Comparative study on gamma-ray detectors for in-situ ocean radiation monitoring system

Large-sized crystals and state-of-the-art photosensors are desirable to cope with low environmental radioactivity (e.g., 1–2 Bq∙m−3137Cs in surface seawater) for homeland security purposes. We compared the performances of two different gamma-ray detector assemblies, GAGG crystal + silicon photomultiplier (SiPM) and NaI(Tl) crystal + photomultiplier tube, for our mobile in-situ ocean radiation monitoring system. We performed energy calibration, followed by water tank experiments with varying the depth of a137Cs point source. Experimental energy spectra were compared with MCNP-simulated spectra with identical setup and the consistency was validated. We finally assessed the detection efficiency and minimum detectable activity (MDA) of the detectors. Both GAGG and NaI detectors exhibited favorable energy resolutions (7.98 ± 0.13% and 7.01 ± 0.58% at 662 keV, respectively) and MDAs (33.1 ± 0.0645 and 13.5 ± 0.0327 Bq∙m−3 for 24-h 137Cs measurement, respectively). Matching the geometry of the GAGG crystal with that of the NaI crystal, the GAGG detector outperformed the NaI detector. The results demonstrated that the GAGG detector is potentially advantageous over the NaI detector in detection efficiency and compactness. •We performed a detector study that would be suitable for our remotely controllable in-situ ocean radiation monitoring system.•Two detector assemblies were compared: GAGG crystal + silicon multiplier (SiPM) and NaI crystal + photomultiplier tube.•24-hr minimum detectable activities (MDAs) for 137Cs measurement of GAGG and NaI detectors were 33.1 Bq∙m−3 and 13.5 Bq∙m−3, respectively.•With the identical crystal volume, the GAGG detector outperformed the NaI detector in detection efficiency as well as MDA.•Considering its high detection efficiency and compactness, GAGG + SiPM would be implementable for our application.