First results of XPAD3, a new photon counting chip for X-ray CT-scanner with energy discrimination

XVAD3 is a single photon counting chip based on hybrid pixel counters, with low noise, high dynamics and high speed readout. Its features have been improved to provide an high counting rate capability, a very low threshold, an energy discrimination and a fast image readout. The chip is designed in 0.25 mum IBM technology, and contains 9600 pixels (130 mum times 130 mum) distributed into 80 columns of 120 elements each. An innovative architecture has been designed in order to prevent the digital circuits from disturbing the very sensitive analogue cells. XPAD3 was realized into two versions. The XPAD3-S version accepts positive input charges and offers an energy range from 4 keV to 40 keV with a single threshold. The XPAD3-C version accepts negative input charge and has an energy range from 6 keV to 60 keV with a windowed energy selection set by two independent thresholds. This last feature will be quite useful for experimentations where high contrast resolution is needed. The XPAD3 circuits can be bump-bonded with Si, CdTe or GaAs sensors to match detection efficiency with increasing photon energy. The aim of this development is to combine several XPAD3 circuits to build an 7.5 cm x 12 cm sensitive area for the development of a small animal micro-CT scanner, PIXSCAN and synchrotron X-ray scattering experiments. First prototypes of single chip detectors bump- bonded with a Si sensor and double chip detectors bump-bonded with a CdTe sensor have been produced and tested and preliminary results are presented.