冷凝氡源活度的2π立体角绝对测量方法研究

The natural radioactive gas, radon, is a major contributor to public radiation dose. In the field of radiation protection, accurate measurement of radon concentration is the basis of radon-related research and applications. Various radon measurement instruments and methods need to carry out metrology verification and traceability. Establishing the primary standard based on an absolute radon measurement method with high accuracy, will provide critical support in the field of measurement of radon protection...Considering the actual needs of establishment of radon concentration measurement metrology verification and traceability system, the program of studying the condensed radon source by a 2π solid angle α spectrum gridded ionization chamber is innovatively presented to develop the absolute measurement method of radon. The main contents in this program include: Building radon gas condensate model, the gaseous radon completely condensed into solid; Building model covering condensate compartment and cold finger, radon source and detector, simulating α particle behavior, studying the absorption and scattering correction α particles; the study on techniques of α spectrum analysis and the algorithms of realizing radon activity...This program is a basic study project through sophisticated modeling and experimental measurements, focusing on the absorption, scattering behavior and related correction method of α particle. The aim of this project is to establish the absolute measurement method of radon. Expected outcomes of the project include building the theory and technical basis for the development of radon primary standard and providing high-precision measurement standards support for radiation protection and related fields’ all radon measurement instruments or methods.

天然放射性气体氡是公众辐射剂量的主要贡献者，氡浓度的准确测量是辐射防护领域氡相关研究和应用的基础。各种测氡仪器与方法都需要进行计量检定与量值溯源。依据高准确度的绝对测量方法建立氡活度基准，将为氡防护领域提供关键计量保障。.本项目从建立我国氡浓度测量的计量检定与溯源体系的实际需要出发，创新性地提出以冷凝成固态的氡源为测量对象，用2π立体角的α能谱屏栅电离室探测器，开展氡活度绝对测量方法研究。主要内容包括：构建氡气冷凝模型，将气态氡完全冷凝成固态；对冷凝舱室及冷指、氡源以及探测器整体建模，模拟计算α粒子的行为，研究α粒子的吸收与散射修正；研究α能谱的解谱技术与氡活度算法等学术问题。.本项目主要通过建模与精密实验测量，研究α粒子被吸收与散射的行为，目标是建立氡活度绝对测量方法。本项目成果将为氡活度基准的建立奠定理论和技术基础，将为辐射防护及相关领域所有测氡仪器的测量准确提供高精度计量标准支持。

天然放射性气体氡是公众辐射剂量的主要来源，氡浓度的准确测量是辐射防护相关研究的基础。各种测氡仪器与方法都需要通过计量检定、校准或测试实现量值溯源，保障准确。本项目研究了以冷凝氡源为对象的高准确度活度绝对测量方法，是为建立氡计量基准而开展的基础性研究工作。项目通过构建气态氡冷凝模型，研究了气态的氡冷凝至镍冷指上、形成固态的机理与过程；通过对冷凝舱室、冷指与探测器整体建模，研究了密封真空舱室内α粒子的散射行为及对活度测量结果的影响；通过设计实验，研究了固态氡源的α粒子自吸收行为及其对测量结果的影响；通过研究冷凝氡源的α能谱，建立了氡活度的绝对测量方法。结合理论与实验研究，得到以下结论：（1）通过制冷，氡原子从气态近似完全转化为固态。如在约4L的舱室内，1Bq的222Rn在293K(20℃)下形成的气压为5.398E-16kPa，而其在40K时的蒸气压为1.0E-40kPa；当冷指的温度达到或低于40K时，即使仅有1Bq的氡气，氡原子也将被吸附冷凝到冷指上。（2）随着冷凝温度降低，冷指吸附杂质气体分子的能力也将增强。研究表明，对于一定体积和结构的系统，存在一最佳冷凝温度，该温度下，氡原子能被完全冷凝到冷指上而同时被冷凝的杂质气体分子最少，从而使所测α能谱的分辨率达到最优，测量准确度最高。在本项目所研究的系统上，对应的最优冷指温度为30 K~45 K。（3）屏栅电离室具有较好的分辨率，具有比PIPS探测器更优的灵敏度，容易实现1.0E+02 Bq~1.0E+03 Bq量级222Rn活度的高准确度测量。（4）采用磷屏成像方法可对冷凝氡源的尺寸进行精准测量。在本项目所研究的系统上，35K条件下氡源的形状近似为圆形，半径为3.138（98）mm，其对活度测量结果的标准不确定度为0.19%（5）α粒子的散射跟衬底的表面抛光程度、源物质厚度等因素密切相关，将在冷指边缘和舱室壁发生散射，散射α粒子能量在全能峰之外呈均匀分布，仅用5.489MeV分析222Rn活度时散射的测量结果的影响为0.02%，选用222Rn+218Po分析222Rn活度的不确定度为0.09%。以上研究为最高准确度等级氡活度基准的建立奠定了基础。