Research On In-situ Labr γ-ray Spectrometer For Seawater Radiation Monitoring

The current method of monitoring of the marineγradiation is field sampling andγenergy spectrum analysis in laboratory, with which it is not possible to carry out in real time on-line monitoring and early warning of marine radioactivity environment. The method of in-situγenergy spectrum analysis under water is widely used abroad to monitor theγradioactive nuclide. The current usedγenergy spectrometers under water are mostly based on sodium iodide detectors with the shortcoming of lack of energy resolution. In recent years, newly developed lanthanum bromide detector with excellent energy resolution is promising to replace the conventional sodium iodide detector on many occasions.First of all, the natural radioactive background forγspectra measurement in seawater and its Monte Carlo simulation method were studied. Sodium iodide, cadmium zinc telluride, HPGe and Lanthanum bromide (not considering the self-activity) detectors were compared through simulations and calculations when used for in-situγspectra measurement in seawater. The energy response functions, detection efficiency, effective detection distance and minimum detectable activity concentration (MDAC) were worked for these detectors. The calculated MDAC of 137Cs in seawater for sodium iodide detector accords with the data in literature, thus partially verifying the method of simulations and calculations.Because the self-activity of LaBr will deteriorate the detection performance, the simulation of the background erergy spectrum of LaBr due to its self-activity was studied. HPGe spectrometry system was utilized to measure the specific activity of 138La in LaBr crystal. The detection limit and MDAC of LaBr detector to radionuclides in seawater such as 137Cs, 60Co, etc were worked out when considering the self-activity issue. The self-activity background heightens the MDAC, but because of its good energy resolution, the MDAC of LaBr detector to 137Cs is merely several times of that of sodium iodide. Experiments with a LaBr detector verified the results of simulation of the background spectrum and calculation of MDAC of LaBr.At last, some preliminary studies and introductions of the applied technology of in-situγspectrometer in seawater were implemented, including the selection of the packaging material of the underwater detector and its influence, the key radionuclides and their bestγrays to be monitored in sea areas of nuclear power plants, and the method of deconvolution to increase the peak counts.