ELECTRICAL MOBILITY AND SIZE DISTRIBUTION OF AGED PB-212 NANOMETER CARRIERS IN NITROGEN GAS

Radon and thoron progeny are positively charged clusters of radioactiv e atoms and other molecules and are neutralized through several mechan isms when they are aged. The charge status of radon clusters has been shown to be a major factor influencing their size distribution. Ln the present work, we attempted to determine simultaneously the activity s ize and charge distributions of thoron progeny using an electrical mob ility spectrometer and a graded diffusion battery. These measurements allow us to study the dynamics of the charge neutralization process. O ur data showed that Pb-212 generated in dry N-2, atmospheres (RH < 3.4 %) had a bimodal distribution. The small cluster mode with a size rang e of 0.5-2.5 nm was comprised of mainly neutral progeny, whereas a sig nificant portion of the large nucleation mode (2.5-10 nm) consisted of charged progeny (36-72% based on activity) depending on the aging tim e. The relative humidity (RH) had a major effect on the charge neutral ization process. As the RH increased from 3.4 to 17%, the proportion o f charged progeny decreased steadily indicating charge neutralization. At the same time, the fraction of the nucleation mode decreased. At R H greater than or equal to 13%, the progeny consisted of essentially n eutral molecular clusters with a single size mode about 1 nm. Therefor e, the mean particle size decreased in the neutralization process, con sistent with previous observations. As the concentration of the nuclea tion Fraction decreased, the charge fraction of these nanometer partic les increased from 72% to essentially 100%. These results show the imp ortance of charged progeny in the Formation and disappearance of nucle ation mode. (C) 1997 Elsevier Science Ltd. All rights reserved.