Simulation of single-event energy-deposition spreading in a hybrid pixellated detector for gamma imaging

摘要

In the framework of the Medipix2 Collaboration, a new photon-counting chip is being developed made of a 256 x 256 array of 55 mum-side square pixels. Although the chip was primarily developed for semiconductor X-ray imagers, we think that this type of device could be used in applications such as decommissioning of nuclear facilities where typical sources have gamma-ray energies in the range of a few hundred keV. In order to enhance the detection efficiency in this energy range, we envisage connecting the Medipix2 chip to a CdTe or CdZnTe substrate (at least I mm thick). The small pixel size, the thickness of the Cd(Zn)Te substrate and the high photon energy motivate us to estimate first the spatial energy spreading following a photon interaction inside the detector. Estimations were made using the MCNP Monte Carlo package by simulating the individual energy distribution for each primary photon interaction. As an illustration of our results, simulating a 660keV gamma source, we found that there are two pixels on average, for each primary interaction, on which the deposited energy exceeds 50 keV. We have also made more accurate simulations using sub-pixels of side I I mum, in order to evaluate the distance between the barycentre of the deposited energy and the photon impact point. As an example, with a 660 keV gamma source, we found that the average of this distance reaches 67 pun when restricted to the events depositing more than 400 keV. If all events are taken into account, the mean distance is 26 mum, even though there is a small proportion of interactions where the scattered photon interacts somewhere else in the detector. Results are presented for different photon energies (60keV, 660keV, 1.25MeV) and different materials (CdTe, GaAs). (C) 2002 Elsevier Science B.V. All rights reserved. References: 9