Preliminary study on the detection efficiency and estimation of minimum detectable activity for a NaI(Tl)-based seawater monitoring system

Abstract

To monitor radioactivity levels in seawater Korea Institute of Nuclear Safety has installed and been operating 18 NaI(Tl)-based gamma detectors around the Korean peninsula. This study was conducted to estimate the detector efficiency and MDA of ¹³⁷Cs in seawater for measurement situations. For this purpose, experiments in the air and a water tank, and Monte Carlo simulations were performed using a seawater radioactivity monitor system with 3 in. × 3 in. NaI(Tl) scintillation detector. In the geometry reliability assessment using certified reference materials in a disc source, the validity of simulations was obtained by comparing measurement and Monte Carlo simulation results. The FWHM of the seawater radioactivity monitor were obtained from the results of the water tank measurement for applying a Gaussian Energy Broadening (GEB) option to Monte Carlo N-Particle (MCNP) radiation transport code. In addition, the detection efficiency of ⁴⁰K in the water tank was measured and compared with the Monte Carlo simulation results in order to estimate the MDA and the detection efficiency of the seawater radioactivity monitoring system. For the based condition of water tank, ⁴⁰K concentration in water tank was controlled to 10.13±0.18 Bq/L, similar to that of real marine. In laboratory water tank experiments, the detection efficiency of the radioactivity monitor for ⁴⁰K was measured at 0.184±0.005 cps/(Bq/L), the Monte Carlo simulations showed the similar result of 0.182±0.002 cps/(Bq/L), and the detection efficiency of ¹³⁷Cs was estimated to be 0.224±0.009 cps/(Bq/L) from the simulations. For 3h measurement in the water tank based condition, the MDA of ¹³⁷Cs was estimated to be 0.077±0.003 Bq/L. Future research will include detailed studies for detector sizes and seawater salinities.