Observation of a low energy nuclear recoil peak in the neutron calibration data of an Al$_{2}$O$_{3}$ crystal in CRESST-III

TL;DR

This study reports the detection of a mono-energetic nuclear recoil peak at around 1114 eV in an Al₂O₃ crystal, caused by thermal neutron capture on aluminum, which could aid low-energy calibration of cryogenic detectors.

Contribution

First observation of a low-energy nuclear recoil peak in Al₂O₃ due to neutron capture, with implications for calibration of dark matter detectors.

Findings

Detected a 1114 eV recoil peak in Al₂O₃ with neutron irradiation.

Confirmed the peak results from neutron capture on aluminum.

Compared experimental data with Geant4 simulations.

Abstract

The current generation of cryogenic solid state detectors used in direct dark matter and CE\textnu NS searches typically reach energy thresholds of $\mathcal{O}$(10)$\,$eV for nuclear recoils. For a reliable calibration in this energy regime a method has been proposed, providing mono-energetic nuclear recoils at low energies $\sim\,$100$\,$eV$\,$-$\,$1$\,$keV. In this work we report on the observation of a peak at (1113.6$^{+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 mono-energetic peak to the radiative capture of thermal neutrons on $^{27}$Al and the subsequent de-excitation via single $\gamma$-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.