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 crystal was coupled to dual-ended 3-inch photomultiplier tubes (PMTs) for scintillation readout. The measured light yields were 28.7 ± 0.9 28.7 ± 0.9 and 29.3 ± 1.0 29.3 ± 1.0 photoelectrons per keV electron-equivalent (PE/keV ₑₑ ee ) 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.