大气降雨俘获空气中短寿命氡子体实验研究

Key words: 
• precipitation /
• short-lived radon daughter /
• radon /
• washout

Abstract: 

为了研究降雨对空气中短寿命氡子体的清洗机制，设计了分别模拟大气降雨的水汽凝结过程和雨粒下降过程的两个实验装置，并进行了多次实验，实验水样采用低本底 能谱仪进行测量。实验结果表明：凝结水的短寿命放射性计数与本底相近，说明水汽凝结不能俘获短寿命氡子体。某一次自来水冲刷空气后获得了高出本底30.9% 的初始 计数，其半衰期为35.7 min，纯净水冲刷空气后获得了高出本底19.6%的初始计数，这些计数主要是由氡子体214Pb 和214Bi 引起的。可见，无论是自来水还是纯净水，它们冲刷空气后都能俘获短寿命氡子体，且半衰期和放射性核素种类均与雨水相似。这一实验结果充分说明：大气降雨的短寿命放射性不是在水汽凝结阶段而是在降落阶段对空气的冲刷获得的，是在降落过程中俘获了空气中的短寿命氡子体214Pb 和214Bi 而引起的。本次实验结果为降雨对空气中短寿命氡子体的冲刷清洗机制提供了最有力的证据。

To investigate the underpinning mechanism of rainfall-scavenging of the atmospheric short-lived radon daughters, two experiments simulating water vapor condensing procedure and raindrop falling procedure were conducted. The radioactivity of the tested water was measured by low-background   γ spectrometry. The obtained results showed that the γ-ray counts of condensed water was on the same level as the background,which proved that water vapor condensation cannot trap short-lived radon daughters. In contrast, the initial γ-ray count of washout water (35.7 min half-life) was increased by 30.9% after washing the atmosphere with tap water. Similarly, the initial  -ray count of washout water was increased by 19.6% after washing the atmosphere with puri ed water. These counts originated mainly from radon daughters 214Pb and 214Bi. Obviously, both tap water and puri ed water can trap short-lived radon daughters when washing the atmosphere. Furthermore, the half-life and the kinds of radioactive nuclides in the washout water are similar to that in the rainfall water.These experimental results are able to ascertain that short-lived radon daughters in precipitation come mainly from scavenging (washout) and not from water vapor condensation within the cloud. It is in the process of falling that raindrops trapped short-lived radon daughters 214Pb and 214Bi. The obtained results as demonstrated here provide a powerful evidence for understanding the washout mechanism of short-lived radon daughters from the
atmosphere by rainfall.