Pixelated CdZnTe Detectors Research Articles

3x3 array module of 8×8×32 mm3 position-sensitive virtual frisch-grid CdZnTe detectors for imaging and spectroscopy of cosmic gamma-rays

We evaluated the performance of the 3 × 3 array of 32-mm long position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors read out by the IDEAS GDS-100 data acquisition system. The array is a mechanically solid module designed for integration into a large effective area gamma-ray telescope proposed for imaging and spectroscopy of cosmic gamma-rays. The array employs the bar-shaped 8 × 8 × 32 mm3 CZT crystals which recently became available from Redlen Technologies, Inc. and Kromek-eV Group PLC at affordable costs. The waveform sampling IDE3421 application-specific integrated circuit (ASIC), developed by the University of Michigan and IDEAS for CZT pixel detectors, was adapted to capture the signals from the array's VFG detectors. We showed good spectral performance of the Redlen detectors with an energy resolution of <1% FWHM at 662 keV and slightly poorer performance of the Kromek-eV detectors with a resolution of <2% measured at a reduced temperature of 13 °C. By testing the same detectors as single devices using hybrid charge-sensitive preamplifiers, we demonstrated improved energy resolutions at a normal temperature of ∼25 °C: <1% and 1.3–1.7%, respectively. However, the Kromek-eV detectors did not perform as well as the Redlen ones. We explained this by the space charge formation (polarization) decreasing the electric field and the electron drift times. The polarization was likely caused by the high injections of holes from the anode contacts. The absence of polarization in Redlen detectors suggested that the quality of the contacts might have played a role in this case.

Preliminary results from the Submarine Gamma Imager

In this work, we present preliminary results of a novel Submarine Gamma Imager (SUGI) being developed based on pixelated CdZnTe detector modules. The instrument, mounted on a remotely operated vehicle (ROV), has been tested in a series of field deployments performed at the hydrothermal fields on the island of Milos, Greece. The analysis of the collected data demonstrate the capabilities of the instrument, while comparison with a reference gamma detector confirms the validity of the results.

Radiation damage of [formula omitted] pixelated CdZnTe due to high-energy protons

Pixelated CdZnTe detectors are a promising imaging-spectrometer for gamma-ray astrophysics due to their combination of relatively high energy resolution with room temperature operation negating the need for cryogenic cooling. This reduces the size, weight, and power requirements for telescope-based radiation detectors. Nevertheless, operating CdZnTe in orbit will expose it to the harsh radiation environment of space. This work, therefore, studies the effects of 61MeV protons on 2×2×1cm3 pixelated CdZnTe and quantifies proton-induced radiation damage of fluences up to 2.6×108p/cm2. In addition, we studied the effects of irradiation on two separate instruments: one was biased and operational during irradiation while the other remained unbiased. Following final irradiation, the 662keV centroid and nominal 1% resolution of the detectors were degraded to 642.7keV,4.9%(FWHM) and 653.8keV,1.75%(FWHM) for the biased and unbiased systems respectively. We therefore observe a possible bias dependency on proton-induced radiation damage in CdZnTe. This work also reports on the resulting activation and recovery of the instrument following room temperature and 60°C annealing.

Modelling Polarization Effects in a CdZnTe Sensor at Low Bias.

Semi-insulating CdTe and CdZnTe crystals fabricated into pixelated sensors and integrated into radiation detection modules have demonstrated a remarkable ability to operate under rapidly changing X-ray irradiation environments. Such challenging conditions are required by all photon-counting-based applications, including medical computed tomography (CT), airport scanners, and non-destructive testing (NDT). Although, maximum flux rates and operating conditions differ in each case. In this paper, we investigated the possibility of using the detector under high-flux X-ray irradiation with a low electric field satisfactory for maintaining good counting operation. We numerically simulated electric field profiles visualized via Pockels effect measurement in a detector affected by high-flux polarization. Solving coupled drift-diffusion and Poisson's equations, we defined the defect model, consistently depicting polarization. Subsequently, we simulated the charge transport and evaluated the collected charge, including the construction of an X-ray spectrum on a commercial 2-mm-thick pixelated CdZnTe detector with 330 µm pixel pitch used in spectral CT applications. We analyzed the effect of allied electronics on the quality of the spectrum and suggested setup optimization to improve the shape of the spectrum.

