Characterization of CdTe Sensor Materials of Different Pixel Sizes with the IBEX ASIC (#1977)
P. Zambon1, M. Rissi1, V. Radicci1, C. Disch1, M. Schneebeli1, P. Trueb1, C. Broennimann1
1 Dectris Ltd., Baden-Daettwil, Aarau, Switzerland
High-Z materials such as CdTe give the opportunity to extend the detector sensitivity up to X-ray energies unattainable with classic Silicon sensors and their combination with single photon counting systems is believed to offer great improvement possibilities e.g. in the field of medical imaging.
In the design of such systems, the pixel size is a key parameter. A smaller pixel allows for higher spatial resolution and higher count rates but at the expenses of the spectral response which is affected by fluorescence and complex charge sharing effects.
In this work, we present the results of the characterization of CdTe sensors of different pixel size – 150 µm and 300 µm – bonded to the IBEX ASIC, the latest-generation photon counting chip developed at DECTRIS and targeted to spectral imaging applications. The measurements were performed both in our in-house laboratories and at synchrotron facilities in the energy range 10-60 keV.
The (prompt) count rate capabilities were assessed at 60 keV both in paralyzable and non-paralyzable counting mode. In the first case the maximum allowed incoming rate can be well above 9.8 Mcts/s/pix (430 Mcts/s/mm2) and 6.65 Mcts/s/pix (78 Mcts/s/mm2) for the 150 and 300 µm pixels, respectively. The energy resolution was measured in the range 10-60 keV for different chip settings and at 40 keV it can be as low as 1.7 and 2.4 keV FWHM for the two pixel sizes, respectively. In order to evaluate the impact of the charge sharing and of fluorescence effects arising above the Cd and Te K-edges (26.73 and 31.82 keV) on the spectral response as a function of the impinging energy we report the comparison of the quantum efficiency (QE) and of a newly-introduced and more straightforward quantity called spectral efficiency (SE), which is basically the ratio of the number of photons counted with the correct energy over the total number of impinging photons. The SE dependence on the incoming photon flux was also measured up to several hundreds of Mcts/s/mm2.
Keywords: cdte, photon counting, pixel detector, IBEX, count rate, energy resolution, quantum efficiency, spectral efficiency
An FPGA Based Algorithm for the Correction of the Instability of High Resolution and High Flux X-ray Spectroscopic Imaging Detectors (#2044)
C. De Cesare1, 2, A. Brambilla1, P. Ouvrier-Buffet1, S. Stanchina1, O. Rossetto2, L. Verger1
1 University of Grenoble Alpes, CEA LETI MlNATEC Campus, Grenoble, France
The emergence of CdTe photon counting detectors (PCD) with energy discrimination capabilities, opens up new perspectives in X-ray imaging. Medical and security applications require very high fluxes and consequently a very fast shaper in order to limit dead time losses due to pileup. However, if the shaper is faster than the collection of the charges in the semiconductor, there is a loss of charge called ballistic deficit. Moreover, variations of the electrical field profile in the detector with time cause a change in the collection time of the charges. According to this, the response of the detector will be affected by these variations of the collection loss due to the ballistic deficit. The instability of the response is visible over time as a channel shift of the spectra, resulting in a false information of the photon energy. The aim of this work is to develop real-time digital algorithms implementable in an FPGA in order to correct these effects and to give a stable response of the detector even at very high fluxes. The method has been tested with a 4x4 pixels detector (CdTe) of 3 mm thickness and 800 μm pitch which is able to measure transmitted X-ray spectra in the energy range of 20-160 kV on 256 energy bins. The developed method was initially tested at low count rate with a source of 57Co. After this first validation, it was tested with an X-ray beam, with a counting rate up to ~106 cps. The results obtained show that the algorithm is effective to improve the stability of the detector response while preserving the good compromise between energy resolution and count rate.
Keywords: CdTe - high flux - high resolution - FPGA - ballistic deficit
Comparison of CdTe and CdZnTe Fine Pitch Pixel Sensors and Construction of a Large Area CdTe Photon Counting Camera (#2056)
J. Kalliopuska1, J. Jakubek2, S. Polansky2, D. Turecek2, P. Soukup2, M. Jakubek2, S. Vähänen1, J. Salmi1
1 Advacam Oy, Semiconductors, Espoo, Finland
During the past three years Advacam has developed a spectral photon counting camera for inspection of static or moving objects. In the previous IEEE RTSD 2016 workshop we presented a spectral imaging double row camera using ten Ohmic CdTe pixel sensors of 1 mm thick flip chip bonded to Timepix readout ASICs and placed next to each other. The constructed CdTe camera showed outstanding properties for X-ray imaging and feasibility to extend the full field of view imaging area of the camera.
Several industrial applications use high energy X-rays and require higher efficiency than 1 mm thick Cd(Zn)Te is able to offer. Recently we have custom fabricated 2 mm thick edgeless CdTe and CdZnTe pixel sensors with 55 μm pitch. The sensors were flip chip bonded to Timepix readout ASICs using low temperature InSn solder micro bumps. Comparison of imaging and spectroscopic performance of the two sensor materials and sensor thicknesses will be presented. The compared properties include point-spread-function, material homogeneity, energy resolution and charge sharing between the pixels.
