ON THE MEASUREMENT OF PO-218 DIFFUSIVITY USING THE 2-FILTER METHOD

Determining the diffusion coefficient of Po-218 is an important applic ation of the two-filter method (Thomas and LeClare, 1970, Hlth Phys. 1 8, 113). Air containing a known concentration of Rn-222 is drawn throu gh a cylindrical tube with filters at both ends. The Po-218 activity o n the downstream filter relative to the Rn-222 concentration in air is interpreted to determine the diffusivity of Po-218. Previously, the i nterpretation scheme was based on a theoretical analysis of Po-218 tra nsport 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). T heir analysis did not account for radioactive decay of airborne Po-218 within the apparatus nor for the perturbation of the fluid flow cause d by the flow resistance of the downstream filter. Also, simplified an alytical air flow profiles were assumed in that analysis. In this pape r, the two-filter method is re-analyzed numerically, by solving the co ntinuity equation, the Navier-Stokes equations and the species conserv ation equation for a tube. Full account is taken of the effects of rad ioactive decay of Po-218, and the flow resistance of the downstream fi lter. The results of the new analysis lead to a downward correction of reported diffusivities of Po-218 by about 10-20%.