Preliminary Work Toward a Transuranic Activity Estimation Method for Rapid Discrimination of Anthropogenic from Transuranic Activity in Alpha Air Samples

Abstract

Radon (Rn) and thoron (Rn) progeny (primarily bismuth and polonium) are known interferents when rapid evaluation of transuranic content on air filters is of interest. These complexities stem from the overlapping energies of the progeny alpha particles onto the transuranic region of interest (3-5.5 MeV) where naturally-occurring alpha emitters can overwhelm the spectra. Due to the immediacy of the alpha counting methods employed, coupled with the half-life of thoron progeny dominated by Pb (t1/2=10.6 h), a conservative transuranic activity estimate with rigorous uncertainty is being sought. A successful transuranic activity estimation method will incorporate any thoron progeny present on the filter providing 95% confidence decision levels in which a filter may be evaluated for emergency response applications. Twenty-three pairs of samples of various duration having no transuranic content were taken over a 2-mo period. The resulting filters were counted in a time series before non-linear least squares decay curve fitting was applied to the decay profile. For the samples considered, a transuranic activity estimator decision level was determined at 0.2 Bq for the given geographic location and months analyzed. Validation of this method for other seasonal and geographic regions could provide enhanced emergency response capability when the presence of transuranic activity is suspected.