Direct measurement of electron drift parameters in pixelated cadmium zinc telluride semiconductor detectors

The single-polarity charge-sensing electrode is widely used in wide-bandgap room-temperature CdZnTe detectors; in such systems, the drift of electrons is the primary contribution to the induced signal at the anode. Thus, it is important to accurately evaluate the parameters that describe the electron transport properties because they affect the performance of detectors. This paper proposes a method to directly measure the electron mobility and their mean free drift time in a pixelated CdZnTe detector using a depth-sensitive technique. The presented method considers the effect of the nonideal weighting potential in an actual detector, which has not been accounted for in previously proposed methods. In addition, by fitting the cathode waveform to avoid the effect of the time–amplitude walk due to the trigger threshold, improved measurement accuracy of the electron drift parameters can be achieved.

Imaging effects due to pixel distortions in CdZnTe detectors—results from the HREXI Calibration Facility

ProtoEXIST2 (P2) was a prototype imaging X-ray detector plane developed for wide-field Time Domain Astrophysics (TDA) in the 5 - 200 keV energy band. It was composed of an 8 $\times$ 8 array of 5 mm thick, 2cm $\times$ 2cm pixelated (32 $\times$ 32) CdZnTe (CZT) detectors with a 0.6 mm pitch that utilize the NuSTAR ASIC(NuASIC) for readout. During the initial detector development process leading up to post-flight examination of the entire detector plane, distortions in expected pixel positions and shapes were observed in a significant fraction of the detectors. The HREXI (High Resolution Energetic X-ray Imager) Calibration Facility (HCF) was designed and commissioned to improve upon these early experiments and to rapidly map out and characterize pixel non-uniformities and defects within CZT detector planes at resolutions down to 50 $\rm \mu$m. Using this facility, the sub-pixel level detector response of P2 was measured at 100 $\rm \mu$m resolution and analyzed to extract and evaluate the area and profile of individual pixels, their morphology across the entire P2 detector plane for comparison with previous measurements and to provide additional characterization. In this article, we evaluate the imaging performance of a coded-aperture telescope using the observed pixel morphology for P2 detectors. This investigation will serve as an initial guide for detector selection in the development of HREXI detector planes, for the future implementation of the 4pi X-Ray Imaging Observatory (4piXIO) mission which aims to provide simultaneous and continuous imaging of the full sky ($\rm 4\pi$ sr) in the 3-200 keV energy band with $\rm \simeq$ 2 arcmin angular resolution and $\simeq$ 10 arcsec source localization, as well as other, future coded-aperture instruments.

Characterization of a CdZnTe detector for a low-power CubeSat application

We report spectral and imaging performance of a pixelated CdZnTe detector custom designed for the MeVCube project: a small Compton telescope on a CubeSat platform. MeVCube is expected to cover the energy range between 200 keV and 4 MeV, with a sensitivity comparable to the one of the last generation of larger satellites. In order to achieve this goal, an energy resolution of few percent in full width at half maximum (FWHM) and a 3-D spatial resolution of few millimeters for the individual detectors are needed. The severe power constraints present in small satellites require very low power read-out electronics for the detector. Our read-out is based on the VATA450.3 ASIC developed by Ideas, with a power consumption of only 0.25 mW/channel, which exhibits good performance in terms of dynamic range, noise and linearity. A 2.0 cm× 2.0 cm× 1.5 cm CdZnTe detector, with a custom 8 × 8 pixel anode structure read-out by a VATA450.3 ASIC, has been tested. A preliminary read-out system for the cathode, based on a discrete Amptek A250F charge sensitive pre-amplifier and a DRS4 ASIC, has been implemented. An energy resolution around 3% FWHM has been measured at a gamma energy of 662 keV; at 200 keV the average energy resolution is 6.5%, decreasing to ≲ 2% at energies above 1 MeV. A 3-D spatial resolution of ≈ 2 mm is achieved in each dimension.

