Observation of charge-sharing in an HPGe double-sided strip detector

Abstract

In double-sided strip high-purity germanium (HPGe) detectors, improved position resolution can be obtained through axial and lateral strip interpolation by means of pulse shape analysis. Yet, only a small fraction of events can be interpolated in both the axial and lateral dimensions, meaning that the best possible imaging performance is delivered at the cost of low imaging efficiency. Lateral position interpolation is complicated by the bipolar nature of induced bystander signals, charge-sharing between neighboring strips, and close interaction sequences. The first two complications were observed in our HPGe double-sided strip detector, and their significance is explored. An algorithm has been developed to calculate detector signals for clouds of drifting charge in three dimensions. Simulated bystander signals are in agreement with the family of waveforms produced in our detector. Based upon simulation, the nature of the bipolar signals and fundamental limits on position resolution are discussed. To determine the significance of charge-sharing, our detector was irradiated with high-energy gamma-ray sources, and then preamplifier signals were digitized and analyzed offline. Charge-sharing between adjacent strips was found to increase with gamma-ray energy, occurring for approximately 18% of all Ba-133 interactions (356 keV) and 30% of all Co-60 interactions (1173 and 1333 keV).