NANOMETER PARTICLE-SIZE AND CONCENTRATION FROM THORON RADIOLYSIS

The activity and number size distributions of Pb-212- ,, and Bi-211-bo rne nanometer particles have been measured by the diffusion battery, B imodal peaks associated with molecular clusters and coagulation partic les were found, The activity median thermodynamic diameter and the geo metric standard deviation were determined, It showed that both the act ivity median thermodynamic diameter and the geometric standard deviati on increased with decreasing total flow rate, This,vas attributed to t he longer mean residence time of thoron progeny in the sampling chambe r for smaller total flow rate, Consequently, thoron progeny ions had b etter chance to collide, combine, and grow to a larger size, It also i ndicated 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 small er neutralization rates and thus larger cluster sizes, A comparison be tween the activity and number size distributions showed that a large f raction of coagulation particles contained no thoron progeny especiall y for the case of large humidity, This might be due to the contributio n of nonradioactive particle formation from hydroxyl radicals produced in the water vapor radiolysis. (C) 1998 American Association for Aero sol Research.