Characterization of the C-MOS Cd-Te Imager PIXIRAD for energy discriminated X-ray imaging

The aim of the present work is to assess the operational characteristics of the PIXIRAD Imaging Counter for use in high-definition energy resolved X-ray imaging for different applications. The PIXIRAD imager was developed by an INFN-Pisa Spin-off. It works in photon counting mode in a wide energy range, soft and hard X-rays (2-100 keV), with pulse discrimination defined by two thresholds. The 650 mu m thick CdTe X-ray sensor is interfaced with a CMOS VLSI chip organized on a 512 x 476 matrix of 55 mu m mexagonal pixels (total active area of 30.7 x 24.8 mm(2)). The experimental characterization was carried out in the range 3.7-80 keV, to assess the energy discrimination capability and detection efficiency of the PIXIRAD. Energy discrimination in bands was investigated using calibrated monochromatic X-ray sources (fluorescence of Ca, Fe, Cu, Br, Mo, Ag, I, Ta) and a BaCs radioactive source. In addition, two absolutely calibrated X-ray sources (Moxtek 50 kV Bullet and Oxford Instruments SB-80-1M) were utilized. The experimental data show that the PIXIRAD energy response is linear up to about 15 keV, beyond which the cluster size becomes larger than the pixel dimension. It produces multiple counts resulting in a tail at lower energy. Energy resolution was estimated to be about 30 %. The effects in term of energy discrimination and a "smooth energy discrimination" in bands has been investigated by studying the separation between different energy lines, acquiring combined images with different energy ranges and setting properly the PIXIRAD threshold.