A robust digital signal processor: Determining the true input rate

In addition to proper spectral deconvolution, accurate quantitative work in energy dispersive X-ray or gamma-ray spectrometry requires information about the number of particles striking the detector in a given measurement time. This requires knowledge of the analysis system dead time as well as events lost from the spectrum due to pileup and event discrimination. This is particularly true at high input rates where the correction factors can be significant. In any system where noise is present and is capable of triggering the detection electronics or algorithm it is not sufficient to simply “count” the triggering events and use assumptions about the distribution of arrival times to make pileup loss corrections in order to determine the incident rate of X or gamma rays on the detector. It is necessary to understand the nature of the rejected events and their spectral distribution. The Cambridge Scientific CSX series of digital signal processors provides a separate spectrum of rejected events in addition to the normal spectrum that allows a more accurate determination of the real event input rate, providing a greater level of quality assurance in the measurement. A method for determining the true input rate is presented and examples are given where the rejected spectra are dominated by noise, noise–event pileup, event–event pileup, single event rejection and unrelated but real event rejections.