Hard X-ray response of CdZnTe detectors in the Swift burst alert telescope

The Burst Alert Telescope (BAT) onboard the Swift gamma-ray burst explorer has a coded aperture mask and a detector array of 32 K CdZnTe semiconductor devices. Due to the small mobility and short lifetime of carriers, the electron-hole pairs generated by gamma-ray irradiation cannot be fully collected. Hence the shape of the measured spectra has a broad low-energy tail. We have developed a method to model the spectral response by taking into account the charge transport properties which depend on the depth of the photon interaction [1]. The mobility-lifetime products for detectors derived from our method vary by more than one order of magnitude among detectors. In this paper, we focus on the nonuniformity of the mobility at the millimeter scale by employing a scanning experiment for a single detector. We reveal almost an order of magnitude variance in the mobility-lifetime product of holes within a single detector, while those of electrons remains fairly uniform.