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

The properties of 32K CdZnTe (4×4mm2 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 (μτ) 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 μτ for electrons ranges from 5.0×10-4 to 1.0×10-2cm2V-1 while the μτ for holes ranges from 1.3×10-5 to 1.8×10-4cm2V-1. We find that this wide distribution of μτ products explains the large diversity in spectral shapes between CdZnTe detectors well. We also find that the variation of μτ products can be attributed to the difference of crystal ingots or manufacturing harness. We utilize the 32K sets of extracted μτ 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.