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The radiation detection performance of CZT epitaxial single crystal grown by the Close Space Sublimation (CSS) method is hindered by inherent defects. However, significant improvement in this work is achieved through annealing in a Te 2 atmosphere. The saturation channel value of the total energy peak of the detector for 241Am α-particles has shown a substantial increase from 1900 to 6000, resulting in a notable enhancement of the carrier collection efficiency. After annealing, the detector's energy resolution to 241Am gamma-rays can reach 8.3%, representing the highest energy resolution achieved by CSS growth of CZT materials. This remarkable achievement holds great promise for its potential application in the field of low-energy detection. Analysis of the laser-beam-induced current (LBIC) curve results reveals that the surface recombination of the detector is more pronounced prior to annealing, adversely affecting the transport process of photogenerated carriers. The low-temperature photoluminescence (PL) results and I–V curve demonstrate a significant compensation effect on the shallow acceptor level V Cd after annealing, leading to a reduced concentration of free carriers and subsequently leading to lower leakage current and improved signal-to-noise ratio for the detector. The concurrent reduction in leakage current and surface recombination contributes collectively to improving the radiation detection performance of the CZT detector. These findings underscore the importance of annealing in a Te 2 atmosphere as a viable method for mitigating defects and optimizing the performance of CZT epitaxial single crystals, showcasing the potential for broader applications in the field of low-energy detection. • The leakage current is reduced from 1.3 nA to 0.59 nA under the applied bias voltage of 0.5 V, which makes the detector obtain a good signal-to-noise ratio. • The surface recombination rate decreased from 13.78 × 106 cm s−1 to 3.82 × 105 cm s−1, significantly improving the carrier collection efficiency. • The energy resolution to 241Am@59.5 KeV gamma-rays of the detector reached 8.3% for the first time, which is expected to be applied in the field of low energy detection. [ABSTRACT FROM AUTHOR]