CdZnTe Detectors Research Articles

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.

Study on the Performance of a Planar CdZnTe Detector with Guard-Ring

The guard-ring of a CdZnTe (CZT) detector can reduce surface leakage current and improve charge collection efficiency. In this paper, COMSOL simulation software is adopted to evaluate the influence of the weighting potential and electric field of a CZT detector with guard-ring under different geometries. We used different bias on the guard-ring and analyzed the side effects. Based on the simulation results, an actual CZT detector with guard-ring used CZT wafers with dimensions of 5 × 5×3 mm3. The ideal distance and width of the guard-ring enhances the weighting potential and weighting field. The detector properties, such as current–voltage curves, energy resolution, and intrinsic detection efficiency were also compared. The results from these measurements provided an optimal guard-ring structure to achieve higher performance of room-temperature CZT radiation detectors.

Characterization of the Uniformity of High-Flux CdZnTe Material.

Since the late 2000s, the availability of high-quality cadmium zinc telluride (CdZnTe) has greatly increased. The excellent spectroscopic performance of this material has enabled the development of detectors with volumes exceeding 1 cm3 for use in the detection of nuclear materials. CdZnTe is also of great interest to the photon science community for applications in X-ray imaging cameras at synchrotron light sources and free electron lasers. Historically, spatial variations in the crystal properties and temporal instabilities under high-intensity irradiation has limited the use of CdZnTe detectors in these applications. Recently, Redlen Technologies have developed high-flux-capable CdZnTe material (HF-CdZnTe), which promises improved spatial and temporal stability. In this paper, the results of the characterization of 10 HF-CdZnTe detectors with dimensions of 20.35 mm × 20.45 mm × 2.00 mm are presented. Each sensor has 80 × 80 pixels on a 250-μm pitch and were flip-chip-bonded to the STFC HEXITEC ASIC. These devices show excellent spectroscopic performance at room temperature, with an average Full Width at Half Maximum (FWHM) of 0.83 keV measured at 59.54 keV. The effect of tellurium inclusions in these devices was found to be negligible; however, some detectors did show significant concentrations of scratches and dislocation walls. An investigation of the detector stability over 12 h of continuous operation showed negligible changes in performance.

Study on the Energy Spectrum Response of a CdZnTe Detector

In order to study the energy spectrum response of a CdZnTe detector, we firstly measured the temperature dependence and the bias dependence of the main characteristic parameters for both a quasi-hemispherical detector and a CAPture™ plus detector. Secondly, we designed a low-noise readout circuit for the CdZnTe detector and measured the noise. Finally, we evaluated the energy spectrum response of the detector to different radioactive sources at different temperatures by connecting the detector to the readout circuit. The research showed that both detectors had low leakage current and junction capacitance, as well as good stability in temperature and bias; the quasi-hemispheric detector had a smaller leakage current and junction capacitance compared to the CAPture™ plus detector; under zero input capacitor, the noise of the readout circuit was 612e, with the noise slope being 5.44e/pF; at room temperature(20 °C), the energy resolutions of the detector reached 3.84% and 1.36% for X-rays from 241Am (59.5 keV) and gamma-rays from 137Cs (662 keV), respectively; the signal-noise ratio of the output signal reached 31:1 with the rise time being 90 ns; at low temperature, the energy resolution reached 3.41% for the X-rays from 241Am (59.5 keV); the detector achieved an excellent spectrum response and was able to distinguish clearly the energy peaks of 152Eu and 226Ra.

The mechanism of defects effect on carrier transport by employing multi-wavelength sub-bandgap lights on CdZnTe crystal

The effect of different point defects in CdZnTe (CZT) crystals on carrier transport performance were discussed. The energy level and concentration of the traps were determined by the photo-induced current transient spectroscopy (PICTS). By adding the infrared light illumination into the PICTS experiment, the effect of sub-bandgap light on defects was observed. Meanwhile, several DC photoconductive experiments were done under different wavelength lights were used to explore the effect of the different defects on carrier transport properties of CZT crystals. Furthermore, the energy spectrum test was used to investigate the effect of sub-bandgap light on the energy resolution of CZT detectors. The data results showed that sub-bandgap light could effectively change the ionization state of defects. The doubly ionized Te antisite (TeCd2+) had the greatest effect on mobility-lifetime product(μτ) of electrons owing to its stronger ability of capturing electrons and energy level located around the midgap. While the secondary ionized Cd vacancy (VCd2−), as considered as a hole trap, had the greatest effect on μτ-product of holes. Consequently, choosing suitable infrared light can improve the detection performance of CZT crystal effectively.

