Cadmium zinc telluride (CZT) single crystals, composed of a ternary system of Cd, Zn, and Te, are among the best-performing semiconductor materials for room-temperature nuclear radiation detection. From a materials science perspective, CZT single crystals are a type of solid solution that evolves from CdTe crystals. CdTe can be considered a face-centered cubic structure composed of Cd atoms, with a Te atom face-centered cubic nested within it by translating along the body diagonal direction by one quarter of the diagonal length. By replacing some of the Cd atoms with Zn atoms, CZT single crystals are formed, meaning CZT is a solid solution formed by CdTe and ZnTe. The solubility is usually represented by the subscript x, that is, Cd1-xZnxTe. There are two common values of x: when x=0.04, the lattice constant of CZT perfectly matches that of the infrared detection material mercury cadmium telluride (MCT), making it an excellent infrared detector epitaxial substrate; when x=0.1, it is considered detector-grade CZT.
Semiconductor detectors typically operate based on the photoelectric effect, where photons striking the semiconductor cause the ionization of electrons within the semiconductor, forming electron-hole pairs, or non-equilibrium carriers. Under the influence of an applied bias voltage, these carriers drift towards the detector electrodes, generating an electrical signal. By amplifying this signal, photon detection can be achieved. Unlike traditional scintillator detectors, semiconductor detectors work by directly converting light into electrical signals, a process known as direct conversion. In contrast, scintillator detectors usually convert high-energy light signals into visible light signals, which are then converted into electrical signals by a photomultiplier tube or a visible light detector, a process known as indirect conversion. Therefore, generally speaking, semiconductor detectors are more efficient and sensitive compared to scintillator detectors.
The working principle of Cadmium Zinc Telluride detector
The efficiency of photon absorption is a crucial factor affecting the detection efficiency. Cadmium zinc telluride (CZT) has a relatively high average atomic number, which contributes to its strong absorption of photons. Additionally, the electron mobility in CZT is about 1000 cm²/Vs, and the lifetime is approximately on the order of 10^-5ns. This ensures that electrons have a fast drift velocity and a long drift length, resulting in a quick response time for the detection signal, roughly on the order of a few to several tens of nanoseconds. The energy linearity of the signal is also relatively good.
Due to its material characteristics, cadmium zinc telluride (CZT) crystals have high detection efficiency for low to medium energy radiation, and also boast high energy and spatial resolution. Energy resolution refers to the ability to differentiate between radiations of closely similar energies. For example, CZT demonstrates superior energy resolution compared to other room-temperature semiconductor materials available on the market, with values such as 3.5% @ 59.5 keV for 241Am, 2.5% @ 122 keV for 57Co, and 1.0% @ 662 keV for 137Cs.
The temperature response characteristics primarily refer to the stability of cadmium zinc telluride (CZT) detectors in maintaining good performance across different operating temperatures. Due to the intrinsic properties of the crystal material, significant photopeak position shifts in CZT detectors are only observed at temperatures above 60°C, indicating excellent stability.
04 . High Spacial Resolution
Cadmium zinc telluride (CZT) crystals have a high detection efficiency for low-energy X and γ rays, and their spatial resolution (indicating the smallest size unit that an image can distinguish) is also high. Particularly in the area of photon counting detectors, lithography technology can be used to create very small pixels, with the minimum spatial resolution reaching up to 70μm, achieving a leading level internationally.
However, the main factor limiting the development of cadmium zinc telluride (CZT) materials is the difficulty of its preparation. When growing CZT single crystals by the melt method, the CZT melt exhibits high kinetic viscosity (about 10 mPa·s) and extremely low thermal conductivity (about 1 W/m·K). This makes it difficult for heat within the melt to be conducted during crystal growth, and for the material to homogenize. Near the melting point, the high vapor pressure of Cd (1.3 atm) leads to the formation of numerous Cd vacancy defects within the crystal, lowering the material's resistivity. The extremely low critical shear stress (0.3 Mpa) makes the material prone to cracking. CdTe-ZnTe can stably exist in any ratio of solid solution, leading to the easy generation of uneven Zn distribution within CZT. Due to these widespread difficulties, achieving a high single-crystal yield is the primary challenge in the preparation of CZT materials. Imdetek not only mastered the single crystal growth technology but also conducted systematic defect engineering research on CZT crystals. From an industrial perspective, the company has managed to control and balance various defects within CZT, significantly improving the yield of finished products. Under the leadership of Professor Jie Wanqi, the company's technical team overcame the difficulties of preparation, adopted an improved Bridgman method for the batch production of detector-grade CZT crystals, placing their single-crystal yield at an internationally leading level.
