  In this study, we conducted a comprehensive characterization and optimization
of a cryogenic pure CsI (pCsI) detector. We utilized a \SI{2}{\centi\metre}
cubic crystal coupled with a HAMAMATSU R11065 photomultiplier tube (PMT),
achieving a remarkable light yield of \SI{35.2}{PE/\keV_{ee}} and an
unprecedented energy resolution of \SI{6.9}{\%} at \SI{60}{\keV}. Additionally,
we measured the scintillation decay time of pCsI, which proved to be
significantly faster than that of CsI(Na) at room temperature. Furthermore, we
investigated the impact of temperature, surface treatment, and crystal shape on
the light yield. Notably, the light yield peaked at approximately \SI{20}{\K}
and remained stable within the range of \SI{70}-\SI{100}{\K}. We observed that
the light yield of polished crystals was approximately 1.5 times greater than
that of ground crystals, while the crystal shape exhibited minimal influence on
the light yield. These results are crucial for the design of the \SI{10}{\kg}
pCsI detector for the future CLOVERS (Coherent eLastic neutrinO(V)-nucleus
scattERing at China Spallation Neutron Source (CSNS)) experiment.