Energy Spectrum Correction and Carrier Mobility Calculation of CdZnTe Pixel Detector Based on the Depth of Interaction

An experimental method was described to calibrate the depth of interaction (DOI) of high-energy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> -ray in large-size thick pixel CdZnTe detector. The experimental results indicated that the ratio of the cathode and anode amplitude strongly showed linear relationship with the DOI of incidence <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> -rays and the detector. The correlation coefficients were optimized. Based on the experimental calibration of DOI, the photoelectric effect signals from the depth range where the energy resolution and the photopeak location are high were screened out to draw the energy spectrum. In this way, the energy resolution of the CdZnTe pixel detector was improved to 0.707% for <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs@662 keV. Further combined with the anode rise time, the electron mobility of CdZnTe in 2-D (1 mm accuracy) and 3-D (2 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 2 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 1 mm accuracy) scales was calculated. At different pixel positions and depths in CdZnTe detector, the electron mobility varies between 1000 and 1150 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> ).

Subpixel Sensing for Charge-Sharing Events in Pixelated CdZnTe Detectors

Subpixel Sensing for Charge-Sharing Events in Pixelated CdZnTe Detectors

Quantitative Measurement and Development of Back-end Processing System-on-Chip for the Pixelated CdZnTe Detector

Quantitative Measurement and Development of Back-end Processing System-on-Chip for the Pixelated CdZnTe Detector

Charge Sharing and Charge Loss in High-Flux Capable Pixelated CdZnTe Detectors.

Cadmium zinc telluride (CdZnTe) detectors are known to suffer from polarization effects under high photon flux due to poor hole transport in the crystal material. This has led to the development of a high-flux capable CdZnTe material (HF-CdZnTe). Detectors with the HF-CdZnTe material have shown promising results at mitigating the onset of the polarization phenomenon, likely linked to improved crystal quality and hole carrier transport. Better hole transport will have an impact on charge collection, particularly in pixelated detector designs and thick sensors (>1 mm). In this paper, the presence of charge sharing and the magnitude of charge loss were calculated for a 2 mm thick pixelated HF-CdZnTe detector with 250 μm pixel pitch and 25 μm pixel gaps, bonded to the STFC HEXITEC ASIC. Results are compared with a CdTe detector as a reference point and supported with simulations from a Monte-Carlo detector model. Charge sharing events showed minimal charge loss in the HF-CdZnTe, resulting in a spectral resolution of 1.63 ± 0.08 keV Full Width at Half Maximum (FWHM) for bipixel charge sharing events at 59.5 keV. Depth of interaction effects were shown to influence charge loss in shared events. The performance is discussed in relation to the improved hole transport of HF-CdZnTe and comparison with simulated results provided evidence of a uniform electric field.

Performance of Larger-Volume 40 × 40 × 10- and 40 × 40 × 15-mm³ CdZnTe Detectors

The dimension of 20 × 20 × 15 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> has been selected as standard pixelated CdZnTe detector size for many years. Energy resolution of 0.31% full-width at half-maximum (FWHM) at 662 keV for single-pixel events was achieved in one of the best direct-attachment detectors based on VAD_UM v2.2 application-specific integrated circuit (ASIC). Energy resolution under 0.40% was obtained for most of the direct-attachment detectors. The consistent good performance drives the desire to build larger crystals. The increment of size is beneficial in many aspects, such as higher detection efficiency, better imaging performance, better spectroscopic resolution, and potentially lower cost. This article presents the performance of recently delivered 40 × 40 × 10 and 40 × 40 × 15-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> CdZnTe crystals manufactured by Redlen Technology and Kromek Group. The result will be analyzed and the feasibility of mass producing 40 × 40-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> crystals will be discussed.

