Cryogenic pure CsI read out by SiPM for CEνNS: Enhanced light yield and anti-quenching dynamics

Coherent elastic neutrino-nucleus scattering (CE ν NS) represents a powerful probe for low-energy neutrino physics. In this work, we present a novel cryogenic detection scheme utilizing pure CsI crystals coupled with silicon photomultiplier (SiPM) readout. While previous experiments have demonstrated the viability of this detector architecture at liquid-nitrogen temperatures (77 K), theoretical models anticipate significant performance enhancements at deeper cryogenic temperatures. To systematically investigate this regime, we developed a custom-built cryogenic facility with continuous temperature regulation from 30 K to 293 K. Building upon prior single-photon calibrations of the SiPMs at 40 K, we characterized the pure CsI detection system using 13.9–59.54 keV X/ γ -rays and 5.5 MeV α -particles. Crucially, we report the observation of a pronounced anti-quenching phenomenon. At 35 K, the scintillation light yield for densely ionizing α -particles reaches 19.2 p.e./keV, unambiguously exceeding the 17.3 p.e./keV measured for electron recoils induced by γ -rays. Because the ionization density profiles of α -particles closely emulate those of low-energy nuclear recoils generated via CE ν NS, this enhanced light yield for high linear energy transfer (LET) interactions is of paramount importance for validating the reliability of this technique. This cryogenic pure CsI platform offers a transformative pathway for achieving the ultra-low thresholds required in next-generation reactor CE ν NS experiments.