Influence of pileup rejection on nuclear counting, viewed from the time-domain perspective

Time-interval density distributions of accepted events in a HPGe gamma-ray detection set-up are measured with a time-interval digitiser. In particular , the effect of pulse-pileup rejection is investigated. Experimental data a re obtained with two types of shaping amplifiers: a classical amplifier wit h semi-Gaussian pulse shaping and a gated-integrator amplifier. A theoretic al model is developed to predict typical time-interval density distribution s for stationary Poisson processes passing through a detector with count lo ss by pulse-pileup rejection. Good agreement is obtained between theoretica l, measured and simulated time-interval spectra. It is found that, when cou nting is affected by pileup rejection, the true incoming count rate cannot simply be determined by fitting an exponential to the time-interval distrib utions. From the Laplace transform of the interval-density distribution, ex pressions are derived for the expectation value and the variance of the cou nts. Good agreement is found with experimental counting statistics for diff erent system configurations, as well as with data from computer simulations . (C) 1999 Elsevier Science B.V. All rights reserved.