Marie Ruat

Characterization of a Pixelated CdTe X-Ray Detector Using the Timepix Photon-Counting Readout Chip

A prototype 1mm thick CdTe detector bonded to a Timepix chip with 256*256 pixels at 55 μ m pitch was evaluated for use as a photon-counting imaging detector at high energy synchrotron beamlines (energy range 30-100 keV). A complete characterization of the system was performed. Powder diffraction experiments have also been conducted using a monochromatic beam at the ESRF. The expected gain in efficiency at energies above 30 keV with reference to silicon pixel detectors and current CCD systems of similar pixel size was demonstrated, together with an improved spatial resolution. Background-free powder diffraction spectra were obtained using the Timepix energy thresholding feature. The energy-resolved detection capabilities are limited by a strong charge sharing. The major limitations preventing a wider use of these devices at synchrotron X-ray sources are the lack of homogeneity of the CdTe crystal which exhibits numerous defects, and the unavailability of large fields of view.

How spectroscopic x-ray imaging benefits from inter-pixel communication.

Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations.

A method for high-energy, low-dose mammography using edge illumination x-ray phase-contrast imaging.

Since the breast is one of the most radiosensitive organs, mammography is arguably the area where lowering radiation dose is of the uttermost importance. Phase-based x-ray imaging methods can provide opportunities in this sense, since they do not require x-rays to be stopped in tissue for image contrast to be generated. Therefore, x-ray energy can be considerably increased compared to those usually exploited by conventional mammography. In this article we show how a novel, optimized approach can lead to considerable dose reductions. This was achieved by matching the edge-illumination phase method, which reaches very high angular sensitivity also at high x-ray energies, to an appropriate image processing algorithm and to a virtually noise-free detection technology capable of reaching almost 100% efficiency at the same energies. Importantly, while proof-of-concept was obtained at a synchrotron, the method has potential for a translation to conventional sources.

3D semiconductor radiation detectors for medical imaging: Simulation and design

In conventional planar detection structures, photon absorption efficiency is limited by the thickness of the detector, which is itself limited by charge transport properties in the chosen material. Therefore a trade-off must be found between photons absorption efficiency and charge collection efficiency. To overcome this compromise, alternative detector architecture has been proposed [1], where charge collection is realized perpendicularly to the photon absorption plane. The volume of the detector is micro-structured with an array of columnar electrodes (alternatively anodes and cathodes) which penetrate through the whole bulk. Thanks to this new 3D architecture of the detectors, and as long as electrodes are sufficiently close, various semiconductor materials can be considered for room temperature use. CdTe and GaAs 3D detectors have been simulated thanks to a complete model coupling Monte-Carlo modeling of interaction of X- and γ-rays with matter (with PENELOPE, [2]) and computation of Charge Induction Efficiency (CIE) with finite elements method. Planar structures of the same material have also been simulated for comparison purposes. CdTe and GaAs single crystal detectors with an electrode pitch of 7μm have demonstrated spectrometric efficiency, i.e. discrimination of the energies of incident rays in the considered radiation energy range (typically 10μ160 keV for medical applications). Detectors with an electrode pitch of 350μm allowed the counting of photons arriving on the detector, without energy discrimination, which is suitable for X-ray imaging in medical applications.

Characterization of a X-ray pixellated CdTe detector with TIMEPIX photon-counting readout chip

A prototype 1mm CdTe detector bonded to a TIMEPIX chip with 256*256 pixels at 55µm pitch is evaluated for use at synchrotron beamlines. A complete characterization was performed. At the tested energies above 50 keV, a 10 times efficiency increase and a higher spatial resolution have been exhibited with reference to Silicon-based pixel detectors and to current CCD systems of similar pixel size. Background-free powder diffraction spectra were obtained, owing to the energy thresholding feature. The energy-resolved detection capabilities are limited by a high charge sharing. The major limitations preventing a wider use of these devices at synchrotron X-ray sources are the lack of homogeneity of the crystal exhibiting numerous defects, and the unavailability of large detection areas.

Investigation of Polarisation in CdTe using TCT

The polarisation effect in CdTe:Cl has been studied using the Transient Current Technique (TCT) in order to quantitatively evaluate the subsequent changes in the charge transport properties as well as the electric field distribution in the sensor volume. The electric field is calculated from TCT pulses using the Schockley-Ramo theorem. The mobility of the charge carriers as well as their average drift velocity in the CdTe material are determined using the TCT pulse width. Infrared illumination demonstrated a temporary restoration of the electric field. However after a few minutes the polarization effect is resumed, even under constant IR illumination.

X-ray imaging characterization of active edge silicon pixel sensors

The aim of this work was the experimental characterization of edge effects in active-edge silicon pixel sensors, in the frame of X-ray pixel detectors developments for synchrotron experiments. We produced a set of active edge pixel sensors with 300 to 500 μm thickness, edge widths ranging from 100 μm to 150 μm, and n or p pixel contact types. The sensors with 256 × 256 pixels and 55 × 55 μm2 pixel pitch were then bump-bonded to Timepix readout chips for X-ray imaging measurements. The reduced edge widths makes the edge pixels more sensitive to the electrical field distribution at the sensor boundaries. We characterized this effect by mapping the spatial response of the sensor edges with a finely focused X-ray synchrotron beam. One of the samples showed a distortion-free response on all four edges, whereas others showed variable degrees of distortions extending at maximum to 300 micron from the sensor edge. An application of active edge pixel sensors to coherent diffraction imaging with synchrotron beams is described.