THE EFFECT OF TEMPERATURE ON THE DIFFUSION-COEFFICIENT OF PB-212

The temperature dependence of the diffusion coefficient of Pb-212 in nitrogen was investigated using a diffusion sampler at a constant relative humidity of 10% (P(H2O) = 2.25 mmHg at 21.5-degrees-C, 5.0 mmHg at 35-degrees-C, 8.0 mmHg at 45-degrees-C and 12.0 mmHg at 55-degrees-C), and a constant water vapour pressure of 15 mmHg (70% RH at 21.5-degrees-C), in the presence of trace SO2 concentrations. At constant relative humidity, both temperature and trace gas concentration had negligible effect on the diffusion coefficient. At constant water vapour pressure, the effect of temperature depends on the concentration of sulphur dioxide. At P(H2O) = 15 mmHg, the diffusion coefficient of Pb-212 at 4 ppm sulphur dioxide in nitrogen increased from 0.010 s-1 at 21.5-degrees-C to 0.070 cm2 s-1 at 55-degrees-C; at 1 ppm sulphur dioxide, the diffusion coefficient increased from 0.037 cm2 s-1 to 0.077 cm2 s-1. The apparent cluster size of Pb-212 was found to be a function of temperature, slightly increasing as the temperature increased at a constant relative humidity of 10% and decreasing at a constant water vapour pressure of 15 mmHg. The relationship between cluster size and temperature was derived from the kinetic theory.