radiation level that the monitoring system initiates the emergency action to prevent the water source from being contaminated early enough to avoid serious social commotion. In this study, the EDD and the MDA of the monitoring system fabricated by the Korea Atomic Energy Research Institute (KAERI) were measured with $^{68}Ga$ source. The $^{68}Ga$ source was mixed with water homogeneously in the experimental device, and detectable area was expanded gradually with detecting gamma emitted from $^{68}Ga$ source for EDD. Then, the gamma was also detected as a function of time for MDA, because the activity of $^{68}Ga$ decreases with its half-life. It turned out that the values were 29 cm and $5Bq{\cdot}l^{-1}$ in the experiments, respectively. Relatively short range of the EDD seems to be natural in water, but it suggests that multiple monitoring system need to be strategically installed in a body of water. the MDA value is far lower than the limitation for drinking water, which is $200Bq{\cdot}l^{-1}$ for $^{137}Cs$.