A Monolithic Amorphous-Selenium/CMOS Single-Photon-Counting X-Ray Detector

We report on the design and characterization of a monolithic single-photon-counting (SPC) X-ray detector integrating an amorphous semiconductor sensor. Our prototype combines amorphous selenium (a-Se), a well-known photoconductor, with a CMOS readout integrated circuit (ROIC) containing two 26\times196 arrays of single-energy-threshold pixels. We achieve high spatial resolution for capturing single 60-keV photons by implementing 11.4- \mu \text{m} -pitch pixels and by integrating a 70- \mu \text{m} -thick a-Se sensor on the ROIC. We present experimental SPC results using a mono-energetic 60-keV radioactive source. Our results also suggest that our prototype a-Se/CMOS detector leverages the small-pixel effect (SPE) due to the large ratio of a-Se thickness to pixel pitch. This effect could overcome the slow inherent transient response of a-Se for high count rate X-ray diagnostic applications like mammography.