Performance of a Large-Volume Cryogenic Pure CsI Detector for CEνNS and Low-Energy Rare-Event Searches

A cryogenic detector system based on two 3.3 kg high-purity CsI crystals was developed and characterized at approximately 95 K. Each wedge-shaped crystal was coupled to dual-ended 3-inch photomultiplier tubes (PMTs) for scintillation readout. The measured light yields were 28.7 ± 0.9 and 29.3 ± 1.0 photoelectrons per keV electron-equivalent (PE/keVₑₑ) for the two crystals, with corresponding energy resolutions of 7.2 % and 7.7 % (FWHM) at 59.6 keV. The detector demonstrated excellent spatial uniformity, low intrinsic radioactivity, and stable operation over a continuous one-month period. Optical photon simulations using Geant4 reproduced the observed light collection trends, providing guidance for detector optimization. These results establish cryogenic pure CsI as a scalable technology for low-threshold rare-event searches.