Observation of a low energy nuclear recoil peak in the neutron calibration data of an <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>Al</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvari

The current generation of cryogenic solid state detectors used in direct dark matter and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>CE</a:mi> <a:mi>ν</a:mi> <a:mi>NS</a:mi> </a:mrow> </a:math> searches typically reach energy thresholds of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi mathvariant="script">O</c:mi> <c:mo stretchy="false">(</c:mo> <c:mn>10</c:mn> <c:mo stretchy="false">)</c:mo> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>eV</c:mi> </c:mrow> </c:math> for nuclear recoils. For a reliable calibration in this energy regime a method has been proposed, providing monoenergetic nuclear recoils at low energies <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mrow> <h:mo>∼</h:mo> <h:mn>100</h:mn> <h:mtext> </h:mtext> <h:mi>eV</h:mi> <h:mi>–</h:mi> <h:mn>1</h:mn> <h:mtext> </h:mtext> <h:mtext> </h:mtext> <h:mi>keV</h:mi> </h:mrow> </h:math> . In this work we report on the observation of a peak at ( <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msubsup> <j:mn>1113.6</j:mn> <j:mrow> <j:mo>−</j:mo> <j:mn>6.5</j:mn> </j:mrow> <j:mrow> <j:mo>+</j:mo> <j:mn>6.5</j:mn> </j:mrow> </j:msubsup> </j:math> ) <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mi>eV</l:mi> </l:math> in the data of an <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:msub> <n:mtext>Al</n:mtext> <n:mn>2</n:mn> </n:msub> <n:msub> <n:mi mathvariant="normal">O</n:mi> <n:mn>3</n:mn> </n:msub> </n:math> 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 <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mrow> <q:mmultiscripts> <q:mrow> <q:mi>Al</q:mi> </q:mrow> <q:mprescripts/> <q:none/> <q:mrow> <q:mn>27</q:mn> </q:mrow> </q:mmultiscripts> </q:mrow> </q:math> and the subsequent deexcitation via single <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mi>γ</s:mi> </s:math> 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 <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:msub> <u:mtext>Al</u:mtext> <u:mn>2</u:mn> </u:msub> <u:msub> <u:mi mathvariant="normal">O</u:mi> <u:mn>3</u:mn> </u:msub> </u:math> 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.