Dynamics of Rn-222 daughter size distribution evolution: modelling and experimental aspects

Size distribution of short lived radon daughters is considered as a fundamental parameter in radiation protection in so far as about 40% of human exposure is due to inhalation of such radioactive particles, in mines as well as in indoor environments. Many experiments have been carried out in various conditions: controlled atmospheres, uranium mine atmospheres, atmospheric air, and more recently in houses because of the increasing interest in exposure to natural radiation. From an experimental point of view, previous measurements dealt with the cumulative size distribution of radon progeny alpha activity. Then, in spite of the difficulty in comparing data of different investigators, it was often found that a significant part of the activity was associated with particles above 0.1 μm in diameter. Otherwise, a bimodality was often observed in the case of airborne radioactive particles. The attachment of small radioactive ions or atoms produced by the radon decay on ambient particles has been studied. Differences arise in the calculation of the attachment coefficient and in the choice of the natural particle size distribution. This paper presents our investigation in this field by studying the theoretical and experimental aspects of the radon daughter behaviour in a cylindrical vessel simultaneously. (author)