Scintillation response of cryogenic CsI to few-keV and sub-keV nuclear recoils

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Abstract
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.
Funding Information
  • European Research Council (101055120)
  • National Science Foundation (PHY-2209579)
  • H2020 Marie Skłodowska-Curie Actions (101026628)
  • Horizon 2020 (RYC2023-045436-I)

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