With the advancement of nuclear science and technology, nuclear detection technology is now widely used in people’s daily lives and scientific research. Examples include nuclear radiation monitoring, safer medical imaging, as well as a more effective non-invasive analysis for security, nuclear safety, or product inspection applications and radiation detection and imaging ^[1][2][3][1,2,3].

For public safety concerns and further exploration in nuclear science and technology, contemporary society is setting higher standards for nuclear radiation detection technology. Early gas detectors exhibited low detection efficiency for high-energy radiation, lacked high energy resolution, and had relatively larger volumes, making them only suitable for detecting medium to low-energy radiation. Scintillation detectors (such as NaI, CsI, MHPs, etc.), although inexpensive and having high detection efficiency for X-rays and γ rays, are limited in application due to their long response times, comparatively lower energy resolution compared to semiconductor detectors, and larger physical dimensions. Moreover, semiconductor materials, owing to their ability to operate at room temperature and their excellent resolution and faster response times, possess unique advantages in nuclear radiation detectors. These advantages have led to their extensive use in various fields such as national security, medical imaging, industrial non-destructive testing, astronomical observations, and cutting-edge physics ^[4][5][6][4,5,6].

In order to achieve a detector with high energy resolution, high detection efficiency for high energy radiation, and the capability to operate at room temperature, the compound semiconductor materials used in the fabrication of such detectors should possess specific physical properties. These properties include the following:

• Low ionization energy: To minimize the impact of statistical fluctuations, the semiconductor material should have a low ionization energy.
• Higher average atomic number: A higher average atomic number enhances the detector’s efficiency in detecting high energy radiation.
• Sufficiently wide bandgap: A wide bandgap allows the detector to function at room temperature with minimal leakage current.

Product of carrier mobility and lifetime (μτ): A high μτ product reduces the impact of carrier capture, improving the detector’s energy resolution [7]. In the context of detectors, the overall efficiency, energy resolution, and time response speed of the backend electronic components should be exceptionally high to effectively process data for image generation.

To meet these requirements, the technology of using Cadmium Zinc Telluride (CZT) semiconductor material to produce array detectors has been proposed and implemented. Detectors made of CZT semiconductor material are characterized by their high resistivity, wide bandgap, and ability to operate at room temperature. Such detectors have found extensive applications in a wide range of fields, including aerospace, astrophysics research, nuclear medicine, environmental monitoring, nuclear counterterrorism, and nuclear emergency response ^[8][9][8,9].

2. The Basic Structure and Advantages of CZT Material

The CZT crystal can be regarded as an infinite solid solution crystal composed of CdTe and ZnTe crystals in a (1-x):x ratio, possessing a cubic zinc blende crystal structure, as illustrated in Figure 1. By varying the value of x, different proportions of CZT crystals can be obtained, leading to changes in their physical and chemical properties. This variation allows for the production of various types of CZT detectors, catering to the evolution of nuclear detection technology. Moreover, as the value of x changes, the bandgap width of the crystal varies between 1.49 and 2.26 eV. In general, the fabrication of large-volume CdTe crystals is relatively easier compared to CZT crystals, with CdTe crystals achievable at a maximum diameter of 50 mm. Additionally, CdTe crystals exhibit good uniformity, devoid of grain boundaries, rendering CdTe relatively inexpensive. However, for CZT materials, the addition of Zn to the CdTe compound stabilizes the Cd-Te bond. Consequently, CZT crystals possess stronger inter-atomic covalent bonds compared to CdTe crystals, resulting in reduced susceptibility to defects [10]. By comparing CZT with other semiconductor materials, it is evident that during the crystal solid solution process, adjusting the Zn composition allows for changes in the bandgap width of CZT. This adjustment facilitates the preparation of CZT crystals with a larger bandgap width, granting CZT a wider bandgap compared to other semiconductor materials. As a result, CZT exhibits higher resistance and consequently lower leakage current in the detectors produced, leading to lower power consumption. Additionally, CZT’s higher average atomic number endows it with strong attenuation properties against γ radiation, high electron mobility, and excellent carrier transport characteristics, ensuring superior detection performance of CZT crystals for γ radiation [11]. In assessing the physical performance of semiconductor materials for radiation detection through statistical analysis, a comparative study was conducted to contrast and analyze the obtained results, as depicted in Table 1.

