Monochromatic neutron emissions from photonuclear sources ${}^{88}\mathrm{Y}/\mathrm{Be}$ and ${}^{124}\mathrm{Sb}/\mathrm{Be}$ are employed to obtain the response of pure (undoped) cesium iodide at 80 K. The use of a low-noise, high-quantum-efficiency avalanche photodiode in combination with a novel wave shifter results in a 70 eV analysis threshold. This reach allows us to observe signals from sub-keV nuclear recoils originating in neutron scattering. The extracted quenching factor drops much faster towards low energy than the extrapolation of a model developed for room-temperature CsI[Na]. We comment on the impact of our measurement on planned use of cryogenic CsI in neutrino physics and dark matter experiments.

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• Elastic scattering reactions
• Neutron physics
• Nucleus-neutrino interactions
• Photonuclear reactions
• Radiation detectors