Observation of a low energy nuclear recoil peak in the neutron calibration data of an Al2O3 crystal in CRESST-III

The current generation of cryogenic solid state detectors used in direct dark matter and CEνNS searches typically reach energy thresholds of O(10) eV for nuclear recoils. For a reliable calibration in this energy regime a method has been proposed, providing monoenergetic nuclear recoils at low energies ∼100 eV–1 keV. In this work we report on the observation of a peak at (1113.6+6.5−6.5) eV in the data of an Al2O3 crystal in CRESST-III, which was irradiated with neutrons from an AmBe calibration source.We attribute this monoenergetic peak to the radiative capture of thermal neutrons on 27Al and the subsequent deexcitation via single γ emission. We compare the measured resultswith the outcome of Geant4 simulations and investigate the possibility to make use of this effect for the energy calibration ofAl2O3 detectors at low energies. We further investigate the possibility of a shift in the expected energy scale of this effect caused by the creation of defects in the target crystal.