Material	Atomic Number	Density (g/cm3)	Bandgap Width (eV)	Ionization Energy (eV)	Resistivity (Ω)	μeτe	μhτh
Si	14	2.33	1.12	3.62	104	>1	1
Ge	32	5.33	0.67	2.96	50	>1	>1
InP	15/49	4.78	1.35	4.2	106	5 × 10−6	<2 × 10−5
GaAs	33/31	5.32	1.43	4.2	107	10−5	10−6
HgI2	80/53	6.40	2.13	4.2	1013	10−4	10−5
PbI2	82/53	6.20	2.3~2.6	4.9	1012	10−6	10−7
TlBr	81/35	7.56	2.68	6.5	1012	10−5	10−6
CdTe	48/52	6.20	1.44	4.43	109	10−3	10−4
Cd0.9Zn0.1Te	48/30/52	5.78	1.57	4.64	1010~1011	10−3~10−2	10−5
Cd0.8Zn0.2Te	48/30/52	6.02	1.5~2.2	5.0	1010~1011	10−3	10−6~10−5

Through the comparison of the performance of various semiconductor materials in the table, it is evident that compared to other types of semiconductor materials, CZT crystals exhibit a well-balanced array of characteristics. With a larger atomic number and bandgap width, high resistivity, and rapid response rate, CZT demonstrates superior overall performance, making it highly suitable for nuclear radiation detection ^[3][15][3,15]. In comparison to other compound semiconductor materials, Cadmium Zinc Telluride (CZT) emerges as a highly competitive material for X-ray and γ-ray detection.

3. Application of CZT Detector in Nuclear Detection

In the detection of special nuclear materials, non-contact detection methods are used to detect radioactive isotopes using X-rays, and γ-passive non-destructive analysis characterized by X-ray spectroscopy is the preferred method ^[16][164]. High-purity germanium (HPGe) detectors have traditionally been the first choice for such applications. However, they require cooling through liquid nitrogen or thermoelectric methods, which can be inconvenient for portable equipment. As a result, the demand is increasing for γ-ray measurement devices that can operate at room temperature ^[17][165]. Three-dimensional array CZT, as a high-energy-resolution, room-temperature-operable, deployable γ-ray imaging spectrometer, is capable of detecting and characterizing Special Nuclear Materials (SNMs) ^[18][166]. The CZT detector exhibits unique capabilities for the detection of SNM due to its ability to locate fast neutrons and γ-rays, its sensitivity to thermal neutrons, and its <1% γ-ray energy resolution at room temperature. This positions it with a growing scope of applications in this field ^[19][20][167,168]. In 1998, Zhong et al. at the University of Michigan ^[21][169] collaborated with the Johns Hopkins University Applied Physics Laboratory to upgrade two three-dimensional position-sensitive CZT spectrometers. They assembled a prototype Compton scattering γ-ray imaging device using the two upgraded CZT detectors. The individual performance of the two γ-ray spectrometers was independently tested. The angle resolution and detection sensitivity of the imaging system were measured using point and line sources of ¹³⁷Cs radiation. The measurements matched the results from Monte Carlo simulations, confirming the potential of room-temperature three-dimensional position-sensitive CZT detectors in nuclear detection. In 2001, Mortreau et al. ^[22][170] investigated the characterization of cadmium zinc detectors’ spectra for spent fuel analysis using room-temperature semiconductor detectors. The respective detectors have been employed in the field of nuclear security for characterizing spent fuels (SFATs), enabling determination of the purity of enriched uranium and verification of the radiation status of nuclear fuels. In 2015, Bolotnikov et al. ^[23][24][25][171,172,173] developed a position-sensitive Virtual Frisch-grid (VFG) CZT detector array based on a 2 × 2 array. They combined this with a robust detector module designed by Brookhaven National Laboratory’s readout ASIC to assemble a large-area, high-energy-resolution γ-ray detection instrument. It was demonstrated that the detector can be divided into sub-arrays, where their cathode connections form a single electrode, enabling signal correction for electron charge loss within the VFG detector and rejecting incomplete charge collection events. The design included position-sensitive edge contacts to correct material defects within CZT detector-grade crystals, significantly improving the acceptance rate of useful CZT crystals and reducing detector costs. In 2020, David Goodman and collaborators from the Idaho National Laboratory ^[26][174] conducted research on the detection of ²³⁹Pu/²⁴⁰Pu isotopes using a digital 3D position-sensitive CZT detector. They used the digital CZT system to detect the isotopic composition and effective percentage of ²⁴⁰Pu in nuclear materials. Through statistical measurements from reactor-grade to weapons-grade nuclear material samples, the plutonium and uranium content exhibited reasonable consistency with uncertainties in the statistical measurements less than 3σ. The designed digital system is applicable for quantitative analysis in γ-ray spectroscopy. In 2021, Xuesong et al. ^[27][175] analyzed the characteristics of γ-ray energy deposition in three key processes that can form Compton edges. They primarily used cadmium zinc telluride (CZT) detectors as the main detectors and bismuth germanate scintillators as anti-coincidence detectors. They designed a new structure for a portable anti-Compton γ detector suitable for measuring strong radiation sources. The Geant4 program was used for simulating and calculating the measurement spectra of 662 keV and 1525 keV γ-rays in two detection systems. The results showed that the overall external dimensions of both systems were controllable. The 4π-structured detection system had Compton suppression factors of 63 for 662 keV and 29 for 1525 keV γ rays, while the quasi-4π structured detection system had suppression factors of 51 for 662 keV and 26 for 1525 keV γ-rays. The simulation results were in line with the design specifications, validating the feasibility of the system. The above findings demonstrate the vast potential of CZT detectors, particularly in the field of nuclear detection, especially for detecting special nuclear materials.