A large buoy-based radioactivity monitoring system for gamma-ray emitters in surface seawater.

A buoy (shallow water light type) -based in situ gamma-ray spectrometry system with a 7.6 cmØ × 7.6 cm NaI(Tl) detector for remote real-time monitoring of gamma-ray emitting radionuclides in surface seawater is presented. To convert measured count rates to radioactivity, the full energy peak efficiency of the detector for radionuclides in seawater was estimated using Monte Carlo simulation with the MCNP code. The efficiency calibration was validated by comparing the results with a sampling analysis of K in seawater at the sites where the monitoring systems were deployed. The minimum detectable activity of the system for Cs, Cs and I with gamma-ray measurement time is discussed.