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

Abstract

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, and radiation monitoring in nuclear facilities. The Medipix2 + CdTe hybrid detector features 256 256 square pixels, a pitch of 55 m, a sensitive area of 14 14 mm 2 . 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 ohmic contacts, showed an ohmic response when negatively biased up to less than 60 V (electrons collection mode). Tests were also performed in holes collection mode, where a nonresistive behavior was observed above +15 V. We performed a series of imaging tests at low voltage bias with gamma radioactive sources and with an X-ray tube. Under uniform irradiation, we observed for all detectors the presence of numerous, stable structures in the form of small circles of about 200 m diameter, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime). Also long filament structures have been observed. Further investigations will reveal whether they are due to an intrinsic detector response (e.g., due to Te inclusions) or to the bump-bonding process.

Characterization of a pixellated CdTe detector with single-photon processing readout

“…The bias current was time-dependent and a diode-like behavior was observed as previously reported in Ref. [2]. The current started at a high value when power was turned on and then stabilized after a couple of minutes.…”

“…[1][2][3]) are interesting because of low noise operation and their ability to record the energy of each individual photon [3]. One such readout chip is the MEDIPIX2 chip [1] with a pixel size of 55 Â 55 mm 2 .…”

A 20kpixels CdTe photon-counting imager using XPAD chip

“…As one can guess looking at the transport properties of CdTe (Table 2), and specially with a hole mobility 20 times lower than in silicon, this working mode seems to be not very suitable in our case (short shaping time) even if some authors report more uniform charge collection in this mode when using ohmic contacts [6]. The only way to circumvent this problem will be to apply a high collection field in high-quality material (hole lifetime).…”

Development of a portable gamma camera with coded aperture

“…Each pixel comprises two individually adjustable thresholds and a 13-bit counter. The main application is X-ray imaging but we participated in the development of CdTe detectors 1 mm thick bump-bonded on Medipix2 chips for higher-energy applications [7]. Fig.…”