Optimization of pulse processing parameters for HPGe gamma-ray spectroscopy systems used in extreme count rate conditions and wide count rate ranges

Abstract

The need to perform gamma-ray spectroscopy measurements at high count rates with HPGe detectors is more common than many believe. Examples exist in safeguards, radiochemistry, nuclear medicine, and neutron activation analysis. In other applications wide dynamic ranges in count rate may be encountered, for example samples taken after a nuclear accident are counted on a system normally used for environmental monitoring. In a real situation, it may not be possible to reduce count-rates by increasing the distance or using collimators. The challenge is to obtain the “best” data possible in the given measurement situation. “Best” is a combination of statistical (number of counts) and spectral quality (peak width and position) considerations over a wide range of count rates. The development of multichannel analyzers (MCA) using digital signal processing (DSP) has made possible a much wider range of values for shaping times as well as the processing of the detector signal in various ways to improve performance with pulse-by-pulse adjustments. The pulse processing time is directly related to the shaping time. The throughput is related to the pulse processing time and the duration of the detector signal. Longer shaping times generally produce better peak resolution. However, the longer shaping times mean larger dead times and lower throughput. The ability to select the best compromise between throughput and resolution is possible with DSP MCAs. In addition, the dead-time-per-pulse can be reduced by changing the digital filter without significant impact on the full-width at half-maximum. To evaluate the improvements and to suggest an approach to optimization of system performance, a small and a large GEM (p-type) coaxial HPGe detector were selected for measurements to determine the performance at various input count rates and wide range of rise times and flattops in the DSPEC 50 MCA. Results will be presented for the throughput measured at dead times from 30 to 99.9 % with and without the use of the ORTEC Enhanced Throughput Mode.