Copper(I) halide-based emitters have recently garnered significant attention for their potential in X-ray imaging applications owing to their efficient emission, facile synthesis, and low toxicity. While several strategies have been proposed to improve scintillation efficiency in these systems, the critical role of Cu–I cores─particularly during ultrafast energy conversion and transport─has received limited attention. In this work, we introduce a unified ligand strategy to construct a series of zero-dimensional copper(I) iodide clusters, including a Cu1I1 monomer, Cu2I2 rhomboid dimer, and Cu4I4 cubane tetramer, all exhibiting near-unity photoluminescence quantum yield (ϕPL). This approach enables a systematic investigation of how the core architecture governs radioluminescence (RL) behavior and efficiency beyond ϕPL. Our results demonstrate that the core geometry has a strong influence on both thermal stability and exciton relaxation pathways. Notably, low-temperature PL–RL differences uncover a previously unrecognized exciton relaxation channel intrinsic to the cubane cluster, allowing a fraction of excitons to directly populate the 3CC state. This process confines exciton generation, transport, and radiative recombination within the Cu–I cubane, thereby potentially increasing the exciton transfer efficiency and enhancing scintillation efficiency. These findings provide critical insights into the fundamental scintillation mechanisms and structure–property relationships of Cu–I clusters, establishing core geometry as a key design principle for the development of next-generation, high-performance scintillators.