Autonomous NaI(Tl) gamma-ray spectrometer for in situ underwater measurements

An automated low-background NaI(Tl) gamma-ray spectrometer for in situ underwater measurements consisting of a submersible unit with a radiation detector and of a surface microcomputer unit is developed and described. Along with an ultra low-background NaI(Tl) radiation sensor, the submersible unit contains a specially designed FPGA-based electronics with low energy consumption and with abilities both for remote data acquisition by a 1200 m conducting cable and for an autonomous operation mode. The unit is enclosed in a waterproof polyacetal housing withstanding a pressure of 4 MPa at a depth of 400 meters. The surface microcomputer unit contains an all-in-box software for acquisition, storage, and processing of data from the detector unit, for real-time monitoring of activities of dissolved radionuclides, for their identification and quantification. The software functions are accessed remotely by a web-interface using Wi-Fi or Ethernet. Several techniques for gain control (including an auto-adjustment of the gain by setting a selected photopeak in a prescribed channel) together with the corresponding approaches to the energy-scale calibration are implemented and examined over a wide temperature range −10÷+50°C in a thermo-chamber calibration experiment with 152Eu point gamma-ray sources. Calibrating the energy resolution of the detector is performed from line-width measurements of reference gamma-ray sources. The detection-efficiency calibration is carried out using Monte Carlo simulations based on the GEANT4 code taking into account the materials and geometry of the detector as well as the water environment. In addition, the efficiency of the detector immersed in a 8m3 water tank with dissolved gamma-ray sources of 139Ce, 137Cs and 40K of known activities is determined experimentally and compared with the theoretical findings. Finally, a trial deployment of the gamma-spectrometer in the Yellow Sea together with test measurements of the activity concentrations of dissolved radionuclides is fulfilled. The Minimum Detectable Activity concentrations (MDA) for 137Cs in fresh and sea water are calculated from the water tank experiment and from the test deployment experiment correspondingly.