Experimental and Simulation Study of Charge Sharing in Pixelated CdZnTe Radiation Detectors Using the PIXIE ASIC.

In recent years CdZnTe has been identified as an attractive alternative semiconductor material for the fabrication of radiation detectors. However, the poor hole transport properties have forced researchers to turn towards pixelated detectors in order to utilise the small pixel effect and minimise the contribution of holes in the output signal. This project focuses on the study of charge sharing and how the performance of a pixelated detector can be improved with the addition of a steering electrode. Silvaco ATLAS, a simulation software package, is used in this work along with the PIXIE ASIC to study these effects theoretically and experimentally. Two different detectors are used our measurements, which include general spectroscopy and collimated X-ray beam experiments. The simulations showed that the use of a steering electrode changes the electric field and potential distributions, which should reduce any charge sharing or charge loss. It is important however, for the steering electrode to have the right bias based on the anode geometry and also not to be shorted to an anode pixel. If the latter occurs then there is significant charge loss through the steering electrode. The two detectors demonstrated good energy resolution for the largest of the four arrays included in the PIXIE anode pattern. It was also shown that the amount of charge sharing is increased with decreasing pixel size and pitch. The performance of pixelated detectors is affected by pixel gain matching and the cathode bias, which determines the charge cloud diffusion and charge collection efficiency. The collimated X-ray beam experiments showed that pixelated detectors suffer from charge loss in the inter-pixel gap and that the cathode bias plays a bigger role in the amount of charge sharing than the incident radiation energy. Finally, our results show that care needs to be taken when devices are bonded to the ASIC, because when the steering electrode is shorted to an anode pixel the performance of the detector drops significantly and the amount of charge loss in the inter-pixel gap can be as high as 80%