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

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 487; no. 1; pp. 142 - 150
• Main Authors: Manach, Erwan, Gal, Olivier
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 11-07-2002
• Subjects: Charge sharing; Detector; Imaging; Pixellated; 28.41.Rc; 28.41.Te; Charge sharing; Detector; γ; Imaging; Pixellated; 29.40.Gx
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/S0168-9002(02)00958-0

In the framework of the Medipix2 Collaboration, a new photon-counting chip is being developed made of a 256×256 array of 55
μm-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 γ-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 1
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 660
keV γ 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 11
μm, 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 γ source, we found that the average of this distance reaches 67
μm when restricted to the events depositing more than 400
keV. If all events are taken into account, the mean distance is 26
μm, 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 (60
keV, 660
keV, 1.25
MeV) and different materials (CdTe, GaAs).