In this study, we comprehensively characterized and optimized a cryogenic pure CsI (pCsI) detector. We utilized a \({2}\,{\textrm{cm}}\times {2}\,{\textrm{cm}}\times {2}\,{\textrm{cm}}\) cube crystal coupled with a HAMAMATSU R11065 photomultiplier tube, achieving a remarkable light yield of 35.2 \(\hbox {PE/keV}_{\textrm{ee}}\) and an unprecedented energy resolution of 6.9% at 59.54 keV. Additionally, we measured the scintillation decay time of pCsI, which was significantly shorter than that of CsI(Na) at room temperature. Furthermore, we investigated the impact of temperature, surface treatment and crystal shape on light yield. Notably, the light yield peaked at approximately 20 K and remained stable within the range of 70–100 K. The light yield of the polished crystals was approximately 1.5 times greater than that of the ground crystals, whereas the crystal shape exhibited minimal influence on the light yield. These results are crucial for the design of the 10 kg pCsI detector for the future CLOVERS (coherent elastic neutrino(V)-nucleus scattering at China Spallation Neutron Source (CSNS)) experiment.
