Spectroscopic x-ray imaging by means of photon counting detectors has received growing interest during the past years. Critical to the image quality of such devices is their pixel pitch and the sensor material employed. This paper describes the imaging properties of Medipix2 MXR multi-chip assemblies bump bonded to 1 mm thick CdTe sensors. Two systems were investigated with pixel pitches of 110 and 165 μm, which are in the order of the mean free path lengths of the characteristic x-rays produced in their sensors. Peak widths were found to be almost constant across the energy range of 10 to 60 keV, with values of 2.3 and 2.2 keV (FWHM) for the two pixel pitches. The average number of pixels responding to a single incoming photon are about 1.85 and 1.45 at 60 keV, amounting to detective quantum efficiencies of 0.77 and 0.84 at a spatial frequency of zero. Energy selective CT acquisitions are presented, and the two pixel pitches' abilities to discriminate between iodine and gadolinium contrast agents are examined. It is shown that the choice of the pixel pitch translates into a minimum contrast agent concentration for which material discrimination is still possible. We finally investigate saturation effects at high x-ray fluxes and conclude with the finding that higher maximum count rates come at the cost of a reduced energy resolution.
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.
CdTe is a promising material for the detection of γ-and X-rays as 1 mm thick CdTe sensors offer an absorption efficiency that is higher than 50 % for photon energies up to 120 keV. Therefore 1mm thick CdTe from Acrorad has been flip-chipped as the sensor material on a Timepix readout-electronics ASIC at the Freiburg Materials Research Center (FMF).The transport properties of the CdTe material are investigated by determining the µτ-product of the charge carriers by illuminating the non-collecting electrode with alpha particles. The method commonly used to determine the µτ-product for non-pixelated devices was adapted for the Medipix pixelated readout-electronics. The position of the alpha particles on the pixel matrix provides spatial information, which was used to create a mapping of the µτ-product.The energy resolutions and the positions of the noise edge of two CdTe-Timepix assemblies, one with a pixel pitch of 110 x 110 µm 2 and another one with a pixel pitch of 55 x 55 µm 2 were investigated by performing a threshold scan of a 241 Am source. A comparison of the energy spectra obtained with the two assemblies with different pixel pitches confirms the strong dependence of charge sharing on the energy spectra, as expected.
We have operated a Medipix2 CMOS readout chip, with amplifying, shaping and charge discriminating front-end electronics integrated on the pixel-level, as a highly segmented direct charge collecting anode in a three-stage gas electron multiplier (Triple-GEM) to detect the ionization from 55 Fe X-rays and electrons from 106 Ru. The device allows to perform moderate energy spectroscopy measurements (20 % FWHM at 5.9 keV X-rays) using only digital readout and two discriminator thresholds. Being a truly 2D-detector, it allows to observe individual clusters of minimum ionizing charged particles in Ar/CO 2 (70:30) and He/CO 2 (70:30) mixtures and to achieve excellent spatial resolution for position reconstruction of primary clusters down to ∼ 50 µm, based on the binary centroid determination method.
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