S. Hustache

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.

K-edge imaging with the XPAD3 hybrid pixel detector, direct comparison of CdTe and Si sensors

We investigate the improvement from the use of high-Z CdTe sensors for pre-clinical K-edge imaging with the hybrid pixel detectors XPAD3. We compare XPAD3 chips bump bonded to Si or CdTe sensors in identical experimental conditions. Image performance for narrow energy bin acquisitions and contrast-to-noise ratios of K-edge images are presented and compared. CdTe sensors achieve signal-to-noise ratios at least three times higher than Si sensors within narrow energy bins, thanks to their much higher detection efficiency. Nevertheless Si sensors provide better contrast-to-noise ratios in K-edge imaging when working at equivalent counting statistics, due to their better estimation of the attenuation coefficient of the contrast agent. Results are compared to simulated data in the case of the XPAD3/Si detector. Good agreement is observed when including charge sharing between pixels, which have a strong impact on contrast-to-noise ratios in K-edge images.

Silicon Absolute X‐Ray Detectors

The responsivity of silicon photodiodes having no loss in the entrance window, measured using synchrotron radiation in the 1.75 to 60 keV range, was compared to the responsivity calculated using the silicon thickness measured using near‐infrared light. The measured and calculated responsivities agree with an average difference of 1.3%. This enables their use as absolute x‐ray detectors.

A large surface X-ray camera based on XPAD3/CdTe single chip hybrids

The XPAD3 chip bump-bonded to a Si sensor has been widely used in preclinical microcomputed tomography and in synchrotron experiments. Although the XPAD3 chip is linear up to 60 keV, the performance of the XPAD3/Si hybrid detector is limited to energies below 30 keV, for which detection efficiencies remain above 20%. To overcome this limitation on detection efficiency in order to access imaging at higher energies, we decided to develop a camera based on XPAD3 single chips bump-bonded to high-Z CdTe sensors. We will first present the construction of this new camera, from the first tests of the single chip hybrids to the actual mechanical assembly. Then, we will show first images and stability tests performed on the D2AM beam line at ESRF synchrotron facility with the fully assembled camera.