R4B  Pixel Detectors 2

Thursday, Nov. 5  10:30-12:05  California

Session Chair:  Simon Procz, FMF Universität Freiburg, Germany

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(10:30) R4B-1, invited, Development of Slim-Edge CdTe Sensors for Large Area Pixelated X-Ray Detectors

M. Ruat1, C. Ponchut1, A. Fauler2, J. Kalliopuska3

1ESRF - The European synchrotron, ESRF, Grenoble, France
2X-ray Imaging Europe, Freiburg, Germany
3Advacam, Espoo, Finland
This work is part of a long-term project aiming at the construction of large-area pixel detectors for high X-ray energy applications on synchrotron beamlines. Single crystal CdTe sensors are currently not available in sizes beyond 45 mm x 30 mm. To overcome this limitation several individual modules must be butted up against each other. The first critical aspect of sensor enhancement for large-area detector development is the reduction of dead areas around the individual modules composing the detector. The second one is the improvement of the crystal edge quality after dicing. Both these aspects have been addressed and 15 mm x 15 mm sensors with an edge region of 100 um width, embedding a guard ring or not, have been fabricated and tested. The first results show that good quality CdTe sensors with edges reduced to 100 $\mu$m with or without guard ring can be obtained. A uniform detector response was measured along with high yield flip-chip bonding. The active area of CdTe sensors is consequently greatly increased. It also demonstrates that guard rings, yet not harmful, are not mandatory for appropriate charge collection in CdTe sensors. By improving the dicing quality the sensors exhibited no distortion of the electric field at their edges. A very good stability of the detector's response with irradiation time was also measured on both sensors. Further assemblies are expected to improve the yield. Such sensors will then be stitched in arrays of 2xn to further evaluate the edge quality.

(10:50) R4B-2, A Comparison of the Temporal Instabilities Found in State-of-the-Art CdTe and CZT Sensors Used in Spectral CT Measurements with the Medipix3RX Detector

M. Zuber1, E. Hamann1, R. Ballabriga2, M. Campbell2, K. Iniewski3, A. Butler4, P. Butler5, S. Vahanen6, M. Fiederle7, T. Baumbach1, T. Koenig1

1Institute for Photon Science and Synchrotron Radiation (IPS) & ANKA Synchrotron Radiation Facility, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany
2European Organization for Nuclear Research, Geneva, Switzerland
3Redlen Technologies, Saanichton, BC, Canada
4Department of Physics and Astronomy, University of Cantebury, Christchurch, New Zeeland
5Center for Bioengeering and Nanomedicine, University of Otago, Christchurch, New Zeeland
6Advacam, Espoo, Finland
7Freiburg Materials Research Center, University of Freiburg, Freiburg, Germany
One of the hallmarks of spectral x-ray imaging are photon counting detectors. In order to achieve high detection efficiencies, these detectors are mostly based on compound semiconductor sensors such as cadmium telluride (CdTe) or cadmium zinc telluride (CZT). Since these sensors consist of at least two chemical elements, inhomogeneities are inevitably present. These can result in spatially varying electric fields and resistivities, which translate into an inhomogeneous distribution of leakage currents within the sensor. Most photon counting detectors feature a leakage current compensation scheme that attempts to compensate this current offset on a pixel level. Pixels pertaining to sensor regions of high quality usually have no troubles compensating the leakage current flowing through them. However, those pixels which are connected to parts of the sensor that experience an unusually high leakage can give rise to wrong or even fluctuating count rates, leading to severe artifacts in spectral CT. Quantitative evaluations of CT measurements like material resolved spectroscopic CT require a stable detector to ensure artifact-free images. We examined the influence of the leakage current compensation for CdTe sensors on the quality of material resolved spectroscopic CT slices and found a significant decrease of artifacts by increasing the leakage current compensation. The possibility to suppress artifacts with the leakage current compensation shows that the artifacts are dominated by transient leakage currents in single pixels. In our conference contribution we will compare these results with those acquired with a CZT sensor. Various image quality metrics will be determined for regular and material resolved CT slices. It will be investigated whether the lower average leakage current present in CZT sensors also translates into reduced artifact levels by means of a suppression of highly localized, transient leakage currents.

(11:05) R4B-3, A Comparison of CdZnTe and CdTe Assemblies with PILATUS3

M. Rissi, C. Christodoulou, T. Donath, V. Radicci, T. Sakhelashvili, M. Schneebeli, G. Tudosie, S. Traut, P. Trueb, P. Zambon, C. Broennimann

DECTRIS Ltd., 5400 Baden, Switzerland
CdTe and CdZnTe sensors were bump bonded to the pixellated PILATUS3 single photon counting ASICs. The sensor thicknesses are 1mm and 1.5mm for CdTe and CdZnTe, respectively. The PILATUS3 ASIC with a pixel size of 172µm×172µm allows for the measurement of both electrons or holes, and is well suited for electron collection mode of the CdTe and CdZnTe sensors. The retrigger feature implemented in PILATUS3 allows for non-paralyzable counting, thus enabling the counting of incoming x-ray fluxes above 107 photons per pixel per second. Both assemblies were characterized and compared in terms of count rate, detective quantum efficiency (DQE) and energy resolution, sensor uniformity and imaging properties, including resolution and the modulation transfer function (MTF) for monochromatic and polychromatic beams. The energy resolution was determined as a function of rate, using extended simulations. Simulations were done for different pixel sizes, followed by cross checks with measurements from PILATUS3 assemblies. It was found that the energy resolution degrades with higher count rate, due to pulse pile-up in each pixel's preamplifier.

