Energy sensitive X-ray phase contrast imaging with a CdTe-Timepix3 detector

The Timepix3 is a photon counting semiconductor detector that enables to simultaneously measure the energy and time of arrival of each incident X- ray photon. These properties, along with the high spatial resolution and high efficiency, due to the CdTe sensor material, can be exploited for several imaging applications, such as X-ray phase contrast imaging (XPCI). XPCI relies on the phase shift suffered by X-rays when traversing the sample. This study focuses on the free-space propagation XPCI and single mask edge illumination XPCI methods, which are two approaches that are well suited for laboratory implementations. Since both techniques are highly sensitive to charge-sharing, the Timepix3 energy and time information for each photon are used to minimize this effect by using pixel clustering methods. In addition, the performance of both XPCI techniques across a 30kVp source spectrum is studied using the energy-resolving capabilities of the detector. In both cases, the phase contrast and signal-to-noise ratio (SNR) are assessed as a function of different energy. Finally, it is demonstrated that phase contrast enhancement is feasible with pixel clustering and energy-selection for both XPCI techniques

CdTe sensor configurations for robot assisted photon counting gamma camera

International audienceIn this work, different Cadmium Telluride (CdTe) sensor configurations are assessed for the usage in a robot assisted portable gamma camera. In the first part, four CdTe sensors, with thickness of 0.45  mm, 1  mm, 2  mm and 3  mm and pixel sizes of 55  μm and 110  μm, are investigated regarding their spectroscopic performance. The photon counting detector Timepix1 is hereby used. The 3  mm CdTe sensor shows increase in count rate up to a factor of 1.25 compared to a 2  mm CdTe sensor, 1.84 compared to a 1  mm CdTe sensor and up to 2.71 compared to a 0.45  mm CdTe sensor in the case of 137Cs. In the second part, the 3  mm CdTe sensor was implemented in a commercially available gamma camera, the iPIX. The system was integrated in the bomb disposal robot and tested in different scenarios. The integrated 3 mm CdTe detector measured 21.5 counts per second emitting from a 60Co source with an activity of 2.8 ± 0.07  Gbq in 20  meters distance in an open environment. The acquisition time was 116  seconds. The angular resolution was sufficient for the user to localize the radioactive isotope inside the test structure

Spectral X-ray phase contrast imaging with a CdTe photon-counting detector

The present study focuses on the implementation of two X-ray phase contrast imaging (XPCI) techniques: free-space propagation (FSP) and single mask edge illumination (SM-EI) with a microfocus polychromatic X-ray source and a Timepix3 photon-counting detector with a CdTe sensor. This detector offers high spatial resolution, high detection efficiency and it is able to simultaneously record information about Time-over-Threshold (ToT) and Time-of-Arrival (ToA) for each X-ray photon. All these features play a key role in enabling an improvement of XPCI image quality, especially through spectral analysis, since it is possible to measure the energy of each incident X-ray photon. Measurements of phase contrast and contrast-to-noise ratio (CNR) are presented for different energy bins within the typical spectrum of soft X-ray imaging. It is shown that a significant enhancement of XPCI image quality can be obtained, for both implemented techniques, by performing pixel clustering to correct for charge sharing and by introducing some degree of energy-weighting

Performance of a Medipix3RX spectroscopic pixel detector with a high resistivity gallium arsenide sensor

High resistivity gallium arsenide is considered a suitable sensor material for spectroscopic X-ray imaging detectors. These sensors typically have thicknesses between a few hundred μm and 1 mm to ensure a high photon detection efficiency. However, for small pixel sizes down to several tens of μm, an effect called charge sharing reduces a detector's spectroscopic performance. The recently developed Medipix3RX readout chip overcomes this limitation by implementing a charge summing circuit, which allows the reconstruction of the full energy information of a photon interaction in a single pixel. In this work, we present the characterization of the first Medipix3RX detector assembly with a 500 μm thick high resistivity, chromium compensated gallium arsenide sensor. We analyze its properties and demonstrate the functionality of the charge summing mode by means of energy response functions recorded at a synchrotron. Furthermore, the imaging properties of the detector, in terms of its modulation transfer functions and signal-to-noise ratios, are investigated. After more than one decade of attempts to establish gallium arsenide as a sensor material for photon counting detectors, our results represent a breakthrough in obtaining detector-grade material. The sensor we introduce is therefore suitable for high resolution X-ray imaging applications

