| The Fukushima nuclear accident in Japan results in the flow of plenty of liquid radioactive material into nearby ocean, which has a bad impact on the marine environmental security and biodiversity. And it also sparks serious social panic. According to "The Strategic Action Plan of Energy Development (2014-2020)", China will focus on the construction of coastal nuclear power station, which also brings new challenges to the monitor of marine radioactivity. In our country, the monitor of ocean radiation takes the measure of collecting samples in the target areas, and then bringing the samples to laboratory in order to take the y spectrometry analysis, while this method cannot realize the goal of real-time monitoring and early warning. The in situ measurement means the detector directly detects the targets so as to get the radioactive content information. This method makes it possible to realize the real-time and continuous monitoring and determine whether there is any radioactive pollution.In order to fully understand the characteristics of marine radioactivity, this essay uses the self-developed seawater in situ y-ray energy spectrum instrument to conduct a series of experiments, including the seawater background, point source and volume source experiment and make the Monte Carlo simulation. The research was completed with the help of Institute of Oceanographic Instrumentation, Shandong Academy of Science. The experiment has four parts:seawater background in water tank, volume source in water tank, seawater in situ background and seawater in situ point source experiment. And the MCNP5 program which is developed by the American Los Alamos laboratory was applied in the Monte Carlo simulation.By comparing experiment results with simulated y energy spectrum of seawater background in water tank and 131I volume source, it can be learned that the main characteristics of energy spectrum, including the shape and peak position of full-energy peak, scattered peak and Compton platform are consistent with each other. It can be learned from the experiment of 131I volume source that the count rate of y energy spectrum and the area of full-energy peak have linear relation with volume source specific activity. The 131I point source experiment indicates that the total count rate and peak area of y energy spectrum decay on the basis of exponential laws when the distance between source and detector is less than 75cm.And the results of experiment and Monte Carlo simulation are consistent with theoretical analysis.The detective efficiency and minimum detectable activity of 131I can be calculated by the use of background values and 131I volume source energy spectrum. The experimental value of detective efficiency is 2.15×10-4m3 and simulated value is 2.20×10-4m3; the experimental value of minimum detective activity is 0.287 Bq/L and simulated value is 0.297 Bq/L. The relative deviation between experimental value and simulated value is less than 5%. The self-developed instrument meets the international standard in the aspects of minimum detectable activity of continuous measuring device. The Monte Carlo simulation of in situ y-ray spectrum is feasible. It can not only complement the experimental results, but also provide reliable parameters for the research of in situ γ-ray spectrum instrument.In situ y-ray spectrum measurement belongs to the low-level radioactive measurement. And the content of man-made radionuclide in the seawater is very low. If the specific activity of man-made radionuclide in the seawater is less than the minimum detectable activity of detector, the instrument will not make accurate measurement and early warning. Thus this paper carries on the study of influence factor of minimum detectable activity by the use of in situ y-ray spectrum instrument and Monte Carlo simulation.According to the theoretical analysis, background count, measurement time, detective efficiency and y-ray emission efficiency can directly influence minimum detectable activity. The experimental method and Monte Carlo simulation indicate that background count can be reduced through placing the y spectrum instrument in the proper position and thus reduce the minimum detectable activity. And the minimum detectable activity can also be reduced by extend the measurement time if the marine background is stable. Besides, the energy of γ-ray, the type and volume of detect crystal, the type and thickness of packaging materials can have an impact on detective efficiency and background count, which will eventually influence the minimum detectable activity. |
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