Properties of CdZnTe detectors in the Burst Alert Telescope (BAT) array

The properties of 32k CdZnTe detectors have been studied in the pre-flight calibration of Burst Alert Telescope (BAT) on-board the Swift Gamma-ray Burst Explorer (scheduled for launch in January 2004). After corrections of the linearity and the gain, the energy resolution of summed spectrum is 7.0 keV (FWHM) at 122~keV. In order to construct response matrices for the BAT instrument, we extracted mobility-lifetime (μτ) products for electrons and holes in the CdZnTe. Based on a new method applied to 57Co spectra taken at different bias voltages, μτ for electrons ranges from 5.0x10-4 to 1.0x10-2cm2V-1, while μτ for holes ranges from 1.0x10-5 to 1.7x10-4cm2V-1. We show that the distortion of the spectrum and the peak efficiency of the BAT instrument are well reproduced by the μτ database constructed in the calibration.