Increasing Quantitation Accuracy of Fluorescent Excitation Analysis (Fea) Systems by Pulse Rise-Time Discrimination

The quantitation accuracy of FEA systems is dependent on the characteristic x-ray to background ratio. This background appears as a continuum of energy lower than the primary Compton scatter peak (PCSP) from the sample being assayed. In a well designed FEA system most of this low energy background does not arise from multiple Compton scatter in the sample or collimators. Rather, it results from interaction between the PCSP and the Si(Li) detector system. Although guard-ring anticoincidence or narrow collimation help to reduce background, in a good detector edge field effects play a small role, and background appears to be a volume effect. Polarized FEA has been shown to improve quantitation by reducing the PCSP to characteristic x-ray ratio, but the basic detector problem remains. We describe a pulse risetime discrimination scheme whereby slow pulses arising in the detector are discarded. For FEA of iodine with Am-241, this addition to the system produces a halving of background with a minimal impact (1-3% loss) on count-rate of iodine K x-rays. Similar results are obtained for FEA of bromine with Cd-109.