Mechanisms and Modeling Approaches of Radon Emanation for Natural Materials

Abstract

Before radon, a radioactive inert gas, migrates through soil and disperses in the atmosphere, it must rst escape from radium-bearing solid grains. Escape of radon atoms from grains into pore space is called as , which should be distinguished from characterized by the emanation of radon and subsequent migration through pore uid. A number of radon emanation measurements have been carried out from the viewpoint of health physics as well as geoscience, because emanation is the very rst process in context of radon exposure. Of radon isotopes, 222Rn and 220Rn are dominant in the environment. Rn-222 with a half-life of 3.8 days, formed by the alpha decay of 226Ra belonging to uranium series, has been of great concern in several decades from the aspect of radiation protection. Recently, interest has also been growing in 220Rn with a half-life of 56 s, as a decay product of 224Ra in thorium series. Uranium, thorium, and radium are ubiquitous elements in natural materials such as rock and soil, implying that radon is always produced in them. A fraction of radon atoms generated in such materials has a possibility to be emanated into interor intra-particle pores. Table 1 indicates the representative values of radon emanation fractions for mineral, rock, soil, mill tailing, and y-ash samples.1, 2) The radon emanation fraction is de ned as the number of radon atoms emanated per number of radon atoms generated; other terms such as emanation coef cient and emanating power have also been used alternatively. This fraction must be in the range 0–1. Among mineral, rock, and soil samples, mineral and soil generally tend to have the lowest and highest radon emanation fractions, respectively, which could be explained Jpn. J. Health Phys., 52 (4), 296~ 306 (2017) DOI: 10.5453/jhps.52.296