XPAD, a new read-out pixel chip for X-ray counting

Hybrid pixel detectors featuring a high signal to noise ratio, real 2 dimensional reconstruction and local intelligence, have now been demonstrated to be one of the most powerful trackers for high energy physics experiments. They have, therefore, gained interest in other physics fields and for medical applications. However, the conversion of pixel detectors from high energy physics to X-ray detection is not completely straightforward and new challenges appear. For example, high counting rates, wide dynamic ranges and low detection levels are required. “XPAD” (X-ray Pixel Chip with Adaptable Dynamics) is a novel concept of a pixel detector read-out chip especially developed for X-ray diffraction. This circuit, submitted in a 0.8 μm CMOS technology, contains 600 square pixels distributed into 24 columns of 25 elements each. The analogue part, gathering 2 cascaded amplifiers, uses the automatic pole-zero cancellation technique in order to keep the baseline stable even at counting rate as high as 10⁷ photons per second. A fast discriminator allows a detection level from 5 keV (1200 electrons) to 50 keV. A new read-out concept based on a special overflow mechanism has been implemented for infinite dynamic range purposes. This technique permits also data transport during acquisition and motion reconstruction provided by some level of time tagging