Effects of detector scatter on photopeak fraction in pixellated solid state detectors

A Monte Carlo simulation of the photopeak fraction in a 1 cm-thick CdTe crystal with square pixel side dimensions ranging between 0.5 and 2.5 mm was performed. Photons of 140 and 511 keV were assumed to impinge only on a “center” pixel, normal to the crystal surface and uniformly distributed. Three families of pixels were defined relative to the center pixel:(1) the four edge pixels; (2) the four corner pixels; and (3) the 16 next-nearest neighbors. For 140 keV photons, as pixel size is reduced from 2.5 to 0.5 mm, the number of total events in the edge pixels increases from 8% to 17% of the number interacting in the center pixel due to the increasing pixel surface area-to-volume ratio. At 511 keV, however, as the pixel size diminishes the number of events in the edge pixels decreases from 13% to 7% of the number interacting in the center pixel, due to the larger mean free path of the scattered photons and the smaller dimensions of the neighbouring pixels. Although the 1 cm crystal attenuates nearly 99% of the incident 140 keV photons, the computed photofraction for a single pixel ranges from 62% to 75% as pixel size increases from 0.5 to 2.5 mm. For the smallest pixel, 19% and 1.7% of the incident photons result in photoelectric events whose fluorescent photon escapes the pixel for 140 and 511 KeV, respectively. At 511 keV only 42% of incident photons interact with 1 cm CdTe. For the 0.5 mm pixel, the photofraction is only 5.7% at 511 keV. For the 2.5 mm pixel the photofraction improves to 8.3%. These values are roughly half the photofraction of 3/8” NaI at this energy. At 511 keV, of the initially interacting photons, roughly 3/4 of them will Compton scatter to the surrounding pixels and deposit less than photopeak energy. To significantly improve the photopeak performance of pixellated detectors, it appears that simultaneous sampling of neighbouring pixels may be necessary. This study involves the simulation of photon transport within the crystal and has not addressed the effects of electron range, charge diffusion, and induced charge signal on the photofraction.