Seungho Song

Radiation detectors operating in extreme environments such as nuclear accident sites, radioactive waste facilities, space missions, and post‐detonation scenarios must maintain reliable radionuclide identification under prolonged high‐dose irradiation. However, radiation‐induced defect accumulation progressively degrades spectroscopic performance, and the mechanisms governing reversible detector operation under extreme irradiation remain not fully understood. Here, we report a zero‐dimensional perovskite‐inspired Cs ₃ Cu ₂ I ₅ single‐crystal scintillator that exhibits fully reversible spectroscopic degradation under ultra‐high‐dose γ‐ray irradiation of ∼200 kGy. The radiation response was systematically investigated by correlating structural, optical, and spectroscopic analyses before and after mild thermal recovery. Structural and spectroscopic analyses reveal that irradiation induces predominantly interstitial‐type lattice disorder while preserving long‐range crystallinity and the intrinsic self‐trapped exciton emission framework. Despite reduced light output, the degraded spectroscopic performance is completely restored through mild thermal activation at 100°C without device refabrication. Under simultaneous six‐radionuclide γ‐ray fields, the scintillator maintains distinguishable spectral features and high post‐irradiation energy linearity (R ² = 0.9973), outperforming conventional NaI(Tl) under comparable conditions. These findings highlight defect‐mediated reversibility as a promising design strategy for radiation‐resilient scintillators operating in extreme environments.