Simulation of Coplanar Proximity Charge Sensing Electrodes in CZT Detectors

The concept of applying proximity charge sensing electrodes to semiconductor radiation detectors is a novel technique that has distinct advantages over directly deposited electrodes. This paper evaluates the application of proximity charge sensing onto a CdZnTe (CZT) semiconductor crystal using ANSYS Maxwell simulation software to calculate the weighting potential across the detector depth, the weighting potential across the detector width, and the electric field (E) generated inside the detector volume for multiple designs. To accomplish this goal, several variables are studied in the simulated designs: (1) a high resistivity thin-film material that is applied to the detector proximity surface from the anode side, (2) an appropriate metal that acts as an Ohmic contact to dissipate generated charges on the CZT anode side, and finally (3) an insulating layer (dielectric) to isolate the CZT detector body from the proximity electrodes. The results of the generated weighting potentials for both directly deposited electrodes and proximity-sensing electrodes, having the same electrode width and pitch, have been quantitatively compared using a figure of merit (FOM). The FOM compares the weighting potential created by each simulated design for weighting potential uniformity and weighting potential similarity.

Effect of Deep-Level Defects on the Performance of CdZnTe Photon Counting Detectors.

The effect of deep-level defects is a key issue for the applications of CdZnTe high-flux photon counting devices of X-ray irradiations. However, the major trap energy levels and their quantitive relationship with the device’s performance are not yet clearly understood. In this study, a 16-pixel CdZnTe X-ray photon counting detector with a non-uniform counting performance is investigated. The deep-level defect characteristics of each pixel region are analyzed by the current–voltage curves (I–V), infrared (IR) optical microscope photography, photoluminescence (PL) and thermally stimulated current (TSC) measurements, which indicate that the difference in counting performance is caused by the non-uniformly distributed deep-level defects in the CdZnTe crystals. Based on these results, we conclude that the CdZnTe detectors with a good photon counting performance should have a larger and Cd vacancy-related defect concentration and a lower A-center and Tei concentration. We consider the deep hole trap Tei, with the activation energy of 0.638–0.642 eV, to be the key deep-level trap affecting the photon counting performance. In addition, a theoretical model of the native defect reaction is proposed to understand the underlying relationships of resistivity, deep-level defect characteristics and photon counting performance.

Adhesion-free Bridgman growth of AlSb

AlSb crystals as a dual-carrier compound semiconductor have a great potential application for the room-temperature detectors, comparative to high-purity Ge and CdZnTe detectors. However, the single crystal growth of AlSb suffers from the high reactivity of Al, which causes strong adheres of the crystal to the crucible walls. In this work, an in-situ synthesis method was developed using a pBN crucible sealed in a quartz crucible. Meanwhile, the reaction of Al and Sb was settled under the melting point with two stages for solid reaction and liquid reaction in turn. The cycles of rapid directional solidification and refusion were implemented for homogenization. Bulk crystals of AlSb with large grain size 5 × 5 mm2 were obtained from the vertical Bridgman method. The results of the XRD and EDS test showed a single zinc blende structure phase with a nearly stoichiometric atomic ratio of 49.7: 50.3 (Al: Sb). AlN was observed between AlSb/BN interface during the crystals with normal overheating synthesis. The sticking mechanism was ascribed to the increase of the adhesion energy with PBN crucible after the production of AlN. The chemical reactivity of Al has been fully inhibited during the Bridgman growth of AlSb.

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.

Prototype test of a portable thyroid dose monitoring system using gamma-ray spectrometers

In case of a severe nuclear accident, radioiodine monitoring of the thyroid must be carried out for a large number of residents and emergency workers. In the present study, we developed a portable thyroid dose monitoring system consisting of two independent gamma-ray spectrometers and a radiation shield, which could be used under high background radiation conditions. Two types of prototype thyroid monitors were realized; one utilizing CdZnTe detectors for emergency workers and the other employing LaBr3(Ce) detectors for members of the public. As a result of the performance test, it was confirmed that both monitors effectively assess thyroid equivalent doses of <10 mSv during a 150 s measurement, even under high background radiation conditions.

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.

A novel asymmetrical peak broadening feature for a CdZnTe detector response function modeling using PHITS particle and heavy ion transport simulation code

In this paper we present the results of a detector response function modeling using a new version of PHITS Particle and Heavy Ion Transport Code with an advanced feature for asymmetrical peak broadening. Tests are performed on a room temperature medium resolution 500 mm3 CdZnTe detector of a quasi-hemispherical design with validation on a set of point source gamma-ray spectra with a range of energies from 59 keV up to 1332 keV. Performance assessment is conducted with respect to built-in spectra broadening capabilities using as default Gaussian as well as an asymmetrical peak shape models. Results of our study indicate that up to 200 keV energy range where peak asymmetry is not severe a default Gaussian peak shape broadening can be used. Beyond 200 keV the degree of CZT photopeak asymmetry becomes significant and requires an asymmetrical peak shape broadening. This asymmetry affects the low-energy side of the photopeaks and has a non-linear behavior with energy. We find that with a built-in feature of a tailed peak shape model introduced in the new version of PHITS code (ver. 3.10) asymmetrical peak shapes of CZT detectors can be approximated. The analysis routines and mathematical formalism are described in detail and results are presented.

