  The bright scintillation of pure CsI operated at liquid-nitrogen temperature
makes of this material a promising dark matter and neutrino detector. We
present the first measurement of its quenching factor for nuclear recoils. Our
findings indicate it is indistinguishable from that for sodium-doped CsI at
room temperature. Additional properties such as light yield, afterglow,
scintillation decay properties for electron and nuclear recoils, and energy
proportionality are studied over the \mbox{108-165 K} temperature range,
confirming the vast potential of this medium for rare-event searches.
