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 (scheduled for launch in May of 2004) has a coded aperture mask and a detector array of 32,768 Cd_0.9Zn_0.1Te_1.0 (4 × 4 mm² large, 2 mm thick) semiconductor devices. Due to small mobility and short lifetime of carriers, the electron-hole pairs generated by irradiation of gamma-rays cannot be completely collected. Since the shape of the measured spectra has the broad low-energy tail, it is very useful for us to estimate the obtained spectra to fit the model which considers the charge transport properties depended on the depth of the photon interaction (G. Sato, 2002). The energy calibration of the BAT array and coded mask experiments were carried out at NASA/Goddard Space Flight Center between December 2002 and March 2003. We applied the model fitting to the calibration spectra, to yield the mobility-lifetime products for each detector and these values differ by over 2 orders of magnitude among detectors. Also using the mobility-lifetime parameters, we can identify the detector energy response as a function of the temperature and illumination angle. But we figure out a difference between the model and the obtained data. To determine the difference between the model and the measured data, we conducted the detailed check experiment for a single CdZnTe, to show that the cause of the excess is due to the areal nonuniformity of the mobility-lifetime parameter.