Nanometer Particle Size and Concentration from Thoron Radiolysis

The activity and number size distributions of 212Pb_ and 212Bi_borne nanometer particles have been measured by the diffusion battery. Bimodal peaks associated with molecular clusters and coagUlation particles were found. The activity median thermodynamic diameter and the geometric standard deviation were determined. It showed that both the activity median thermodynamic diameter and the geometric standard deviation increased with decreasing total flow rate. This was attributed to the longer mean residence time of thoron progeny in the sampling chamber for smaller total flow rate. Consequently, thoron progeny ions had better chance to collide, combine, and grow to a larger size. It also indicated that at constant flow rates the number concentration measured by the ultrafine condensation particle counter was greater for lower humidity. A lower humidity suggested less electron scavengers or smaller neutralization rates and thus larger cluster sizes. A comparison between the activity and number size distributions showed that a large fraction of coagulation particles contained no thoron progeny especially for the case of large humidity. This might be due to the contribution of nonradioactive particle formation from hydroxyl radicals produced in the water vapor radiolysis