FACTORS THAT AFFECT ALPHA-PARTICLE DETECTION IN CONTINUOUS AIR MONITOR APPLICATIONS

An experimental study was conducted to characterize the full-width hal f-maximum values, peak shapes, and peak shifts of the energy spectra f rom alpha emitters in the forms of particulate matter on sampling filt ers and electro-deposited plated sources. Monodisperse 1.0-mum anhydro us uranium acetate aerosol particles were collected on seven types of sampling filters. Full-width half-maximum values at atmospheric pressu re varied from 373 keV for a 3-mum pore size Fluoropore filter to 584 keV for a glass fiber filter. Monodisperse uranium acetate aerosols fr om 1.2-8.1 mum were collected on Millipore 1.2-mum pore size membrane filters to examine the self-absorption effect. Under vacuum, the corre sponding full-width half-maximum values ranged from 241-1,011 keV. Suc cessively heavier mass loadings of monodisperse 1.8-mum uranium acetat e particles from 13.7-127 mug cm-2 caused the values to increase from 420 to 580 keV. With an electroplated 23.9-mm-diameter Pu-239 source a nd a 25.4-mm detector, the distance between source and detector was in crementally increased from 3.2 to 6.4 mm-a range of distances that is typical of those found in alpha continuous air monitors. At atmospheri c 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.