USE OF A GRADED DIFFUSION BATTERY IN MEASURING THE ACTIVITY SIZE DISTRIBUTIONS OF THORON PROGENY

This paper describes the performance of a graded diffusion battery in measuring the activity size distribution of Pb-212. The diffusion battery consisted of five stainless steel screens with 24,35,50,200 and 635 mesh numbers and a backup filter. Calibration of the screens indicated that the collection of ultrafine particles on these screen follows the diffusion battery theory if the flow rate is kept at a level at which the flow Reynolds number (Re=d(w)U/nu, where d(w) is the wire diameter, U is the face velocity and nu is the kinematic viscosity of air) is less than 1. The activity size distribution of Pb-212 in an aging chamber was measured in ultra-pure air with dry (< 10% r.h.) and humid (40-50% r.h.) conditions. The particle size of Pb-212 (about 1 nm) did not change significantly in the humid air when the mean residence time was between 0.33 and 8.3 min. However, in thc dry air, the thoron progeny grew with aging time from about 1 to 3 nm. By adjusting the relative humidity and the aging time, Pb-212 of activity median diameters from 1 to 3 nm could be produced. This allowed us to study the effect of the particle size of 'unattached' thoron progeny on the deposition efficiency in the upper respiratory tract.