Identification of single-pixel incomplete charge collection events by using cathode waveform in pixelated CdZnTe detectors

Common grid can steer the electron cloud toward anode pixels to improve the charge collection efficiency in pixelated CdZnTe detectors. Due to insufficient steering and the absence of pixels on the outside of the edge pixels, the whole electron cloud cannot be collected completely by anode pixels for some events, resulting in the incorrect energy reconstruction by the deficient anode signal. Since single-pixel events are mainly used to obtain an excellent spectrum, incomplete charge collection events increase incorrect low-energy counts. However, the cathode signals are intact for incomplete charge collection events, and the cathode slope is related to energy deposition. Therefore, incomplete charge collection on the anode electrode can be identified by using the cathode slope to anode amplitude ratio (S/A). The experiments were performed on a pixelated 20 × 20 × 15 mm3 CdZnTe detector manufactured by Redlen Technologies. Incorrect low-energy counts are significantly decreased by rejecting single-pixel incomplete charge collection events, and the photopeak counts are improved by reconstructing the single-pixel incomplete charge collection events.

Development of a multi-detector readout circuitry for ultrahigh energy resolution single-photon imaging applications

In this paper, we present the design and preliminary performance evaluation of a novel external multi-channel readout circuitry for small-pixel room-temperature semiconductor detectors, namely CdZnTe (CZT) and CdTe, that provide an excellent intrinsic spatial (250 and 500μm pixel size) and an ultrahigh energy resolution (∼1% at 122 keV) for X-ray and gamma-ray imaging applications. An analog front-end printed circuit board (PCB) was designed and developed for data digitization, data transfer and ASIC control of pixelated CZT or CdTe detectors. Each detector unit is 2 cm × 2 cm in size and 1 or 2 mm in thickness, being bump-bonded onto a HEXITEC ASIC, and wire-bonded to a readout detector module PCB. The detectors’ front-end is then connected, through flexible cables of up to 10 m in length, to a remote data acquisition system that interfaces with a PC through USB3.0 connection.We present the design and performance of a prototype multi-channel readout system that can read out up to 24 detector modules synchronously. Our experimental results demonstrated that the readout circuitry offers an ultrahigh spectral resolution (0.8 keV at 60 keV and 1.05 keV at 122 keV) with the Cd(Zn)Te/HEXITEC ASIC modules tested. This architecture was designed to allow easy expansion to accommodate a larger number of detector modules, and the flexibility of arranging the detector modules in a large and deformable detector array without degrading the excellent energy resolution.

Intrinsic photopeak efficiency measurement and simulation for pixelated CdZnTe detector

Intrinsic photopeak efficiency of pixelated CdZnTe detectors is an important property for quantitative measurements and analysis, especially for detection and characterization of special nuclear material (SNM). The intrinsic photopeak efficiency of a single 20 mm × 20 mm × 15 mm CdZnTe detector from 59 keV to 2614 keV are measured in experiment. However, compared to the simulated ideal intrinsic photopeak efficiency, the measured efficiency showed deficit. A detailed simulation model including gamma-ray interaction physics, readout electronic as well as signal reconstruction process is built to explain this observed efficiency deficit. The simulated efficiencies of the new model match with experimental results well after taking these factors into account. Efficiency loss due to the guard ring and anode side inaccurate reconstruction are the most important factors. Other miscellaneous efficiency loss mechanisms are discussed too.

