R4A  Pixel Detectors 1

Thursday, Nov. 5  08:30-10:00  California

Session Chair:  Elias Hamann, IPS, Karlsruhe Institute of Technology (KIT), Germany

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(08:30) R4A-1, High Z Photon Counting Large Area Camera for Scanning of Continuously Moving Objects

J. J. Kalliopuska1, J. Jakubek2, D. Turecek2, P. Soukup2, M. Jakubek2, S. Vähänen1, J. Salmi1

1Advacam Oy, Espoo, Finland
2Widepix s.r.o, Prague, Czech Republic
In the previous IEEE RTSD 2014 workshop we presented a high frame rate spectral imaging camera using a CdTe pixel sensor developed in the Mineral Analysis using X-ray Imaging (MAXI) project. The presented CdTe sensors showed outstanding properties for X-ray imaging and spectroscopy applications. During the past year we have developed an imaging camera for scanning of continuously moving objects. In order to obtain high efficiency and thus higher scanning speed, the sensor material of the camera need to be based on high Z materials, such as CdTe or CdZnTe, instead of Si. The approach of the camera is based on a novel row structure that can be extended indefinitely or to cover a large area with a full field of view. We have demonstrated a continues digital reading concept using the Timepix readout chip, where the digital readout is adapted to the scanning speed of the sample using a Time Delayed Summation (TDS). First results of the TDS will be presented. We have custom fabricated edgeless 1 mm thick CdTe and 2 mm thick CdZnTe pixel sensors with 55 µm and 110 µm pitch, respectively. The sensors were flip chip bonded to Timepix readout chips using low temperature InSn solder micro bumps. Spectroscopic measurements with threshold scans of Am-241 show an energy resolution of 1.88 keV and 3.61 keV FWHM at 60 keV for the 1 mm thick Schottky CdTe sensor and 2 mm thick CdZnTe sensor, respectively. A comprehensive comparison of imaging and spectroscopic performance of the two sensor materials is presented. In addition, we have demonstrated the capability to scale up the imaging area by tiling sensor assemblies as continuous full field of view row. Properties of the camera will be demonstrated with various scientific and industrial samples.

(08:45) R4A-2, Low Energy Gamma Ray Imaging with a Detector Array

F. Pan1, W. Dixon1, S. Katz1, B. Yanoff1, B. Bures2

1GE Global Research, Niskayuna, NY, USA
2Morpho Detection, Andover, MA, USA
Methods such as Compton imaging can be used to image a radiation field, including localizing different radioactive isotopes. Depth of interaction detection enables Compton imaging using Cadmium Zinc Telluride (CZT) detector for gamma ray energies above ~300 keV. We have developed an imaging technique for low energy gamma rays, using an array of CZT sensors, based on the non-uniform penetration of radiation into the interior of individual crystals, and radiation “shadows” cast by one detector crystal onto its neighbors. This mask-free imaging technique is ideal for a hand-held detector system because it avoids the need for the heavy collimator or masks typically used to image at low energies. A radiation source direction is reconstructed from the unique geometric shadow that develops when illuminating detector array. This shadow is based on the path length attenuation of the gamma as it passes through the detector material. This self-shielding technique can also be combined with high energy gamma ray Compton imaging to enhance radioisotope localization. We demonstrate this approach using simulations and measurement on a multi-crystal CZT detector.

(09:00) R4A-3, A High Frame Rate Pixel Readout Chip Design for Synchrotron Radiation Applications

W. Wei1,2, J. Zhang1,2, Z. Ning1,2, Y. P. Lu1,2, L. Fan1,2, H. S. Li1,2, X. S. Jiang1,2, A. K. Lan2, Q. Ouyang1,2, Z. Wang1,2, K. J. Zhu1,2, Y. B. Chen1,2, P. Liu1

1Institute of High Energy Physics, CAS, Beijing, China
2State Key Laboratory of Particle Detection and Electronics, Beijing, China
A hybrid pixel detector working in the single photon counting mode was designed for the High Energy Photon Source (HEPS) in China. Aiming for diffraction and protein crystallography applications, the pixel readout chip works in single photon counting mode in each pixel. It contains an array of 104 ×72 pixels with a pixel size of 150 μm ×150 μm, each owning a counting depth of 20bits. Different from the conventional readout structure based on the pseudo-random counter, an independent shift register chain was inserted, separated with the counter. Therefore this design allows high frame rate counting and data transfer simultaneously. The chip measurement showed 94 e- equivalent noise and non-uniformity less than 55 e- after threshold equalization. All functions were proved to be normal at a frame rate of 1.2 kHz with a negligible dead-time, which are greatly improved compared with the existing pixel system.

(09:15) R4A-4, High Flux Performance of a Digital Readout Electronics for Room Temperature Energy-Resolved Photon Counting Detectors

L. Abbene, G. Gerardi, F. Principato

Dipartimento di Fisica e Chimica, University of Palermo, Palermo, Italy
Photon counting detectors with energy resolving capabilities are desired for high flux X-ray imaging. In this work, we present the performance of a custom-designed digital readout electronics for high flux measurements. The 16-channel readout electronics is able to continuously digitize and process the signals from each pixel, performing multi-parameter analysis (event arrival time, pulse shape, pulse height, pulse time width, etc.) even at high fluxes and at different throughput and energy resolution conditions. High flux measurements with CdTe/CZT pixel detectors were presented.

(09:30) R4A-5, Artifact Reduction Techniques for Medipix Based Computed Micro Tomography with Si, GaAs and CdTe Sensors

F. Fischer1, S. Procz1, S. Moser2, M. Pichotka3, M. Fiederle1

1Freiburg Materials Research Center (FMF), Freiburg, Germany
2Fraunhofer Institute for High Speed Dynamics, Ernst-Mach-Institut (EMI), Freiburg, Germany
3Institute of Experimental and Applied Physics (IEAP), Praha, Czech Repuplic
Computed tomography (CT) is an X-ray imaging tool widely established in medical diagnostics and non-destructive material analysis. Over the last years, hybrid semiconductor photon-counting detectors of the Medipix family have proven themselves beneficial for this kind of imaging. An advantage all Medipix detectors have in common is their hybrid structure which allows the use of sensor materials different from Si. Especially materials with high atomic-numbers like GaAs and CdTe are materials of interest, since they allow the effective detection of high energy photons. Moreover, the adjustable energy thresholds lead to a high signal-to-noise-ratio (SNR). Together with the pixel pitch of 55 µm this allows both good contrast and high resolution imaging. In cone-beam geometry, the spatial resolution can be further increased and is then mainly limited by the spot size of the X-ray tube employed. However, the fact that planar images, used for reconstruction, are acquired under real conditions gives raise to inconsistencies during the measurement. The result of these inconsistencies is the formation of artifacts in the reconstructed CT image which leads to the loss of image information and misinterpretation. Thus, this paper focuses on several techniques with the objective to reduce artifacts such as ring artifacts, beam-hardening and metal artifacts. In addition, the impact of the sensor choice on artifact formation and reduction possibilities is examined by using different sensor materials (Si, GaAs, CdTe) for the acquisition of the raw data. The methods applied to diminish these artifacts are performed both before the actual data acquisition, for example by filtering, as well as part of the reconstruction procedure, where special filters and regularizations are applied. It can be shown, that these procedures decrease the appearance of the artifacts mentioned earlier and therefore CT image quality can be improved and misinterpretation can be prevented.