海水原位γ能谱快速检测方法初步研究

发布时间：2018-10-19 10:45

【摘要】：为贯彻落实海洋强国战略部署,提升我国海洋环境安全保障能力,围绕“沿海核电安全”的科学发展目标,针对沿海核电站邻近海域可能发生的放射性泄漏事件,建立核电站在正常和事故情况下液态排放物的监测方法和自动监测系统,以及开展应急响应、快速跟踪监测的研究。而当前我国的海洋γ放射性监测方式是现场采样和实验室γ能谱分析的方式,无法对海洋放射性环境进行实时在线的监测和预警。水下原位γ能谱分析的方法已经在国外被广泛用来监测海洋中的γ放射性核素。本论文基于国家重点研发计划重点专项子课题“主要放射性核素现场快速检测设备研究”展开。目前使用的水下原位γ能谱仪基本是基于碘化钠探测器的,具有能量分辨率不足的缺陷。近年来,新型溴化镧探测器以其优异的能量分辨率而具有在很多场合代替传统的碘化钠探测器的可能,但溴化镧自身含有放射性。高纯锗也是近年发展起来的测量能量范围宽,分辨率高的探测器,但在使用时需要液氮制冷或电制冷,并且针对高纯锗探测器前端精密电子学和后续数字化处理等相关技术方法的研究,国内的研究水平还远远落后于国外先进技术。所以文章选用溴化镧探测器作为主探测器。由于溴化镧探测器自身放射性的问题,本课题通过符合测量来降低溴化镧自身放射性影响。并通过研制前端快电子学电路和后续脉冲成形、基线恢复、重叠脉冲分离和谱线处理等数字处理方法,最终实现基于溴化镧探测谱仪的数字化多道能谱处理系统。海水中放射性检测,除了自身单元仪器结构和性能指标的影响,系统还会受到的工作环境本底、康普顿散射和宇宙射线的干扰,影响测量的精确度。在海水原位γ能谱测量的过程中,会受到较强的康普顿闪射影响,即使是单能的γ射线,测量到的能谱也会变成一个很分散很宽的能谱。针对该情况本课题拟采用数字多道脉冲幅度反符合处理方法,抑制康普顿坪台,降低系统积分本底,提升系统峰康比,进而提高系统的综合技术指标。本文采用的反符合模型是外面用Ф130×400mm的井形塑料闪烁体探测谱仪作为符合探测谱仪,主探测器采用Ф75×75mm溴化镧。主要工作原理是塑料闪烁体上的脉冲信号先进行符合处理再与溴化镧探测器中的信号进行反符合处理,由于塑料闪烁体只做γ计数而不能做能谱分析,所以只有当溴化镧探测器采集到信号而塑料闪烁体符合后无脉冲信号输出时,FPGA中才会对主探测器中的信号进行记录,并进行后续的脉冲成谱。通过实验分析可知,测量时间、探测距离以及核素γ射线发射概率会直接影响探测器测量结果。通过实验发现,将双符合谱仪放置在合适的测量位置可减小本底计数,从而降低最小可探测活度;γ射线能量、探测器晶体类型及体积会影响探测效率和本底计数,从而对最小可探测活度产生影响。
[Abstract]:In order to carry out the strategic deployment of a powerful marine power country and enhance the safety and security capability of our country's marine environment, focusing on the scientific development goal of "coastal nuclear power safety", the possible radioactive leakage incident in the coastal nuclear power plant adjacent sea area will be addressed. The monitoring method and automatic monitoring system of liquid emissions from nuclear power plants under normal and accident conditions are established, as well as the research of emergency response and rapid tracking and monitoring. At present, the methods of marine 纬 -radioactivity monitoring in China are field sampling and laboratory gamma-ray spectrum analysis, so it is impossible to carry out real-time on-line monitoring and early warning of marine radioactive environment. Underwater in situ 纬-ray spectroscopy has been widely used to monitor 纬-radionuclides in the ocean. This paper is based on the research of the main radionuclide field rapid detection equipment, which is one of the key subtopics of the National key R & D Program. At present, the underwater in situ gamma spectrometer is based on sodium iodide detector, which has the defect of insufficient energy resolution. In recent years, novel lanthanum bromide detectors have the potential to replace traditional sodium iodide detectors in many occasions with their excellent energy resolution, but lanthanum bromide itself contains radioactivity. High purity germanium is also developed in recent years to measure a wide range of energy, high resolution detector, but in use needs liquid nitrogen refrigeration or electrical refrigeration, According to the research of precision electronics and subsequent digital processing of high purity germanium detector, the domestic research level is far behind the foreign advanced technology. Therefore, the lanthanum bromide detector is chosen as the main detector in this paper. Due to the radioactivity of lanthanum bromide detector, the effect of lanthanum bromide self-radioactivity is reduced by coincidence measurement. The digital multichannel energy spectrum processing system based on lanthanum bromide detector is realized by developing digital processing methods such as front-end fast electronic circuit and subsequent pulse shaping, baseline recovery, overlapping pulse separation and spectral line processing. In addition to the influence of the structure and performance index of its own unit instrument, the system will also be affected by the background of the working environment, Compton scattering and cosmic ray interference, which will affect the accuracy of the measurement. In the process of in-situ measurement of 纬 -ray spectrum in seawater, it will be affected by Compton flicker. Even if the single energy 纬 -ray is measured, the measured energy spectrum will become a very dispersive and very wide energy spectrum. In view of this situation, this subject intends to adopt the digital multi-channel pulse amplitude anti-coincidence processing method to restrain the Compton plateau station, reduce the system integral background, raise the system peak to health ratio, and then improve the comprehensive technical index of the system. The anti-coincidence model used in this paper is a well shaped plastic scintillator detector of 130 脳 400mm. The main detector is 75 脳 75mm lanthanum bromide. The main working principle is that the pulse signal on the plastic scintillator is treated with coincidence first and then with the signal in the lanthanum bromide detector. Because the plastic scintillator only counts 纬 and can't do the energy spectrum analysis, So only when the lanthanum bromide detector acquires the signal and the plastic scintillator accords with the plastic scintillator can the signal in the main detector be recorded and the subsequent pulse spectrum is obtained in the FPGA. The experimental results show that the measurement time, detection distance and 纬 -ray emission probability of nuclide will directly affect the measurement results of the detector. It is found by experiments that the background count can be reduced by placing the double coincidence spectrometer at a suitable measuring position, thus the minimum detectable activity can be reduced, and the 纬 ray energy, the type and volume of the detector crystal will affect the detection efficiency and the background count. Thus the minimum detectable activity is affected.
【学位授予单位】：成都理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：X837

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