A shipboard real-time gamma-ray measurement system for detecting radionuclides in seawater

A shipboard gamma-ray measurement system equipped with a 7.6 cmØ $\times$ 7.6 cm NaI(Tl) detector for remote real-time radioactivity monitoring of gamma-ray emitting radionuclides in surface seawater is presented. The full energy peak detection efficiency of the detector to convert measured gamma-ray counts to radioactivity concentration was estimated using Monte Carlo simulation with the MCNP code. The detection efficiency calibration was validated by sampling and laboratory analysis for ⁴⁰K in seawater from the site where the ship with the measurement system stayed. The minimum detectable activity of this measurement system for ¹³¹I, ¹³⁴Cs and ¹³⁷Cs in seawater with the varied spectrum accumulation time is discussed.