Observation of a low energy nuclear recoil peak in the neutron calibration data of an Al$_2$O$_3$ crystal in CRESST-III

The current generation of cryogenic solid state detectors used in direct dark matter and CE⁢𝜈⁢NS 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 Al$_2$⁢O$_3$ 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 $^{27}$Al and the subsequent deexcitation via single 𝛾 emission. We compare the measured results with the outcome of Geant4 simulations and investigate the possibility to make use of this effect for the energy calibration of Al$_2$⁢O$_3$ 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.