Intercomparison of activity size distribution of thoron progeny and a mixture of radon and thoron progeny

Abstract

The First International Intercomparison and Intercalibration Meeting for Thoron and its Progeny and Thoron/Radon Mixture was organized to compare techniques for determining concentrations and particle size distributions of 220 Rn and mixtures of 222 Rn and 220 Rn . The meeting was held at Elliot Lake, Canada, November 2–6, 1992. We have reported results of activity size distributions and measurements of unattached fractions of both thoron progeny and mixtures of thoron/radon. Four teams, including the US Bureau of Mines in cooperation with Canada Center for Mineral and Energy Technology, the US Environmental Measurement Laboratory (EML), the Lovelace Respiratory Research Institute, and the Swedish Radiation Protection Institute (SSI) participated in the intercomparison of activity size distributions. Each team had a different diffusion sampling system and counting protocol. Results for all four systems were in good agreement when thoron gas was used in the test chamber. However, in the case of thoron and radon gas mixtures, the agreement among the different sampling systems was not as good. One reason for the inconsistency of results was that the sensitivity of the systems was different. Calculation of size distribution for individual progeny ( 218 Po , 214 Pb , 214 Bi , 212 Pb , and 212 Bi ) requires sampling systems with improved sensitivity. Our results suggest those diffusion batteries with a minimum of four stages and more likely five stages are optimal. In addition, a system for continuously counting the total alpha particles, such as the EML system, or alpha spectrometry, such as the SSI system, is needed. Our results provide a relationship between the unattached fraction and condensation nucleus concentration over a wide range of particles from 300 to 16,000 cm −3. These findings are consistent with measurements obtained in indoor environments, mines, and underground caves, indicating that it is possible to establish a more universal relationship.