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

TL;DR

This study measures the scintillation response of cryogenic CsI to very low-energy nuclear recoils, revealing a rapid decrease in quenching factor at low energies, which impacts its use in neutrino and dark matter detection.

Contribution

It provides the first detailed measurement of cryogenic CsI's response to sub-keV nuclear recoils, highlighting differences from room-temperature models.

Findings

Quenching factor drops faster at low energies than previous models predicted.

Achieved a 70 eV analysis threshold using novel detection techniques.

Results influence the design of cryogenic CsI detectors for neutrino and dark matter experiments.

Abstract

Monochromatic neutron emissions from photonuclear sources $^{88}$Y/Be and $^{124}$Sb/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 waveshifter results in a 70 eV analysis threshold. This reach allows 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.