High-Purity Germanium Spectroscopy at Rates in Excess of $10^{6}$ Events/s

In gamma spectroscopy, a compromise must be made between energy resolution and event-rate capability. Some foreseen nuclear material safeguards applications require a spectrometer with energy resolution typical of high purity germanium (HPGe) detectors, operated at event rates up to and exceeding 10⁶ per second. We report the performance of an HPGe spectrometer system adapted to run under such conditions. Our system consists of a commercial semi-coaxial HPGe detector, a modified high-voltage-rail, resistive-feedback, charge-sensitive preamplifier and a continuous waveform digitizer. Digitized waveforms are analyzed offline with a novel time-variant trapezoidal filter algorithm. Several time-invariant trapezoidal filters are run in parallel and the slowest one not rejected by instantaneous pileup conditions is used to measure each pulse height. We have attained full-width-at-half-maximum energy resolution approximately 8 keV measured at 662 keV with 1.03 ×10⁶ per second incoming event rate and 39% throughput. An additional constraint on the width of the fast trigger filter removes a significant amount of rising edge pileup that passes the first pileup cut, reducing throughput to 25%. While better resolution has been reported by other authors, our throughput is an order of magnitude higher than any other reported HPGe system operated at such an event rate.