First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip

We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, radiation monitoring in nuclear facilities. The Medipix2+CdTe hybrid detector features 256x256 square pixels, a pitch of 55 μm, a sensitive area of 14x14 mm². Here we analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt contacts, showed an ohmic response when negatively biased to less than 100 V (electrons collection mode). Tests were also performed in holes collection mode, where a non-resistive behaviour was observed above +15 V. We performed a series of imaging tests with gamma radioactive sources and with an X-ray tube. In flood illumination, we observed for all detectors the presence of numerous, stable small-scale structures in the form of small circles, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime).