Energy and DOI Calibrations for High Spatial Resolution CZT Detectors

Abstract

In this paper we investigate the energy calibration methods of a high spatial resolution CdZnTe pixel detector, across a wide energy range, along with a fine estimation of the depth of interaction (DOI). A 24x24 CdZnTe detector (160 um pixel size, 300 um pitch) is successfully calibrated in energy, from 32 keV to 662 keV, dealing with three effects, which have a strong impact on the acquired data: charge sharing, dependence with the DOI and incomplete charge induction. The CdZnTe detector is bump-bonded to a VATA210P ASIC, which outputs the induced charge on the square of nine pixels and on the common cathode. The ASIC noise is characterized by an automatic adjustment of the internal threshold available for every pixel. The channels are linearized in charge units in order to improve the resolution when the complete induced charge is computed as the sum of the adjacent pixel contributions. For energy calibration, the variation of the photopeak histogram with the cathode-to-pixel (C/P) ratio is analyzed. Two calibration profiles are generated for 32 and 662 keV for a 137Cs complete histogram. The obtained resolution, full width at half maximum (FWHM), is 24% for 32 eV (7.9 keV) and 4% for 662 keV (30 keV). The higher charge sharing for 662 keV events into many pixels explains the different energy resolutions. The DOI calibration is performed using simulation tools. A linear function with an exponential term is found suitable to characterize the DOI. Finally, the parametric fit obtained by simulation is applied to the experimental data with good results.