A modified diffusion model for characterization of real I–V curve and charge collection efficiency of CdZnTe detectors

In the present paper, a modified diffusion model describing I–V curve of CZT detectors is proposed and used to characterize the electric field distribution. The detectors are considered to be fully depleted at high bias and incompletely depleted at low bias. For incompletely depleted condition, the inner electric field of CZT detector was separated into depletion region and neutral region. The neutral region was usually neglected in the traditional model. Two fitting parameters VD and α are included in the proposed model, which can be fitted from I–V curve. The model is also extended to describe the electric field distribution. Furthermore, we figured out the calculated charge collection efficiency equations, which are consistent with experimental results. Experimental data from planar detectors indicate that the model could describe I–V curve precisely throughout a broad bias range. Serious electric field distortion in some planar detectors is attributed to the Schottky barrier. The relationship between charge collection efficiency (CCE) and bias voltage in these devices is not consistent with the Hecht equation. The difference may arise from the non-uniform electric field. The stronger effect of the Schottky barrier on electron collection efficiency is found in pixel detectors according to the non-uniform weighting potential, because for pixel detectors the induced charge is mainly contributed by the near anode region, which has low electric field under high cathode barrier.

Effects of the inductively coupled Ar plasma etching on the performance of (111) face CdZnTe detector

The effect of inductively coupled Ar plasma etching (ICP-Ar) instead of traditional Br–MeOH etching on the performance of CdZnTe detectors was studied. The optimal ICP etching parameters were determined by experiments. The XPS results indicated that the surface composition of CdZnTe etched by ICP-Ar had no obvious TeOx peak and was closer to stoichiometric ratio than that etched by Br–MeOH. The leakage current of ICP-Ar etched CdZnTe surface was significantly reduced, and the detection performance with the energy resolution was improved by 12%, as compared to Br–MeOH etched sample. Moreover, ICP-Ar etching abandoned the drawbacks of Br–MeOH by producing corrosive gases, making it safer and more environmentally friendly. Therefore, inductively coupled Ar plasma etching is expected to replace Br–MeOH etching and become an effective method for CdZnTe surface treatment.

Investigation on X-Ray Photocurrent Response of CdZnTe Photon Counting Detectors.

Counting rate is an important factor for CdZnTe photon counting detectors as high-flux devices. Until recently, there has been a lack of knowledge on the relationship between X-ray photocurrent response and the photon counting performance of CdZnTe detectors. In this paper, the performance of linear array 1 × 16-pixel CdZnTe photon counting detectors operated under different applied biases is investigated. The relation between experimental critical flux and applied bias show an approximate quadratic dependence, which agrees well the theoretical prediction. The underlying relationship among X-ray photocurrents, carrier transport properties, and photon counting performance was obtained by analyzing X-ray current–voltage and time current curves. The typical X-ray photocurrent curve can be divided into three regions, which may be explained by the photoconductive gain mechanism and electric field distortion characteristics. To keep CdZnTe photon counting detectors working in a “non-polarized state”, the applied bias should be set on the left side of the “valley region” (high bias direction) in the X-ray I-V curves. This provides an effective measurement for determining the proper working bias of CdZnTe detectors and screening photon counting detector crystals.

An analysis of defects induced by high dose γ-ray irradiation and their influence on the charge transport properties in CdZnTe detectors

Radiation damage effects were important issues for the development of CdZnTe detectors in space-exploration field applications. In this study, we explored the radiation damage mechanisms on CdZnTe crystals based on the Kinchin-Pease model. The type and evolution of irradiation induced defects under different gamma ray doses were investigated by the photoluminescent technique. Laser beam induced transient current measurement was applied to investigate the influence of defects on the charge transport property. Our results demonstrated that the radiation induced defects accumulated negative space charges, which significantly distorted the electric field distribution and reduced electron mobility by enhancing the ionized impurities scattering effect. By correlating the results with 241Am gamma ray radiation response spectra measurement, the effects of radiation damage on the detector performance were discussed.