Chromium-Compensated GaAs Detector Material and Sensors

Results obtained from numerical calculations of and experimental studies on the pulse height distribution inherent in ionizing radiation gallium arsenide sensors as a function of the design features of the devices and electrophysical characteristics of the detector material are presented. It is shown that the pulse height distribution is defined by the distribution pattern of the nonequilibrium charge carrier lifetime and by the electric field profile in the bulk of the sensor. Investigations on the detector sensitivity to X-ray energies in the range between 40 and 150 keV were performed. The sensor polarization was found to produce only a marginal effect compensated by an increase in the bias voltage. Prototype pixel sensors measuring 256 × 256 and 512 × 768 pixels with a 55 μm pitch and a 500 μm thick sensitive layer were produced. The dependence of the photocurrent and count rate on the X-ray radiation intensity and bias voltage applied to the sensor was examined. In the 40–80 keV energy range, the maximum count rate amounted to 800 kHz/pixel for a negative sensor bias voltage of 800 V. The sensors are demonstrated to provide spatial resolution varying with the pixel pitch and to enable high-quality X-ray images to be obtained

Performance of a Medipix3RX spectroscopic pixel detector with a high resistivity gallium arsenide sensor

High resistivity gallium arsenide is considered a suitable sensor material for spectroscopic X-ray imaging detectors. These sensors typically have thicknesses between a few hundred μm and 1 mm to ensure a high photon detection efficiency. However, for small pixel sizes down to several tens of μm, an effect called charge sharing reduces a detector's spectroscopic performance. The recently developed Medipix3RX readout chip overcomes this limitation by implementing a charge summing circuit, which allows the reconstruction of the full energy information of a photon interaction in a single pixel. In this work, we present the characterization of the first Medipix3RX detector assembly with a 500 μm thick high resistivity, chromium compensated gallium arsenide sensor. We analyze its properties and demonstrate the functionality of the charge summing mode by means of energy response functions recorded at a synchrotron. Furthermore, the imaging properties of the detector, in terms of its modulation transfer functions and signal-to-noise ratios, are investigated. After more than one decade of attempts to establish gallium arsenide as a sensor material for photon counting detectors, our results represent a breakthrough in obtaining detector-grade material. The sensor we introduce is therefore suitable for high resolution X-ray imaging applications

Investigation of GaAs:Cr Timepix assemblies under high flux irradiation

High resistivity, chromium compensated gallium arsenide (HR-GaAs:Cr) has recently shown to be a promising sensor material for X-ray detectors due to its high resistivity, its fully active volume, the good electron transport properties and good absorption properties for photon energies up to around 40 keV. These properties make this material a favorable candidate for producing sensors for photon counting X-ray imaging detectors. Such detector systems have also gained increasing attention to be used for high flux applications as found at synchrotrons or in medical applications like computed tomography. Whereas other interesting high-Z materials such as CdTe have already been studied under high flux irradiation, the behavior of HR-GaAs:Cr under such conditions has yet to be investigated

Investigation of GaAs:Cr Timepix assemblies under high flux irradiation

High resistivity, chromium compensated gallium arsenide (HR-GaAs:Cr) has recently shown to be a promising sensor material for X-ray detectors due to its high resistivity, its fully active volume, the good electron transport properties and good absorption properties for photon energies up to around 40 keV. These properties make this material a favorable candidate for producing sensors for photon counting X-ray imaging detectors. Such detector systems have also gained increasing attention to be used for high flux applications as found at synchrotrons or in medical applications like computed tomography. Whereas other interesting high-Z materials such as CdTe have already been studied under high flux irradiation, the behavior of HR-GaAs:Cr under such conditions has yet to be investigated

The Medipix3RX: A high resolution, zero dead-time pixel detector readout chip allowing spectroscopic imaging

The Medipix3 chips have been designed to permit spectroscopic imaging in highly segmented hybrid pixel detectors. Spectral degradation due to charge sharing in the sensor has been addressed by means of an architecture in which adjacent pixels communicate in the analog and digital domains on an event-by-event basis to reconstruct the deposited charge in a neighbourhood prior to the assignation of the hit to a single pixel. The Medipix3RX chip architecture is presented. The first results for the characterization of the chip with 300 μm thick Si sensors are given. ~ 72e(−) r.m.s. noise and ~ 40e(−) r.m.s. of threshold dispersion after chip equalization have been measured in Single Pixel Mode of operation. The homogeneity of the image in Charge Summing mode is comparable to the Single Pixel Mode image. This demonstrates both modes are suitable for X-ray imaging applications

Characterization of the Medipix3 pixel readout chip

The Medipix3 chip is a hybrid pixel detector readout chip working in Single Photon Counting Mode. It has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture charge deposited in overlapping clusters of four pixels is summed event-by-event and the incoming quantum is assigned as a single hit to the summing circuit with the biggest charge deposit (this mode of operation is called Charge Summing Mode (CSM)). In Single Pixel Mode (SPM) the charge reconstruction and the communication between neighbouring pixels is disabled. This is the operating mode in traditional detector systems. This paper presents the results of the characterization of the chip with electrical stimuli and radioactive sources