Applications of Digital ASIC Array System for Noise Analysis, Non-linearity Correction, Event Classification and Reconstruction.

Abstract

Wide band gap semiconductor detectors have been studied over decades for gamma-ray spectroscopy and CdZnTe has attracted most attention. Compared to HPGe detectors, the room temperature operation of CdZnTe detectors is an appealing feature. With the help of a 3-D position sensing technique and the use of low electronic noise analog ASIC, the energy resolution of pixelated CdZnTe detectors has been improved to 0.48% FWHM which is close to the theoretical limit of 0.2% FWHM at 662 keV achieved by HPGe detectors. However, the lateral position resolution of the pixelated CdZnTe detectors is determined by the pixel pitch size if using an analog ASIC. In addition, analog ASIC can only report the signal amplitude and triggering time information for each pixel that is not adequate to describe the charge transportation and induction details. These issues can degrade the spectroscopic performance for multiple pixel events, high energy events, and Compton scattering imaging angular resolution. In order to overcome these issues, a different version of the ASIC, the digital ASIC, has been developed, which can provide the digitized pre-amplifier signal waveforms. After several design iterations, the current digital ASIC system contains four digital ASIC modules in a 2 by 2 array and has demonstrated 2 keV FWHM electronic noise. This work discusses the performance of the digital ASIC array system and its non-linear response. The non-linear response of the digital ASIC array system can be corrected experimentally and the performance of the non-linearity correction has been demonstrated. The energy resolution degradation of single-detector multiple-pixel events has also been studied. By using the pre-amplifier waveforms, three types of events can be identified and reconstructed properly, including (1) single-pixel multiple interaction events, (2) incomplete charge collection events, and (3) side-neighbor charge leak events. The photopeak efficiency is increased and the shape of the photopeak is more symmetric. The material defect diagnosis has also been demonstrated with the analog ASIC system and a superior performance of the diagnosis has been achieved using the digital ASIC system.