Development of a spectral model based on charge transport for the Swift/BAT 32K CdZnTe detector array

Abstract The properties of 32K CdZnTe ( 4 × 4 mm 2 large, 2 mm thick) detectors have been studied in the pre-flight calibration of the Burst Alert Telescope (BAT) on-board the Swift Gamma-ray Burst Explorer (scheduled for launch in November 2004). In order to understand the energy response of the BAT CdZnTe array, we first quantify the mobility-lifetime ( μ τ ) products of carriers in individual CdZnTe detectors, which produce a position dependency in the charge induction efficiency and results in a low-energy tail in the energy spectrum. Based on a new method utilizing 57Co spectra obtained at different bias voltages, the μ τ for electrons ranges from 5.0 × 10 - 4 to 1.0 × 10 - 2 cm 2 V - 1 while the μ τ for holes ranges from 1.3 × 10 - 5 to 1.8 × 10 - 4 cm 2 V - 1 . We find that this wide distribution of μ τ products explains the large diversity in spectral shapes between CdZnTe detectors well. We also find that the variation of μ τ products can be attributed to the difference of crystal ingots or manufacturing harness. We utilize the 32K sets of extracted μ τ products to develop a spectral model of the detector. In combination with Monte Carlo simulations, we can construct a spectral model for any photon energy or any incident angle.