Abstract
In order to predict the radon emanation fraction of materials, model calculations have been studied. Most previous studies attempted to express mathematically the behavior of radon atoms and so cannot deal with a complicated model of grains such as soil. In this study, we developed three models of soil and then calculated the radon emanation fraction of soil grains by Monte Carlo simulation. In the three models, soil grains are packed in the simple cubic, body-centered cubic and faced-centered cubic structures. Some assumptions were introduced to simplify the calculation: for example, all grains are just composed of quartz (SiO2) with the same diameter and the radon emanation results only from alpha recoil by the decay of radium atoms. The radon emanation fraction was calculated as a function of grain size, moisture content and radium distribution because these three factors are important in the radon emanation fraction. As a result, on the model of soil grains packed in the simple cubic structure, the radon emanation fraction greatly increased with increasing the grain size in the range of 10−100 μm and then was saturated when the moisture content was 0%. As the moisture content became higher, the radon emanation fraction increased and then was saturated at smaller grain size. Water in pore enhanced the radon emanation fraction because it plays an important role to stop radon atoms before embedded into adjacent grains. We are currently discussing the radon emanation fractions for the other models. On the other hand, the validity of the present model was evaluated in comparison with some experimental data. Since the model calculations were comparable with the experimental data, the present model would be more practical and available to estimate the radon emanation fraction of soil of the single grain structure.
