Determining the diffusion coefficient of 218Po is an important application of the two-filter method (Thomas and LeClare, 1970, Hlth Phys.18, 113). Air containing a known concentration of 222Rn is drawn through a cylindrical tube with filters at both ends. The 218Po activity on the downstream filter relative to the 222Rn concentration in air is interpreted to determine the diffusivity of 218Po. Previously, the interpretation scheme was based on a theoretical analysis of 218Po transport and production in a tube by Tan (1969, Int. J. Heat Mass Trans.12, 471), and Tan and Hsu (1970, Int. J. Heat Mass Trans.13, 1887). Their analysis did not account for radioactive decay of airborne 218Po within the apparatus nor for the perturbation of the fluid flow caused by the flow resistance of the downstream filter. Also, simplified analytical air flow profiles were assumed in that analysis. In this paper, the two-filter method is re-analyzed numerically, by solving the continuity equation, the Navier-Stokes equations and the species conservation equation for a tube. Full account is taken of the effects of radioactive decay of 218Po, and the flow resistance of the downstream filter. The results of the new analysis lead to a downward correction of reported diffusivities of 218Po by about 10–20%.
BT - Journal of Aerosol Science DA - 06/1994 DO - 10.1016/0021-8502(94)90010-8 IS - 4 LA - eng N2 -Determining the diffusion coefficient of 218Po is an important application of the two-filter method (Thomas and LeClare, 1970, Hlth Phys.18, 113). Air containing a known concentration of 222Rn is drawn through a cylindrical tube with filters at both ends. The 218Po activity on the downstream filter relative to the 222Rn concentration in air is interpreted to determine the diffusivity of 218Po. Previously, the interpretation scheme was based on a theoretical analysis of 218Po transport and production in a tube by Tan (1969, Int. J. Heat Mass Trans.12, 471), and Tan and Hsu (1970, Int. J. Heat Mass Trans.13, 1887). Their analysis did not account for radioactive decay of airborne 218Po within the apparatus nor for the perturbation of the fluid flow caused by the flow resistance of the downstream filter. Also, simplified analytical air flow profiles were assumed in that analysis. In this paper, the two-filter method is re-analyzed numerically, by solving the continuity equation, the Navier-Stokes equations and the species conservation equation for a tube. Full account is taken of the effects of radioactive decay of 218Po, and the flow resistance of the downstream filter. The results of the new analysis lead to a downward correction of reported diffusivities of 218Po by about 10–20%.
PY - 1994 SP - 689 EP - 697 ST - Journal of Aerosol Science T2 - Journal of Aerosol Science TI - On the Measurement of 218Po Diffusivity Using the Two-Filter Method VL - 25 SN - 00218502 ER -