Design, Fabrication and Characterization of a Unipolar Charge Sensing Amorphous Selenium X-ray Detector

Abstract

Amorphous Selenium is a direct conversion photoconductor that has been widely used in X-ray imaging applications. Due to its high spatial resolution A-Se plays an important role in breast cancer screening and diagnostics, allowing for the detection of small and subtle lesions. However, a-Se has poor collection efficiency due to low carrier mobility and charge trapping resulting from its amorphous structure. The trapped charges can cause memory artifacts, including photocurrent lag, which can persist for several seconds after the X-ray pulse has ended. As a result, a-Se is a challenging material for dynamic imaging applications that require high spatial resolution. The research discussed in this thesis aims to investigate and address the temporal behavior of a-Se photoconductors, specifically the issue of lag, which can lead to image artifacts and degradation of image quality in dynamic imaging applications. The research involves the design of unipolar charge sensing detectors with pixel sizes of 20, 40, 80 and 150 microns to improve energy resolution and the temporal response compared to conventional a-Se detectors. Theoretical analysis and simulations are presented for the unipolar charge sensing detector including weighting potential, charge collection efficiency, pulse height spectroscopy and energy resolution which range from 5% to 2% . The work further discusses the fabrication process of the designed detector in the G2N lab at the university of Waterloo. It discusses the experimental results obtained and the challenges that were faced while fabricating the detector and how they can be overcome in the future