Charge Sharing and Interaction Depth Corrections in a Wide Energy Range for Small Pixel Pitch CZT Detectors

The CSTD project aims at developing a high resolution pixel gamma detector based on CdZnTe for Compton imaging applications. Our research group has been working recently on the design and characterization of a new pixel detector with specifications focused at high energy SPECT for medical imaging applications. The detector pitch, 0.3 mm, and its thickness, 5 mm, allows to reach high spatial resolution and high detector efficiency. Non-ideal performance appears with more strength in small pixel pitch CdZnTe detectors, below 1 mm, affecting at the spectroscopic results. In order to recuperate the shared charge, the customized ASIC simultaneously collects the charge in the triggering pixel and its eight neighboring pixels per event. The detector design, readout electronics, acquisition software and data analysis have been completed at CIEMAT. Data has been taken by irradiating the CdZnTe detector with high and low energy gamma-ray sources. The high energy events of the Cs-137 source suffer from a great proportion of charge sharing in the neighboring pixels. Two Cs-137 spectra, with and without energy correction, are shown and compared. To obtain the corrected spectra offline, the collected charge at the neighboring pixels is added to the trigger pixel collected charge. The corrected spectra show that the 662 keV photopeak is reconstructed. Interaction depth correction follows to improve the energy resolution by data segmentation of the 662 keV energy peak according to fifty cathode to pixel ratios. The computed interaction depth correction profile is the inverse of the charge collection efficiency. Energy resolution can be improved discarding the segmented data which do not achieve an acceptable energy resolution. Several interaction depth correction profiles at 81, 356 and 662 keV are shown and reveal a second correlation between the charge collecting efficiency and the collecting energy.