In addition, we have studied the possibility to scale up the full field of view imaging area by tiling the CdTe sensor assemblies next to each other to identify how the neighbouring tiles effect to each other when offset. The approach used in the construction is similar that has been used to construct the large area 5x5 Si camera with the Si edgeless sensor assemblies. The offset of the two adjacent CdTe sensors affects electric field shape in proximity of the tile edges causing risk of build-up of trapped charge. The presentation describes how such effect can be stabilized.
Finally the presentation describes a construction and challenges of a continuous full field of view 1.3 Mpixel (4x5) CdTe camera, made out of 20 pieces of 1 mm thick CdTe Timepix assemblies. The X-ray imaging properties of the CdTe large area camera will be demonstrated on various scientific and industrial samples.
Keywords: CdTe, CdZnTe, pixel, sensor, detector, X-ray, imaging, large area, full field of view, edgeless, photon counting
Effects of Non-uniform Electric Fields in Pixelated CdZnTe Detectors (#2057)
M. W. Streicher1, J. Xia1, Y. Zhu1, Z. He1
1 University of Michigan, Nuclear Engineering and Radiological Sciences, Ann Arbor, Michigan, United States of America
An electric field is required in a semiconductor-based spectrometer to drift electron-hole pairs and induce charge on an electrode. In planar devices, the electric field is presumed to be of constant intensity throughout the device. However, measurements of electron drift velocity indicate that CdZnTe detectors exhibit non-uniform electric fields. In general, the measured electric field is weak in the detector bulk and strong near the cathode and anode electrodes. The weak electric field in the bulk may be caused by incomplete annealing. Regardless of their cause, electric field non-uniformities can be measured using digitized pulse waveforms from 3-D position-sensitive CdZnTe detectors. Non-uniform electric fields can lead to lower measured electron mobility-lifetime products, (μτ)e, and significantly reduces the charge induced on neighboring anode pixels. Detectors with long electron trapping/de-trapping time constants also exhibit reduced neighbor signal amplitude. Reduced neighbor signal amplitude causes poorer sub-pixel position resolution which degrades imaging performance.
Keywords: CdZnTe, Sub-pixel Position Sensing, Non-uniform Electric Field
Microscale X-ray mapping of CZT arrays: spatial dependence of amplitude, shape and multiplicity of detector pulses (#2740)
L. Abbene1, F. Principato1, G. Gerardi1, G. Benassi2, N. Zambelli2, A. Zappettini3, M. Bettelli3, P. Seller4, M. C. Veale4
1 University of Palermo, Dipartimento di Fisica e Chimica, Palermo, Italy
CZT arrays with photon counting and energy resolving capabilities are widely proposed for high flux X-ray imaging applications. In this work, we present the results of a microscale 1D&2D mapping of CZT arrays with collimated synchrotron X-ray sources at the Diamond Light Source (U. K.). CZT pixel prototypes, with different thicknesses and pixel pitches (500 µm and 250 µm) are used, flip-chip bonded to a fast and low noise ASIC (PIXIE ASIC) and characterized only by the preamplifier stage. The detector signals are on-line processed through a custom 16-channel digital readout electronics, able to perform a fast pulse shape and height analysis (PSHA) (event arrival time, pulse shape, pulse height) of each output channel from the PIXIE ASIC. By using a 10 x 10 µm microbeam, the detector response was mapped for X-ray energies both above (50 keV) and below (20 keV) the K-shell absorption energy of the CZT material to study the spatial dependence of the amplitude, the shape and the multiplicity of the detector pulses and the role of fluorescence X rays in these events. High rate measurements at 700 kcps/pixel are also performed, pointing out the key role of the pulse shape analysis in the detection of both charge sharing and pile-up events.
Keywords: CZT pixel detectors; digital pulse processing; microscale X-ray mapping; charge sharing; pile-up; X-ray imaging
Using time-correlated transient signals in pixelated CdZnTe detectors to achieve sub-pixel resolution using a focused laser beam and varying pixel sizes (#4181)
L. A. Ocampo Giraldo1, 2, A. E. Bolotnikov2, G. Camarda2, J. Fried2, G. De Geronimo2, R. Gul2, A. Hossain2, K. Unlu1, E. Vernon2, G. Yang2, R. B. James3
1 Pennsylvania State University, Mechanical & Nuclear Engineering, University Park, Pennsylvania, United States of America
Pixelated detectors are able to correct response non-uniformities in Cadmium Zinc Telluride (CZT) crystals by virtually subdividing the detectors area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. An innovation proposed for pixelated detectors is to use the induced (transient) signals from neighboring pixels to achieve high sub-pixel position resolution while keeping relatively large pixel sizes. As previously demonstrated, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used multiples pixelated detectors from different commercial vendors and a focused pulsed laser. The laser generated a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution. We also present the sub-pixel resolution measurements for different pixel sizes.
Keywords: CdZnTe, High-granularity detectors, 3D pixelated detectors, Crystal defects, Charge sharing