Primary Study On Rapid Detection Of Seawater In Situ ?-ray Spectrum

For the implementation of strategy deployment of a marine power,to lift the capacity for security insurance of territorial seas of China,with the aim of scientific development of coastal safety nuclear power,concerning the probability of radioactive spill of coastal nuclear power plant adjacent waters,it is necessary to take the construction of monitoring method and automatic monitoring system into action for liquid waste disposal system while the nuclear power plant were both at regular operation on in accidental case,and to carry out the research of emergency response,rapid tracking and monitoring.While up to now,the method of seawater gamma radiation monitoring can be divided into two ways: field sampling and laboratory gamma ray spectrum analysis in China.It cannot carry out real-time monitoring and give early warning of radioactive in marine environment.In situ gamma ray spectrum analysis has been widely used in foreign countries to monitor the gamma radiation in the ocean.This thesis is based on the national key R&D plan of key special sub subjects of study on field rapid detection equipment of main radionuclides.NaI(Tl)spectrometer is extensively using in in-situ gamma ray spectrum measurement,but,it has the disadvantage of its energy resolution.In recent years,a new type of LaBr3 detector which owns excellent energy resolution has a chance to replace the conventional NaI(Tl)detector on many occasions.Lanthanum bromide,however,contains a certain degree of radioactivity,which will have an effect in measurement results.HPGe detector has wide measurement range,high energy resolution,which has been used for in situ seawater ? radioactivity measurement.But,it requires liquid nitrogen refrigeration or electric refrigeration.In addition,domestic research level on the precision electronics and subsequent digital processing of high purity germanium detector is far behind foreign advanced technology.So the LaBr3 detector is used as the main detector of this article.Due to the radioactivity of the LaBr3 detector,we can reduce the radiation of LaBr3 by coincidence measurement.And add the digital processing methods such as front-end fast electronics circuit and subsequent pulse forming,baseline recovery,overlapping pulse separation and spectral line processing,a digital multi-channel spectrum processing system based on LaBr3 detector is realized.The seawater in situ ? radioactivity detection,not only of its own unit instrument structure and performance index will affect the measurement accuracy,the system energy resolution will also be affected by the background of the working environment,Compton scattering and cosmic ray.During the process of in situ gamma ray spectrum measurement,the results will be under the strong influence of Compton scatter,so even one single energy gamma ray that penetrate into seawater,the energy spectrum will gain a large width peak as the measurement result.Therefore,when it comes in situ gamma ray spectrum measurement,even one single energy gamma ray that penetrate into seawater,the energy spectrum will gain a large width peak as the measurement result.To improve the measurement system,digital multi-channel pulse amplitude anti coincidence processing method has been used in this paper.As a result,this method will suppress of Compton platform,reducing system integration background and will improve the comprehensive technical indicators of the system.This article uses ?130×400mm plastic scintillator detector as coincidence detector,and uses ?75×75mm LaBr3 detector as the main detector.The main working principle of the double coincidence system is the pulse signal on the plastic scintillator coincidence first and then anti coincidence lanthanum bromide detector signal.Because the plastic scintillation detector only counts,only when the lanthanum bromide detector collects the signal and the plastic do not collect signal,FPGA will record the signal in the main detector.The experiment results show that the measurement time,the detection distance and radionuclide gamma ray emission probability will affect the measuring result of the detector.The minimum detectable activity can be reduced by setting double coincidence spectrometer at an appropriate depth which can decrease the background counting.The energy of gamma ray,the type and volume of the detector crystal will affect the detection efficiency and the background count,and thus affect the minimum detectable activity.