Qinzeng Hu (a (b), Lingyan Xu (a) (b), Zhixin Tan (c) (d), Ming Hao (a (b), Lu Liang (a (b), Yingming Wang (a (b),
Zhentao Qin (a (b), Lixiang Lian (a) (b), Chongqi Liu (a) (b), Yanyan Lei (a) (b), Wei Zheng (a b), Wanqi Jie (a (b)
(a) State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China
(b) MIIT Key Laboratory of Radiation Detection Materials and Devices, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, PR China
(c) Chinese Academy of Sciences, Institute of High Energy Physics, Beijing 100049, PR China
(d) China Spallation Neutron Source, Dongguan 523803, PR China
## Abstract
Semiconductor radiation detectors used in nuclear power plants and other environments are inevitably exposed to neutron, γ-ray and other high-energy radiation, which can damage the crystal structure of semiconductors and thus degrade the detector performance. Here, we investigate the effects of neutron irradiation on the microstructure, photoelectric and radiation detection performance of CdZnTe detectors. Low-temperature photoluminescence (PL) spectra show that the dislocation related defect concentration in the irradiated crystals increases with increasing fluence. The infrared (IR) transmittance of the irradiated crystal decreases compared with that of the unirradiated crystal, which also indicates an increase in the dislocation density. The presence of stacking faults, stacking fault dipoles and dislocation locks in the irradiated CdZnTe crystals has been revealed by transmission electron microscopy (TEM). The energy resolution of γ-ray from 241Am@100 V is degraded from 5.86 % before irradiation to 10.72 % after irradiation at 5.6 × 1010 n/cm2. In addition, the mobility-lifetime product of electron (μτ)e in CdZnTe detectors is reduced from 4.8 × 10-3 cm2/V before irradiation to 7.02 × 10-4 cm2/V after irradiation at 5.6 × 1010 n/cm2. I-V test show that the barrier height of the CdZnTe detector decreases with the increase of neutron irradiation fluence, leading to a decrease in resistivity. Time-of-flight (TOF) tests demonstrate that the electron mobility after irradiation decreases with increasing irradiation fluence. Notably, the maximum neutron fluence used in this study is 3.9 × 1011 n/cm2, at which the CdZnTe radiation detector is not completely damaged. This study mainly investigates the radiation damage mechanism, induced defect characteristics and performance degradation of CdZnTe crystals by neutron irradiation, aiming to provide theoretical guidance for improving the radiation-resistant properties of detectors.
## Introduction
As the global transition to clean energy accelerates, low-carbon emission energy sources, such as nuclear energy, will become an essential means of meeting future growth in global electricity demand. Nuclear power has clear advantages in terms of cleanliness and stability. From a cleanliness perspective, nuclear power generation produces negligible greenhouse gases in the process of generating electricity and has extremely low carbon emissions compared to fossil fuel power generation. In addition, nuclear energy is stable and less affected by external conditions such as climate and geographical location than other clean energy sources such as wind and solar energy. In recent years, taking into account energy demand and resource conditions, more and more countries and regions have begun to develop nuclear energy for power generation in order to achieve energy diversification and sustainable development.
According to data released by the International Atomic Energy Agency's Power Reactor Information System, as of January 31, 2024, there were 413 nuclear reactors in operation globally, accounting for about 10 % of total electricity generation and roughly one-third of clean energy output. Global nuclear power generation is expected to reach a record high by 2025. At the 28th Conference of the Parties (COP28) to the United Nations Framework Convention on Climate Change (UNFCCC), a variety of countries announced that they would accelerate the development of nuclear power generation, committing to triple global nuclear power generation by 2050 to the current level.
In nuclear power plants, the power of a nuclear reactor is typically controlled by the neutron flux. Under normal operation, the neutron flux inside the radiation-resistant shell of a nuclear reactor is in the range of 1 to 105 n/cm2·s. If the neutron flux exceeds the standard, the chain reaction becomes uncontrollable, leading to serious nuclear safety accidents. Therefore, accurate monitoring of neutron radiation inside nuclear reactors is one of the keys to ensuring the safety of nuclear power plants. In addition, the neutron moderation and decay processes in nuclear reactor release various types of radiation, including α-ray, β-ray, γ-ray and neutron stream. Nuclear radiation detectors are inevitably exposed to various types of radiation during service, which can cause damage and affect detector performance. Since α-rays and β-rays are weakly penetrating and of low energy, they can be completely shielded with thin metal plates. γ-ray and neutron are the main concerns for radiation protection in nuclear power plants.
