Sensing Gamma-Rays in Thick Pixelated Thallium Bromide Detectors with Application-Specific Integrated Circuitsy Detection
Hall, Erik
2024
Abstract
Pixelated semiconductor gamma-ray detectors are uniquely suited to perform both gamma-ray spectrometry and imaging with the same device. These detectors can precisely record the energy deposited by an incident gamma ray and their planar-pixel electrode configuration enables three-dimensional position-sensing of the interaction. CZT has demonstrated great commercial success and research materials such as TlBr and certain Perovskites have potential as alternatives to CZT. However, readout electronics and algorithms that can mate with these alternative materials are key components to their radiation detection capability. This work seeks to identify the gaps in the fabrication of pixelated TlBr gamma-ray detectors, using current readout technology and techniques, and suggests areas for further development. Electrodes made from various combinations of materials to include chromium, gold, palladium, and platinum provided the best spectroscopic performance. However, their performance generally worsened over time. Some samples showed a small improvement later, but not enough for their further use as gamma-ray detectors. Electrode refabrication failed to provide good results later, so electrode materials are likely diffusing into the detector bulk. Additionally, a detector with thallium electrodes provided similarly good performance until failure, then was operated with a reverse-polarity positive bias for a similar duration. When tested again with a negative bias, it showed good performance again. In larger detectors bonded to a carrier board, over days of operation, some event waveform tails show an increasing amount of extra signal, which degraded spectroscopic resolution. Events occurring near the pixel edges have this extra signal while events in the pixel center have a more consistent shape. Increased electron de-trapping around the pixel edges is a possible cause. A pixel-like pattern has also been observed on their planar cathodes, where patches of the cathode have disintegrated. The silver epoxy used to bond the pixelated anode of the detectors to their carrier board may be reacting with the planar cathode and causing the separation. The VAD-UMv2.2 ASIC is failing to record many events at its smallest dynamic range. This problem is exacerbated for detectors with greater electron trapping. Increasing the preamplifier feedback resistance enabled the recording of more events in the middle depths, but events near the planar cathode were still missing. At this dynamic range, this ASIC’s trigger shaper time constant may be too short for slow-rising waveforms in lower-mobility materials like TlBr. However, this ASIC’s second-smallest dynamic range seemed to be better able to record more events up to the planar cathode. Also, this ASIC's sampling window at a 2.5 MHz sampling rate is barely long enough to fully record events in TlBr at more sustainable operating biases like 1 kV/cm. The slowest sampling rate of 1.25 MHz would be better for recording complete event waveforms but was not functioning. The H3DD-UMv4 ASIC is more suitable for slower materials such as TlBr. It has better reliability at slow sampling rates, a longer sampling window, and a wider span of trigger shaping time constants. Its slower trigger shaper time constant enables the recording of more low-amplitude events, which helps correctly categorize multi-pixel events and thus improves the energy resolution for single-pixel events. One detector showed consistent performance when operated from 0°C to +30°C. However, an extra signal loss was observed repeatedly at +40°C. Faster preamplifier decay at +40°C played a role in that signal loss.Deep Blue DOI
Subjects
thallium bromide pixelated semiconductor radiation detectors waveform signal processing digital ASIC
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