Determination of minimum detectable activity for underwater radiation detection system

An underwater 137Cs (662 keV) and 131I (364 keV) radioisotopes monitoring system has been developed using a 3'' × 3'' NaI(Tl) detector. To estimate the minimum detectable activity and Bq/L conversion factor, an experiment was first conducted with 68Ga (511 keV) in a water tank. For energy spectrum comparison, Monte Carlo simulations were performed for 68Ga, 137Cs, and 131I to determine the conversion factor. The minimum detectable 137Cs and 131I activities for the system are 0.77 and 0.89 Bq/L, respectively.