  An excess of events of unknown origin at low energies below 1 keV has been
observed in multiple low-threshold dark matter detectors. Depending on the
target material, nuclear recoil events at these energies may cause lattice
defects, in which case a part of the true recoil energy is stored in the defect
and not observed in the phonon detector. If the threshold for defect creation
is sharp, this effect leads to a prominent feature in the observed recoil
spectrum. Electronic recoils at low energies do not create defects and
therefore the feature in the observed spectrum is not expected in that case. We
propose to use the sharp defect creation threshold of diamond to test if the
low energy events are due to nuclear recoils. Based on simulated data we expect
the nuclear recoil bump in the observed spectrum to be visible in diamond with
just ~0.1 gram days of exposure.
