Factors that affect alpha particle detection in continuous air monitor applications

An experimental study was conducted to characterize the full-width half-maximum values, peak shapes, and peak shifts of the energy spectra from alpha emitters in the forms of particulate matter on sampling filters and electro-deposited plated sources. Monodisperse 1.0-microns anhydrous uranium acetate aerosol particles were collected on seven types of sampling filters. Full-width half-maximum values at atmospheric pressure varied from 373 keV for a 3-microns pore size Fluoropore filter to 584 keV for a glass fiber filter. Monodisperse uranium acetate aerosols from 1.2-8.1 microns were collected on Millipore 1.2-microns pore size membrane filters to examine the self-absorption effect. Under vacuum, the corresponding full-width half-maximum values ranged from 241-1,011 keV. Successively heavier mass loadings of monodisperse 1.8-microns uranium acetate particles from 13.7-127 micrograms cm-2 caused the values to increase from 420 to 580 keV. With an electroplated 23.9-mm-diameter 239Pu source and a 25.4-mm detector, the distance between source and detector was incrementally increased from 3.2 to 6.4 mm--a range of distances that is typical of those found in alpha continuous air monitors. At atmospheric pressure, the values increased from 280 to 330 keV and the detector efficiency decreased from 30.5% to 20.9%. Tests with various sizes of sources and detectors suggest that a continuous air monitor should be designed so that the two are of approximately equal size.