基于NaI(Tl)谱仪的海水放射性现场测量效率刻度、高分辨率解析和优化方法研究

The NaI(Tl) spectrometer for in situ radioactivity measurement in the marine environment must be accurate efficiency calibrated, spectrum decomposed and sensitivity improved in order to provide the fast and exact activity measurement of the radionuclides in the sea water. For many difficulties in the efficiency calibration experiments using the standard sources, the Monte Carlo method is used in this project to calculate the marine detection efficiencies based on the complete and exact descriptions of the spectrometer geometry, the marine in situ measurement and the absorption of gamma rays in the different media. The non-experimental efficiency calibration method is validated to be reasonable and reliable by the seawater experiments. Then in view of the high background in the low energy region mainly originating from the Compton effect of the high energy gamma rays of natural radionuclides in seawater and the poor energy resolution of NaI(Tl) spectrometer, a kind of high-resolution decomposition method is studied for the radionuclide identification and activity calculation using the improved Richardson-Lucy deconvolution to reconstruct the spectrum, in combination with the spectral response matrix which is calculated based on the non-experimental efficiency calibration method. At last, the Monte Carlo simulation model is further used based on the experimental design, in combination with the multi-objective genetic algorithm optimizing based on the response surface modeling, to study the practical optimization problem of the NaI(Tl) spectrometer for in situ radioactivity measurement in the marine environment. The nonlinear relationships between the detection efficiency, the minimum detectable activity and design parameters are established and analyzed. This project provides the important theoretical and methodological basis for solving the key problems of in situ radioactivity measurement in the marine environment, and so very significant for the marine radioactivity pollution early-warning and emergency response.

海水放射性现场监测NaI(Tl)谱仪必须进行准确的效率刻度、解析能谱并提高探测灵敏度，才能实现海水中核素活度的现场、快速和准确测量。本项目针对标准源实验刻度难题，拟采用蒙特卡罗方法对谱仪几何结构、海水测量环境、γ射线和海水各种原子的相互作用及在谱仪不同结构材料中的吸收进行完整的计算，建立准确、通用的非实验效率刻度方法并开展试验验证。然后，针对海水中天然放射性核素使现场测量γ能谱低能端表现大量的康普顿散射及NaI(Tl)谱仪能量分辨率不高的缺点，研究海水现场测量能谱响应矩阵和改进Richardson-Lucy反褶积重构谱线，提出一种核素甄别和活度计算的高能量分辨率解析方法。最后，将仿真实验设计和基于响应面的多目标遗传算法结合，研究探测性能与谱仪多设计参数的非线性映射并建立优化方法。研究为解决海水放射性现场监测的关键问题提供方法和计算依据，对海水放射性污染的及时预警和事故应急有重要意义。

福岛核电站爆炸引起的海洋放射性污染事故后果严重，基于NaI(Tl)谱仪的海水放射性现场测量研究和仪器研制成为热点。NaI(Tl)谱仪必须进行准确的效率刻度、解析能谱并提高探测灵敏度，才能实现海水中核素活度的现场、快速和准确测量。本项目针对标准源实验刻度难题，采用蒙特卡罗完整描述谱仪的几何和结构特征及海水的开放测量环境，详细计算γ光子在海水中传播和与海水中原子的相互作用，以及在谱仪各种几何材料和结构中的吸收。研究建立的通用、非实验效率刻度方法经海水现场测量试验能够为海水中核素活度的定量解析提供准确依据。然后，项目针对海水中天然放射性核素使现场测量γ能谱低能端表现大量的康普顿散射及NaI(Tl)谱仪能量分辨率不高的缺点，研究海水现场测量能谱响应矩阵和改进Richardson-Lucy反褶积重构谱线，提出了一种核素甄别和活度计算的高能量分辨率解析方法。最后，通过系统研究NaI(Tl)谱仪封装材料、厚度和晶体类型与大小等诸多设计参数对探测效率和探测限这两个关键性能指标的影响进行计算和评估，提出海水放射性现场监测NaI(Tl)谱仪现场探测性能的优化方法，用于指导现场监测仪器的性能优化和同类仪器研发。项目还对海上测量放置位置和积分时长等重要应用问题进行了现场实验研究。研究为解决海水放射性现场监测的关键问题提供了重要的方法和计算依据，对指导现场监测技术的研发具有重要的指导意义。