²²²Rn是一种自然界普遍存在的放射性惰性气体, 可以从其产生的土壤介质中迁移析出并进入大气。分析土壤中氡析出的物理过程, 进行氡析出量的测量和评价, 对于铀尾矿、伴生放射性矿开发利用是十分重要的, 且与天然放射性物质(NORM)现场监管等辐射环境安全密切相关。该文阐述了氡析出过程的机理, 介绍了氡析出过程的数学表达, 分析了此过程中的影响因素, 并对土壤表面氡析出率测量方法进行了总结。最后从辐射防护监管角度对氡析出率这一物理量的评价做了尝试性探讨。

Significance: Radon(²²²Rn) is a naturally occurring radioactive noble gas. ²²²Rn is produced from the alpha decay of radium (²²⁶Ra), with a relatively long half-life (i.e., 3.8 days). Thus, ²²²Rn can diffuse and migrate from the rock and soil where it is generated and can enter the air. According to surveys conducted by the World Health Organization, exposure to ²²²Rn and its short-lived progeny is the second leading cause of lung cancer. The measurement and evaluation of ²²²Rn release involves not only occupational exposure but also public exposure. Therefore, the measurement of ²²²Rn exhalation and the dose assessments of ²²²Rn exposure have always been important concerns in radiation protection. Progress: The entire process of ²²²Rn moving from the soil to the atmosphere can be divided into three steps: emanation, migration, and exhalation. During the emanation process of ²²²Rn, ²²²Rn will obtain a recoil energy of 8.6 × 10⁴ eV from the decay of ²²⁶Ra, which will make ²²²Rn atoms travel through the soil grains at a distance of no more than 50 nm. If produced near the surface of soil grains, ²²²Rn will leave the grain and stop in the interstitial space (pore), becoming freely movable ²²²Rn. The fraction of freely movable ²²²Rn is usually expressed by the emanation coefficient (dimensionless). The emanation coefficient of soil generally ranges from 0.1 to 0.3. In soil with a relatively stable internal environment, the migration of free ²²²Rn in soil mainly relies on molecular diffusion caused by concentration gradients, eventually entering the atmosphere through the soil-air interface. If only diffusion transport is considered, then Fick's law can be used to describe the migration process of ²²²Rn and establish a model for ²²²Rn flux at the soil-air interface. The ²²²Rn exhalation rate at the soil surface is the ²²²Rn flux. On-site measurement methods of the soil ²²²Rn exhalation rate can generally be divided into three categories: accumulation, flow-through, and activated carbon adsorption methods. In actual measurements, different methods can be chosen according to the needs. Conclusions and Prospects: The understanding of the ²²²Rn diffusion exhalation mechanism and influencing factors is becoming comprehensive, and the measurement methods of the ²²²Rn exhalation rate for different purposes have been developed. Analyzing the physical processes of ²²²Rn exhalation from soil and measuring and evaluating the exhalation rate of ²²²Rn are important for assessing environmental radiation and managing uranium tailings and associated radioactive minerals. Moreover, the exhalation rate of ²²²Rn is closely related to the radiation environmental safety of on-site supervision of naturally occurring radioactive materials. Because of the complexity and diversity of ²²²Rn measurement sites, even though the amount of ²²²Rn release can be measured and calculated relatively accurately, nearly no quantitative deterministic correlation is detected between it and the indoor ²²²Rn concentration. Thus, the ²²²Rn exposure dose for key populations is difficult to estimate. The ²²²Rn exposure dose and health risks for key populations can only be estimated and controlled through measurements of the indoor ²²²Rn levels. Therefore, although the ²²²Rn exhalation rate is an important parameter that can be measured and calculated on-site, establishing a fixed exhalation rate limit for regulatory purposes is unsuitable.