Determination of F-Value In Uranium Mine Air

Parts of 218Po, one of the daughter products of 222Rn, suspended in air free from condensation nuclei are so active, owing to their high diffusion velocity, that they a re likely to become attached to the respiratory system when they are inhaled. The maximum permissible concentration for exposure to radon and daughter products in air is determined by the so-called f-value , which is a ratio of the amount of 218Po unattached to condensation nuclei to the amount of 218Po in the secular equilibrium with 222Rn in air. A practicable measurement of the f-value has been designed on the basis that the mean diffusion coefficient of such particles is about 0.05 cm2/s and the size distribution of the particles in uranium mine air has a broad line spectrum at a corresponding size. The latter fact has been proved indirectly by the ion mobility distribution of particles in mine air. When a sample of air containing radon daughter products is passed for 3 min through a diffusion tube 43 cm long and 1.7 cm inner diam., a part of the particle group having a high diffusion coefficient is deposited on the wall of the tube. The attaching rate F of the particles equals 0.26 for the sampling rate Q = 1.73 x 102 cm3/s . When the sampling was over an inner electrode was inserted Into the diffusion tube to count the alpha activity due to the daughter products on its wall, in the proportional region under the flow of counting gas. The f-value can be calculated by the following equation: f = (alpha activity due to the attached 218Po at the end of sampling counts/s)/(radon concentration μCi/cm3 x F x Q x 2.174 x ηα x 3.7 x 104) where ηα is the counting efficiency for alpha particles (ηα = 0.375). The f-values determined by this method in uranium mines are 6-13% at working places and 9-25% at some dead-end drifts left for a long period without working. (author)