B. Dinkespiler

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.

Detective quantum efficiency, modulation transfer function and energy resolution comparison between CdTe and silicon sensors bump-bonded to XPAD3S

XPAD3S is a single-photon-counting chip developed in collaboration by SOLEIL Synchrotron, the Institut Louis Néel and the Centre de Physique de Particules de Marseille. The circuit, designed in the 0.25 microm IBM technology, contains 9600 square pixels with 130 microm side giving a total size of 1 cm x 1.5 cm. The main features of each pixel are: single threshold adjustable from 4.5 keV up to 35 keV, 2 ms frame rate, 10(7) photons s(-1) mm(-2) maximum local count rate, and a 12-bit internal counter with overflow allowing a full 27-bit dynamic range to be reached. The XPAD3S was hybridized using the flip-chip technology with both a 500 microm silicon sensor and a 700 microm CdTe sensor with Schottky contacts. Imaging performances of both detectors were evaluated using X-rays from 6 keV up to 35 keV. The detective quantum efficiency at zero line-pairs mm(-1) for a silicon sensor follows the absorption law whereas for CdTe a strong deficit at low photon energy, produced by an inefficient entrance layer, is measured. The modulation transfer function was evaluated and it was shown that both detectors present an ideal modulation transfer function at 26 keV, limited only by the pixel size. The influence of the Cd and Te K-edges of the CdTe sensor was measured and simulated, establishing that fluorescence photons reduce the contrast transfer at the Nyquist frequency from 60% to 40% which remains acceptable. The energy resolution was evaluated at 6% with silicon using 16 keV X-rays, and 8% with CdTe using 35 keV X-rays. A 7 cm x 12 cm XPAD3 imager, built with eight silicon modules (seven circuits per module) tiled together, was successfully used for X-ray diffraction experiments. A first result recently obtained with a new 2 cm x 3 cm CdTe imager is also presented.

Diffraction studies under in situ electric field using a large-area hybrid pixel XPAD detector

A new experimental approach to perform in situ electric field diffraction on single crystals using an on-then-off pump–probe mode in situ (i.e. the field-switching method) with a synchrotron or a laboratory X-ray source is presented. Taking advantage of the fast readout of the XPAD hybrid pixel two-dimensional detector and its programmable functionalities, the operation mode of the detector has been customized to significantly increase the efficiency of the method. The very weak electric field-induced structural response of a piezoelectric crystal can be accurately measured. This allows the piezoelectric tensor to be precisely obtained from Δθ shifts while the structural variations can be modelled using a full set of ΔI/I data. The experimental method and methodology are detailed and tested as a case study on pure piezoelectric compounds belonging to the α-quartz family (SiO2 and GaAsO4 single crystals). Using the two scan modes developed, it is demonstrated that tiny Bragg angle shifts can be measured as well as small field-induced Bragg intensity variations (<1%). The relevance of measurements performed with an X-ray laboratory source is demonstrated: partial data sets collected at synchrotrons can be completed, but more interestingly, a large part of the study can now be realized in the laboratory (medium to strong intensity reflections) in a comparable data collection time.

Performance and Applications of the CdTe- and Si-XPAD3 photon counting 2D detector

The XPAD3 is the third generation of a single photon counting chip developed in collaboration by SOLEIL Synchrotron, the Institut Neel and the Centre de Physique de Particules de Marseille (CPPM). The chip contains 9600 pixels of 130 μm side and a counting electronic chain with an adjustable low level threshold in each pixel. Imaging and detection performance (detective quantum efficiency, modulation transfer function and energy resolution) of the XPAD3 detectors hybridized with Si and CdTe sensors have been evaluated and compared using monochromatic synchrotron X-rays beam. A second version of the chip, optimized for pump-probe experiments, has been realized and successfully tested. Three 7.3 cm x 12.5 cm Si-XPAD3 imagers, composed of 8 silicon modules (7 chips per module) and one 2.1 cm x 3.1 cm CdTe-XPAD3 imager (4 chips) have been constructed and successfully used for synchrotron diffraction experiments and biomedical imaging.

Design and construction of the ClearPET/XPAD small animal PET/CT scanner

In this study, we present a small animal hybrid PET/CT scanner for simultaneous X and gamma ray scans of the same field of view. It is based on the high resolution PET scanner demonstrator ClearPET developed within the Crystal Clear Collaboration and on the prototype X-ray hybrid pixel detector XPAD3 developed at CPPM. The complete hybrid system has been studied using the GATE Monte Carlo simulation platform. The final design includes the PET detectors appropriately shielded, the XPAD3 detector and a collimated RTW X-ray tube. First, we present experimental measurements that demonstrate the feasibility of acquiring PET data in presence of an X-ray beam scattered by a water phantom hosting a positron emitter point source. Then, we present first images of simultaneous PET and CT acquisitions of several phantoms and a mouse.