Energy-loss correction in charge sharing events for improved performance of pixellated compound semiconductors

Abstract

The sharing of charge between multiple pixels can significantly degrade the energy resolution of small pixelated compound semiconductor detectors. This paper describes an energy calibration and reconstruction technique to correct for absorbed energy that is split over two neighbouring pixels, defined as bipixel events. Results were obtained with a 1 mm thick CdTe detector with 250 µm pixel pitch and an inter-pixel spacing of 50 µm, using the STFC HEXITEC ASIC. The proportion of charge sharing events was found to be 54% for photons at 59.5 keV when applying a noise threshold of 3 keV. Across the energy range investigated, bipixels were the predominant shared event type and the absolute fraction of shared events was found to be dependent on the noise threshold used. The reconstruction technique described reduces the degradation of energy resolution due to charge sharing in bipixel events compared to simple charge sharing summing techniques. This improved counting efficiency compared to using only isolated events and improved energy resolution compared to pixel addition techniques. When only isolated pixels were included, a FWHM energy resolution of 1.42 keV at 140.5 keV was achievable; inclusion of bipixel events using pixel addition results in an energy resolution of 3.33 keV whereas the reconstruction technique described here results in an energy resolution of 2.14 keV. When bipixel events are combined with single pixel events, the number of counts within the 140.5 keV photopeak of 99mTc increased by over 100%.