Nuclear Technology / Volume 168 / Number 1 / October 2009 / Pages 35-40
Detectors / Special Issue on the 11th International Conference on Radiation Shielding and the 15th Topical Meeting of the Radiation Protection and Shielding Division (Part 1) / Radiation Protection
This paper describes an algorithm intended for use in the U.S. Navy's next-generation air particle detector designed for measuring 60Co air contamination. The algorithm measures both alpha and beta activity from an air filter utilizing passivated implanted planar silicon detectors for spectrometry of both particle types and is designed to compensate for radon progeny to discriminate this from the beta emissions of 60Co. This is done by correlating the specific alpha emissions with their beta emission parents, or their beta emission progeny, as appropriate. In addition, the algorithm is unique in that by using region of interest (ROI) windows, it is less sensitive to spectral smearing due to dust or humidity effects on the particle depositions or more specifically to variable energy loss of alpha particles to the detector from deposited material on the filter. A weakness of this approach is that thoron B (212Pb) does not have a detectable alpha parent and the next alpha progeny must decay through an isotope (212Bi) with a half-life of 60.6 min. This causes predictions of the 212Pb activity to lag in time to some extent. Mitigation of this effect is realized by using a first-order correction utilizing appropriate mathematical equations to account for the physics of this buildup and decay. This paper concludes by demonstrating that the beta assay value is a linear superposition of the alpha ROI values from the three dominant alpha peaks. Initial estimates on the coefficients of the alpha ROI values are derived with final values recommended to be determined from operational measurements.