Effect of deep level defects on space charge distribution in CdZnTe crystals

CdZnTe recently emerged as a leading semiconductor crystal for fabricating room-temperature x- and gamma-ray imaging detectors, due to its excellent energy resolution and sensitivity. However, its wide deployment is hampered by the low availability of high-quality CdZnTe crystals. As-grown CdZnTe crystals generally encounter the problems arising from the impurities and defects, especially deep level defects. The presence of impurities and defects leads to severe charge trapping, which significantly affects detector performance. Especially for high counting rate imaging detector used in medical imaging and tomography, the accumulation of space charge at deep levels significantly deforms the electric field distribution and subsequently reduces the charge collection efficiency. Therefore, a considerable interest is focused on the investigation of the space charge accumulation effect in CdZnTe crystal, which is the key factor to improve the performance of high counting rate imaging detector. Thus, the goal of this work is to investigate the effects of deep level defects on space charge distribution and internal electric field in CdZnTe detector. In order to reveal the major problem therein, Silvaco TCAD technique is used to simulate the space charge and electric field distribution profile in CdZnTe detector with considering the typical deep level defects &lt;inline-formula&gt;&lt;tex-math id="Z-20201111102325-1"&gt;\begin{document}$ \rm Te_{Cd}^{++} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20200553_Z-20201111102325-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20200553_Z-20201111102325-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;in CdZnTe crystals with activation energy of &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;v&lt;/sub&gt; + 0.86 eV and concentration of 1 × 10&lt;sup&gt;12&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt; at room temperature. The simulation results demonstrate that the Au/ CdZnTe /Au energy band tilts intensively with the increase of applied bias, which makes the deep level ionization fraction increase. The space charge concentration also increases in the crystal. Meanwhile, the dead layer of electric field distribution decreases, which is of benefit to the carrier collection of CdZnTe detector. In addition, under the premiseof the high resistivity of CdZnTe crystal, the reduction of deep level defect concentration located at &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;v&lt;/sub&gt; + 0.86 eV can narrow the internal dead layer moderately. The deep level defect located at &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;v&lt;/sub&gt; + 0.8 eV can also reduce the space charge concentration near the cathode, which flattens the electric field distribution with narrower dead layer, thus significantly improving the carrier collection efficiency of CdZnTe detector. These simulation results will provide meaningful theoretical guidance for further optimizing the CdZnTe crystal growth, device design and fabrication.

Application of Cdznte Detectors for Control of Initial Enrichment of Fresh Nuclear Fuel at Refueling

The task of determining the initial enrichment of nuclear fuel (NF) at nuclear power plants in real time is relevant for both economic indicators of operation of the power unit and for all components of nuclear safety (nuclear safety, security and safeguards). The technology used by nuclear fuel cycle companies is not suitable for use at nuclear power plants. It is proposed to use a measuring system based on CdZnTe semiconductor detectors designed to control the burn-up depth of nuclear fuel for estimating the initial enrichment of fresh nuclear fuel in fuel assemblies (FA). The initial enrichment is estimated with help of the gamma radiation of uranium isotopes of fresh nuclear fuel. A distinctive feature of the system that has been developed is the use of passive tomography technology to take into account the self-absorption of radiation in the FA. The promise of the proposed method for estimating initial enrichment is confirmed by experimental measurements and simulation results. It is shown that the system developed makes it possible to determine the enrichment of nuclear fuel, taking into account its profiling by the volume of FA. Also, the results of measurements confirmed the possibility of using the proposed technical solutions for CdZnTe detectors to create portable monitoring systems for special nuclear material in the field. The solution of this task is of great importance for national security.

The remote-controlled spectrometric system for searching and characterization of high-level radioactive waste

In 2003, the work on environmental rehabilitation of the former coastal technical base of the Russian Navy in Gremikha town, with technical and financial support of the State Atomic Energy Corporation “Rosatom” and the French Alternative Energies and Atomic Energy Commission had begun. In 2017, the task of searching, characterization and packaging in transport containers for export to the long-term storage of high-level waste containing 152Eu was set as part of this program. These wastes were presented as fragments of reactor control and safety system rods of first generation nuclear submarine reactors and were placed in temporary storage containers mixed with other radioactive waste and construction garbage on the territory of the Gremikha branch of the North-West Center “SevRAO”. To accomplish the task, the spectrometric system “Gamma-Pioneer” was developed and made at National Research Center “Kurchatov Institute”. The system consist of a collimated spectrometric gamma-ray CdZnTe detector 500 mm3 volume, a dosimeter for continuous measurement of ambient equivalent dose rate at the instrument location and a color video camera with zoom lens. The device is placed in a protective frame and installed as an attachment implements on robot Brokk-90. Communication between the measuring system and the control computer is carried out using USB extender and 100 meters cable. The video camera is connected to the control computer via Ethernet. To search for the most active sources of gamma radiation, an equivalent dose rate calculation program using the results of measured instrumental spectra was developed. The characterization of the extracted fragments was carried out by activity evaluation of the main radionuclides polluting the samples under investigation. The paper describes design and operating principles of the system, as well as methods for the equivalent dose rate calculation and the activity evaluation of the fragments of reactor control and safety system rods based on spectral measurements obtained using the developed radiometric system