A diffusion sampler is designed based on Mercer's theory of diffusional deposition from a fluid flowing radially between two coaxial, parallel circular plates. Experimental measurement of the diffusion coefficient for Th-B atoms indicated that the distortion of the flow patterns at the entrance and at the vicinity of the exit does not have a significant effect. It is, therefore, useful for measuring the diffusion coefficients. The diffusion coefficient for the Th-B atoms measured in condensation nuclei free atmosphere comes out to be 0.055 cm2/sec. This instrument when used at a sampling rate of 1.41 l/min is found to collect 65% of the unattached decay products of thoron on the collection plates suitable for direct analysis. It is, therefore, useful for measuring the unattached fractions of radon and thoron decay products in mines and mills, a measurement of significance in assessing the hazards of radon and thoron. Such measurements done in thorium processing areas indicated an unattached fraction as defined by ICRP to be (0.027 ± 0.021)% compared to 0.05% assumed by ICRP in arriving at the MPCa for thoron. Data reported also include the measured concentrations of thoron (Tn) and the ratios, Tn:Th-B:Th-C.
©1975Health Physics Society