The mechanism of edge effect in CdZnTe pixel detector and an improved design of pixel sizes of anode

In this paper, the mechanism of the edge effect in Cadmium Zinc Telluride (CZT) pixel detectors is studied, and the energy spectrum performance and the charge collection efficiency of peripheral pixels with reduced sizes are analysed by computer-aided simulation and measurement. The results show that the distribution of the weighting potential, which is determined by the electrode structure, has significant influence on the edge effect. Reducing the sizes of peripheral pixels can optimize the weighting potential and improve the detection performance. A detector with reduced peripheral pixel sizes is prepared. We build a gamma imaging system using this kind of detector, test the spectrum response to 137Cs@662 keV and image a M10 nut under the collimated 241Am@59.5 keV . The measured spectrum of the edge/corner/intermediate pixels is consistent with the simulated spectrum and the obtained countingtype gray-scale image of the nut clearly shows its shape and sizes. These results prove that reducing peripheral pixel sizes has practical value in gamma imaging technology.

Performance optimization of pixelated CdZnTe detector readout by analog ASIC using cathode waveform

Traditional analog ASICs use filtered signals passing a constant threshold to obtain the trigger time of induced signals from a pixelated CdZnTe detector. However, poor signal-to-noise ratio (SNR) and variable leading edge of the cathode signal cause higher trigger threshold and lower timing resolution. In this study, only the anode channels of analog ASICs are used to trigger the system, and the corresponding cathode waveforms are sampled by an external high-speed analog-to-digital converter (ADC). Rise time and amplitude of the cathode signal can be obtained by digital signal processing (DSP), which achieves a good timing resolution and eliminates the ballistic deficit. The test was carried out on a system with 2 × 2 detector array. The reconstructed depths are improved by using cathode waveforms, and the optimal energy resolution of 0.54% full width at half-maximum (FWHM) at 662 keV for all single-pixel events has been observed in one detector.

A Prototype VP-PET Imaging System Based on Highly Pixelated CdZnTe Detectors.

We investigated a prototype virtual-pinhole positron emission tomography (PET) system for small-animal imaging applications. The PET detector modules were made up of 1.3 mm lutetium-yttrium oxyorthosilicate (LYSO) arrays, and the insert detectors consisted of 0.6 mm pixelated cadmium zinc telluride (CdZnTe). To validate the imaging experiment, we did a Monte Carlo simulation for the virtual-pinhole PET (VP-PET) system in the Geant4 Application for Emission Tomography (GATE). For a point source of 22Na with a 0.5 mm diameter, the filtered back-projection algorithm-reconstructed PET image showed a resolution of 0.7 mm full-width-at-half-maximum. The system sensitivity was 0.46 cps/kBq at the center of the field view of the PET system with a source activity of 0.925 MBq and an energy window of 350 to 650 keV. A rod source phantom and a Derenzo phantom with 18F were also simulated to investigate the PET imaging ability. GATE simulation indicated that sources with 0.5 mm diameter could be clearly detected using 0.6 mm pixelated CdZnTe detectors as insert devices in a VP-PET system.

Time-Encoded Gamma-Ray Imaging Using a 3-D Position-Sensitive CdZnTe Detector Array

The gamma-ray response of thick pixelated CdZnTe detectors varies spatially but is relatively constant as a function of time. Detector irregularities arise from flaws in the material, electrode fabrication, and gaps between the detectors in an array. These nonuniformities generate artifacts in coded aperture images that can obscure a source of interest. This article presents the theory, design, and demonstration of a 3-D CdZnTe-based time-encoded imager. Experimental results show the gamma-ray images of point-like and extended 57Co and HEU-metal sources formed using a spatially nonuniform 3-D CdZnTe detector array and moving coded aperture mask. Image signal-to-noise ratio (SNR) for a point source was within 10% of its statistical limit at 250k counts.

Feasibility study of Compton imaging for PGAA

We are developing a one-sided tomographic method for imaging prompt gamma rays, without moving the source, sample, or detector, in a standard prompt gamma activation analysis setting. Dual-plane pixelated CdZnTe detectors developed from a commercial system with external timing synchronization electronics are used to measure prompt gamma rays that are scattered in the first plane and subsequently absorbed in the second plane. Tomographic images have been reconstructed from the 2.2 MeV gamma rays from H in 3 mm plastic balls using inter-plane events for the first time, demonstrating the feasibility of spatially resolving mm scale H features.