Electrical mobility and size distribution of aged ²¹²Pb nanometer carriers in nitrogen gas

Radon and thoron progeny are positively charged clusters of radioactive atoms and other molecules and are neutralized through several mechanisms when they are aged. The charge status of radon clusters has been shown to be a major factor influencing their size distribution. In the present work, we attempted to determine simultaneously the activity size and charge distributions of thoron progeny using an electrical mobility spectrometer and a graded diffusion battery. These measurements allow us to study the dynamics of the charge neutralization process. Our data showed that ²¹²Pb generated in dry N₂ atmospheres (RH < 3.4%) had a bimodal distribution. The small cluster mode with a size range of 0.5-2.5 nm was comprised of mainly neutral progeny, whereas a significant portion of the large nucleation mode (2.5-10 nM) consisted of charged progeny (36-72% based on activity) depending on the aging time. The relative humidity (RH) had a major effect on the charge neutralization process. As the RH increased from 3.4 to 17%, the proportion of charged progeny decreased steadily indicating charge neutralization. At the same time, the fraction of the nucleation mode decreased. At RH ≤ 13%, the progeny consisted of essentially neutral molecular clusters with a single size mode about 1 nm. Therefore, the mean particle size decreased in the neutralization process, consistent with previous observations. As the concentration of the nucleation fraction decreased, the charge fraction of these nanometer particles increased from 72% to essentially 100%. These results show the importance of charged progeny in the formation and disappearance of nucleation mode.