| The Parameter of Semiconductor Detector Materials | |||||||||
| Material | Density （g·cm-3) | Maximum Atomic Number | Resistivity （Ω·cm） | Bandgap Width （eV） | Average Ionization Energy （eV） | Electron Mobility (cm2·V-1·s-1) | Electron Lifetime （μs） | Hole Mobility (cm2·V-1·s-1) | Hole Lifetime （μs） |
| CdZnTe | 5.75 | 52 | ＞1010 | 1.55-1.6 | 4.6 | 1100-1600 | 1-10 | 30-80 | 0.1-1 |
| CdTe | 5.85 | 52 | 109-1011 | 1.44 | 4.43 | 800-1200 | 0.33-4 | 90 | 2 |
| Diamond | 3.5 | 6 | >1016 | 5.5 | 13.6 | 1800 | - | 1200 | - |
| CdMnTe | 5.81 | 52 | 1010 | 1.59-1.725 | 5 | >1000 | 1.17 | 60 | 1 |
| TIBr | 7.56 | 81 | 1010-1011 | 2.68 | 5.85-6.5 | 10-6-10-3 | 10-5-10-4 | ||
| Si | 2.33 | 14 | 105 | 1.12-1.16 | 3.62-3.76 | 1350 | 103-104 | 480 | 500 |
| Sic | 3.2 | 14 | 5.1*1010 | 3.26 | 7.4 | 450 | - | 115 | - |
| HPGe | 5.35 | 32 | 50 | 0.27-0.746 | 2.96 | 3900 | >103-104 | 1900 | 2*(103-104) |
| BN | 2.27-3.487 | 7 | 5*1010-1016 | 6-6.5 | - | 10-4 | 10-4 | ||
| GaAs | 5.32 | 33 | 107-109 | 1.43 | 4.51 | 8000 | 400 | ||
| InSb | 5.78 | 51 | 4.6*103 | 0.165 | 3.5 | 78000 | 750 | ||
| Amorphous Se | 4.81 | 34 | 1012-1014 | 2-2.3 | 2.3-2.45 | 2-7 | 10-103 | 0.13-0.14 | 10-500 |
| HgI2 | 6.4 | 80 | 1013 | 2.1-2.13 | 4.3 | 100 | 400 | 4 | 10 |
| PbI2 | 6.2 | 82 | 1011-1012 | 2.27 | 4.9 | 8 | 0.01-0.1 | 2 | 0.01-0.1 |
| Bil3 | 5.8 | 83 | (2-5)*109 | 2 | - | 600 | 20 | - | |
The evolution of X-ray imaging has progressed through several stages, from the earliest photographic plates to the combination of film/screen systems, and now to the current digital X-ray imaging technologies. Since the late 1960s and early 1970s, with the rapid development of computer and microelectronics technology, the global wave of digitalization, along with computer networks and communication technologies, has had a widespread and profound impact on X-ray imaging equipment.
In the 1980s, Fujifilm from Japan introduced digital X-ray imaging technology, known as Computed Radiography (CR). CR technology uses an imaging plate instead of the traditional film/screen system to record X-rays. The imaging plate is then stimulated with a laser, and a specialized reading device reads the digital signals stored in the imaging plate. The signals are then processed and imaged by a computer. By the 1990s, Direct Digital Radiography (DR) technology emerged. DR technology's detectors can quickly convert detected X-ray signals directly into digital signals for output, eliminating the need for the laser scanning and specialized reading devices used in CR.
The concept of photon counting has played a significant role in the detection of weak and faint light since its introduction in the 1980s. In recent years, with the rapid development of X-ray photon counting detectors and advances in high-speed processing electronics, it has become possible to achieve X-ray imaging with high count rates in short times, making it a hot topic in the field of X-ray imaging research. Photon counting X-ray detectors can eliminate low-energy noise from imaging results by setting electronic thresholds to filter out lower-energy pulses. By setting multiple electronic thresholds, the detectors can discriminate the pulse signals generated by each incident photon's interaction with matter, identify the energy information of the incident photons, and accumulate it in different energy ranges. This approach allows for counting in different energy spectra of X-rays and directly obtaining imaging results for different energy ranges. It essentially introduces spectral information into traditional X-ray imaging, hence it's also known as "X-ray color imaging."
Compared to traditional charge integration detectors, photon counting detectors offer numerous advantages. Firstly, they can eliminate the interference of low-energy noise on imaging, effectively improving image quality. Additionally, by incorporating spectral information, a single scan can yield imaging results for different energy ranges, upgrading X-ray imaging from "black and white" to "color" and achieving functional imaging for material identification. Moreover, multi-spectral photon counting X-ray imaging technology is significant for improving image quality, signal-to-noise ratio, and detection accuracy, as well as reducing radiation doses, marking the future trend in X-ray imaging development.
The most critical aspect of photon counting imaging technology lies in the selection of detector materials and device fabrication. Among detector materials, semiconductor detector materials have a significant advantage in directly converting X-ray signals into electrical signals, compared to scintillator materials. Cadmium zinc telluride (CZT), in comparison with other semiconductor radiation detection materials, offers advantages such as the ability to operate at room temperature (due to its wide bandgap), high count rates, high energy resolution, high spatial resolution, and high detection efficiency for high-energy γ-rays, making it considered the best semiconductor radiation detection material in terms of overall performance. Additionally, CZT has a high electron mobility, a high signal-to-noise ratio, and a large absorption coefficient for 20keV-200keV, which aligns perfectly with the requirements for photon counting imaging. Decades of research have shown that CZT detectors are the optimal choice for realizing multi-spectral photon counting X-ray detectors. Currently, worldwide, CZT-based photon counting CT, ore sorting equipment, security inspection devices, and other X-ray imaging instruments have become hot topics in research and product development.
Traditional Medical Imaging vs. Photon Counting Imaging Principles
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