A New X-Ray Imaging System Based on Chromatic Photon Counting Technology
The continuous progress and scale reduction of the CMOS technology has allowed the realization of pixellated VLSI ASICs with direct integration in each pixel of electronics of increased complexity and functionality. By coupling a custom, large area, pixellated ASIC with a thin pixellated CdTe crystal, we have realized an innovative X-ray imaging sensor (PIXIRAD) based on Chromatic Photon Counting technology. The detector is able to count and separate in energy the X-ray photons transmitted or scattered by the sample and converted in each pixel of the CdTe sensor to produce two ‘color’ images in a single exposure. The individual block of the PIXIRAD imaging system is buttable on two opposite sides and consists of a solid-state sensor (CdTe) connected to the readout ASIC by flip-chip bonding (bump bonding) technique. The system therefore has a hybrid architecture in which the sensor and readout electronics are manufactured and processed separately.
The CdTe sensor is a 650 m thick Schottky type diode with electrons collection on the pixels. It is characterized by a very low leakage current at 400-500 V bias-voltage. The pixels are arranged on a hexagonal matrix at 60 m pitch and match an analogous pattern on the top metal layer of the VLSI ASIC. In depth testing of several configurations of the PIXIRAD imaging system has been performed. Modules of one, two, four and eight PIXIRAD tile units with almost zero dead space between the blocks have been assembled. They are able to deliver extremely clear and highly detailed X-ray images for medical, biological, industrial and scientific applications in the energy range 1-100 keV. Images are obtained at very high count rate (> 30 GHz for a 4 tiles module).
Color X-ray images from 3-2.5 to 25-25cm2 area have been obtained. Photon counting, color mode and a very high spatial resolution (more than 10 l.p./mm at MTF50) allow to have an optimal ratio between image quality and absorbed dose. The X-Ray imaging system is the technological platform of PIXIRAD Imaging Counters s.r.l., a recently constituted INFN spin-off company.