Improved performance in germanium detector gamma-spectrometers based on digital signal processing

Abstract

In an HPGe spectroscopy system, Digital Signal Processing (DSP) replaces the shaping amplifier, correction circuits, and ADC with a single digital system that processes the sampled waveform from the preamplifier with a variety of mathematical algorithms. DSP techniques have been used in the field of HPGe detector gamma-ray spectrometry for some time for improved stability and performance over their analog counterparts. Recent developments in HPGe detector construction and new liquid nitrogen-free cooling methods have resulted in HPGe detectors which are better adapted to the needs of the application. Some of these improvements in utility have degraded the spectroscopy performance. With DSP, it is possible to reduce the changes, in real time, in several aspects of detector performance on a pulse-by-pulse basis, which is not possible in the old analog environment. In the past, in designing for the analog regime, flexibility was limited by issues of component size, number and cost. In the digital domain, the problem translates to the need for a DSP with enough speed and an efficient algorithm to achieve the desired transformation or correction to the digitally determined pulse shape or height, event-by-event. The use of DSP allows the peak processing to be tuned to the preamplifier peak shape from the detector rather than being set to an average value determined from several detectors of the type in question. The selection of the filter can be automatic or manual. The following corrections are now possible: ballistic deficit correction, peak resolution improvement by reducing the impact of microphonic noise, increase throughput by reducing pulse processing time, and loss-free (zero dead time) counting.