Therefore, neutron irradiation damage poses a significant problem for neutron detectors [1]. When irradiated by neutrons, a certain amount of energy is transferred to the lattice atoms by collisions. If the energy transferred to the primary knock-on atom (PKA) exceeds the displacement threshold energy of the lattice atom, at least one of the lattice atoms will be displaced during the subsequent cascade of collisions, forming a Frenkel defect pair [2]. However, these formed defects are unstable at room temperature and some of them may recombine. Interstitials or vacancies that have not undergone reorganization will continue to diffuse within the lattice, potentially interacting with other impurity atoms or defects, thus severely affecting the photoelectric performance and detection capabilities of the detectors [[3], [4], [5]].
Cadmium Zinc Telluride (CdZnTe) is known to be the most promising material for radiation detection. With the advantages of good room temperature performance, fast response, high sensitivity, high mobility-lifetime, CdZnTe nuclear radiation detectors have a wide range of applications in the fields of astrophysics, nuclear safety, medical imaging and nuclear reactor monitoring [[6], [7], [8]].
Previous studies of neutron radiation damage in CdZnTe detectors have focused on the effect of neutron irradiation on the macroscopic properties. Bartlett et al. [9] exposed a CdZnTe planar detector to the radiation field of a 252Cf neutron source and found that the spectral performance of the detector degraded when the cumulative neutron injections was greater than 1010 n/cm2. Cavallini et al [3,10] found that high-energy neutron irradiation leads to more severe lattice disorder and affects the carrier mobility-lifetime product. At high-energy neutron fluence of 1013 n/cm2 and low-energy neutron fluence of 1013 or 1014 n/cm2, the defect concentration increases to the point where the detector performance is completely deteriorated. Moreover, Eisen et al. [11]investigated neutron-irradiated CdZnTe pixel detectors from 1 to 7 MeV, and revealed that the detector energy resolution of the 133Ba γ source energy resolution decreased from 2.1 % to 6 %. In addition, Chen et al. [12] irradiated CdZnTe crystals with neutrons at 14.1 MeV and found that the effect of irradiation on the detector was essentially negligible at a neutron fluence of 1010 n/cm2 and a maximum flux of 106 n/cm2·s.
Typically, Plyatsko et al. [13] found that deep defects appeared in different locations in the bandgap of the P-type CdZnTe single crystals at neutron fluence up to 1 × 1015 n/cm2. In addition, isothermal annealing at 350∼400 K will decompose deep defects. Bao et al. [14] found that the energy resolution of the CdZnTe:In detector for the 241Am source decreased from 8.8 % to 17.5 % after neutron irradiation with a fluence of 1010 n/cm2. Additionally, after neutron irradiation at a fluence of 5.0 × 1010 n/cm2, the concentration of vacancy-interstitial related defects increased from approximately 1014 cm-3 to 1015 cm-3, and the carrier mobility-lifetime product decreased from 1.0 × 10-3 cm2/V to 1.6 × 10-5 cm2/V. Besides, Bao et al. [15] investigated the effect of 2 MeV neutron irradiation on the properties of Au-CdZnTe interface and found that the Schottky barrier height on the neutron irradiation side decreased from 0.892 eV to 0.826 eV after neutron irradiation at a fluence of 1 × 1011 n/cm2, indicating that a large number of defects and electron traps were introduced on the irradiation side.
All these studies have shown that with increasing neutron fluence, irradiation damage can seriously affect the photoelectric and detection performance of the detectors, or even render them completely ineffective. At the microscopic level, these radiations damage the crystal structure of semiconductor radiation detectors, introducing various crystal defects such as dislocations and stacking faults. At the macroscopic level, these radiations degrade the detector performance, such as increasing the leakage current and reducing the energy resolution. However, studies on the effect of neutron irradiation on the microstructure of CdZnTe crystals are quite limited. The core of this study is to analyze the defects induced by neutron irradiation through transmission electron microscopy (TEM) and their effects on the performance degradation. The aim of this study is to elucidate the defect generation mechanism of CdZnTe detectors under neutron radiation field, which is hoped to provide guidance and support for the extension of service life and the improvement of the material's radiation resistance.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/damage-generation-mechanism-and-performance-degradation-of-cdznte-radiation-detectors-in-neutron-radiation-field.html