DETERMINATION OF ATMOSPHERIC PB-210 BY ALPHA-SCINTILLATION COUNTING OF AIR FILTERS

Alpha-counting of filters used to sample large volumes of air provides a convenient way of determining atmospheric concentrations of Pb-210. Following decay of short-lived Rn-222 and Rn-220 progeny, alpha activ ity of the filters increases as Pb-210 decays to Po-210. After transie nt equilibrium is reached at about 3 y, alpha activity diminishes with the 22.3 y halflife of Pb-210. The degree of equilibrium between Pb-2 10 and Po-210 can be calculated subsequent to sampling, and the averag e concentration of Pb-210 in the air during the sampling period can be computed. Contributions to the total Pb-210 from ambient short-lived radon progeny are small typically 2-4%. Using high volume air samplers with collection rates of 1.1-1.7 m3/min for 24 h periods, and using c ounting times of 2 h for 20 cm2 filter sections, we measured alpha cou nts ranging from 0.0100 +/- 0.0050 to 0.200 +/- 0.0200 dps. Periodic m easurements on 100 of these, filters over a 4 y period yielded mean Pb -210 levels with standard deviations less than +/- 15%. The method req uires minimal sample preparation and can be used to determine past atm ospheric Pb-210 concentrations on filters stored for up to 20 y and mo re.