(11:20) R4B-4, First Measurements with the X-KIT Readout for Multi-Module Timepix Detectors Using a GaAs:Cr Sensor

E. Hamann1, T. Koenig1, M. Zuber1, D. Bormann2, M. Vogelgesang2, S. Smale2, M. Balzer2, A. Fauler3, M. Fiederle3, T. Baumbach1

1IPS/ANKA, Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany
2IPE, Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany
3FMF, University of Freiburg, Freiburg, Germany
In order to extend the use of photon counting semiconductor pixel detectors to a wider range of applications, mainly three aspects have to be addressed: having large active areas, being able to work at higher X-ray energies and having sufficiently high frame rates. Since many photon counting readout ASICs like the Timepix are rather limited in size, the first aspect can be solved by fabricating multi-chip arrays bump-bonded to larger area monolithic semiconductor sensors. Since the Timepix is buttable at three sides, 2×N assemblies can be built. However, the size of many available “high-Z” semiconductor wafers like GaAs or CdTe, which are needed to fulfil the need of high absorption efficiencies at higher photon energies, is limited to a few inches. Thus, often 2×3 chip assemblies (so-called “Hexas”) have been fabricated in the recent years. Concerning the third aspect of high readout speeds, several systems already exist which are able to read out multi-chip Medipix arrays at kHz frame rates. However, since these systems mostly require dedicated hardware and are rather expensive, a novel readout system called “X-KIT” was developed by institutes at KIT and the FMF Freiburg aiming to read out Timepix Hexa modules at moderate frame rates of 50-100 fps. Since all chip communication and data readout is realized by an FPGA via a standard 1 Gb Ethernet link, the system does not require special hardware and can be operated stand-alone by most modern PCs/Laptops. In addition, this enables to build the system very compact and cost-efficient. In this contribution, the hard- and software developments of the X-KIT readout are presented and first measurements with a Timepix Hexa assembly employing a 28×42 mm˛ high-resistivity GaAs sensor are shown, a sensor material which has recently shown to be very promising for X-ray applications up to 60 keV.

(11:35) R4B-5, CdTe Sensors Used in Charge-Integrating Pixel Array Detectors for High-Speed Imaging

M. W. Tate1, K. S. Shanks1, H. T. Phillip1, J. Becker1, J. T. Weiss1, P. Purohit1, D. Chamberlain2, S. M. Gruner1,2

1Department of Physics, Cornell University, Ithaca, NY, USA
2Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, NY, USA
Hybrid Pixel Array Detectors (PADs) are constructed by bonding an x-ray sensor layer, pixel-by-pixel, to an underlying pixel-processing chip, allowing both the sensor and processing electronics to be independently tailored to the imaging problem. For energies above 20 keV, high-Z sensor materials provide increased quantum efficiency of x-ray detection relative to silicon sensors. In high-speed imaging applications, the photon flux per pixel often requires a charge-integrating architecture, as pulses from individual photons can no longer be resolved by a photon counting architecture. We describe hybridizing 750 µm thick CdTe sensors for use at high x-ray energy to two different charge-integrating readout chips, the Keck PAD and the Mixed-Mode PAD (MMPAD), both developed previously in our laboratory with silicon sensors. Each type of detector chip is a 128×128 pixel array with 150 µm pitch. The CdTe detectors are fabricated as Schottky diodes with the pixel electronics collecting holes from the sensor. The Keck PAD chip includes in-pixel storage for 8 frames which can be captured with framing periods as low as 150 ns. This allows x-ray images from successive synchrotron bunches to be captured in rapid succession with a full well of about 10,000 photons/pixel (at 8 keV) in each frame. The MMPAD detector was developed for high-dynamic range imaging. This chip incorporates circuitry to remove a fixed charge (equivalent to about 200 photons) from the integration node when the integration stage exceeds a set threshold. An 18-bit in-pixel counter records the number of charge removal steps and is read at frame end along with the signal remaining on the integration stage. Signal levels ranging from the single photons to tens of millions of photons/pixel can be recorded in a single frame. The MMPAD can image at sustained framing rates of over 1kHz.

(11:50) R4B-6, Characterization of Spectrometric Photon-Counting X-Ray Detectors at Different Pitches

M. Jurdit1, A. Brambilla1, V. Moulin1, P. Ouvrier-Buffet1, L. Helfen2,3, L. Verger1

1CEA, LETI, MINATEC Campus, F-38054 Grenoble, France
2Laboratory for Application of Synchrotron Radiation, ANKA, Karlsruhe Institute of Technology, Karlsruhe, Germany
3European Synchrotron Radiation Facility, CS40220, F-38043 Grenoble, France
There is growing interest in energy-sensitive photon-counting detectors operating at high flux, for applications like medical imaging, non-destructive testing and security. Such innovative detectors should count individual photons and sort them into selected energy bins, at several millions counts per second, which raises a compromise between pixel pitch and count rate. We have assembled Cd(Zn)Te detectors with a thickness of 1.5 to 3mm to interposers able to generate varying pitches from 800µm down to 200µm. These platforms have been tested with our full-digital fast readout electronics, counting at 2Mcounts/pixel. Such 16-channel demonstrators, with 128 energy bins, have been experimentally characterized in terms of energy resolution, count rate, and charge sharing that becomes challenging at low pitch. Results have been compared to simulation, and charge sharing correction has been implemented, significantly correcting X-ray spectra. To investigate the bonding quality at low pitch, flip-chip interconnections were imaged three-dimensionally by synchrotron-radiation computed laminography.