RESUMO
Alpha particle counting is based on the response of an electronic counting system to an incident alpha particle. Alpha spectrometry is used in our employee surveillance bioassay program to measure the concentration of isotopes of Am, Pu and U contained on sample source preparations. Nuclides of Am, Pu and U are separated from the sample matrix by anion exchange and are electroplated on a stainless steel disc (Gautier and Gladney 1986). The plated source diameter is 12.7 mm. A tracer is added to the sample before anion exchange as a quality control procedure to provide a measure of chemical yield. Tracer alpha-particle emissions are recorded in a preassigned calibrated area of the energy spectrum and chemical recovery is calculated by the ratio of tracer counts per second divided by the tracer activity in becquerels (Bq). Percent tracer recovery may also be calculated by introducing the average counting efficiency factor in the denominator. Tracer yield is then used to provide a reliable estimate of the sample's analyte counts that are recorded in other preassigned energy dependent areas of the spectrum. The tracer spectrum in the presence or absence of other nuclides also provides evidence of the performance characteristics of the alpha spectrometer, for example, chamber vacuum and electronics. Electroplated samples are counted in any one of 96 detectors. The backgrounds of these detectors are maintained at less than 3 counts per 70,000 s over a 190 keV energy window to provide a limit of detection of less than 0.37 mBq per sample at the 95% confidence level. In this paper, resolution of the photopeak is shown to be a function of the source to detector distance and a function of degraded alpha energies due to Fe or other extraneous materials on the plated surface. Since our program is concerned only with low activity samples (less than 0.2 Bq) we encounter no significant problem with tailing when the resolution is less than 50 keV FWHM.