Fluoroscopic x-ray demonstrator using a CdTe polycrystalline layer coupled to a CMOS readout chip

Dynamic X-ray imagers require large surface, fast and highly sensitive X-ray absorbers and dedicated readout electronics. Monocrystalline photoconductors offer the sensitivity, speed, and MTF performances. Polycristalline photoconductors offer the large surface at a moderate cost. The challenge for them is to maintain the first performances at a compatible level with the medical applications requirements. This work has been focused on polycristalline CdTe grown by Close Space Sublimation (CSS) technique. This technique offers the possibility to grow large layers with a high material evaporation yield. This paper presents the results obtained with an image demonstrator using 350μm thick CdTe_css layers coupled to a CMOS readout circuit with Indium bumping. The present demonstrator has 200 × 200 pixels, with a pixel pitch of 75μm × 75μm. A total image surface of 15mm × 15mm has then been obtained. The ASIC works in an integration mode, i.e. each pixel accumulates the charges coming from the CdTe layer on a capacitor, converting them to a voltage. Single images as well as video sequences have been obtained. X-ray performance at 16 frames per second rate is measured. In particular a readout noise of 0.5 X ray, an MTF of 50% at 4 lp/mm and a DQE of 20% at 41p/mm and 600 nGy are obtained. Although present demonstrator surface is moderate, it demonstrates that high performance can be expected from this assembly concept and its interest for medical applications.