Improved Optical Quantum Efficiency and Temporal Performance of a Flat-Panel Imager with Avalanche Gain

Active matrix flat panel imagers (AMFPIs) with thin film transistor (TFT) arrays are becoming the standard for digital x-ray imaging due to their high image quality and real time readout capabilities. However, in low dose applications their performance is degraded by electronic noise. A promising solution to this limitation is the Scintillator High-Gain Avalanche Rushing Photoconductor AMFPI (SHARP-AMFPI), an indirect detector that utilizes avalanche amorphous selenium (a-Se) to amplify optical signal from the scintillator prior to readout. We previously demonstrated the feasibility of a large area SHARP-AMFPI, however there are several areas of desired improvement. In this work, we present a newly fabricated SHARP-AMFPI prototype detector with the following developments: metal oxide hole blocking layer (HBL) with improved electron transport, transparent bias electrode for increased optical coupling, and detector assembly allowing for a back-irradiation (BI) geometry to improve detective quantum efficiency of scintillators. Our measurements showed that the new prototype has improved temporal performance, with lag and ghosting below 1%. We also show an improvement in optical coupling from 25% to 90% for cesium iodide (CsI) scintillator emissions. The remaining challenge of the SHARP-AMFPI is to reduce the dark current to prevent dielectric breakdown under high bias and further increase optical quantum efficiency (OQE) to CsI scintillators. We are proposing to use a newly developed quantum dot (QD) oxide layer, which shows to reduce the dark current by an order of magnitude, and tellurium doping, which could increase OQE to 85% to CsI at avalanche fields, in future prototype detectors.

Automated summary

This study introduces a new prototype SHARP-AMFPI detector with improved electron transport, optical coupling, and back-irradiation structure, leading to enhanced performance. The new prototype shows significant improvements in temporal performance and optical coupling for CsI scintillator emissions, but challenges remain in reducing dark current and increasing optical quantum efficiency. Future prototype detectors may utilize quantum dot oxide layers and tellurium doping to address these challenges.