  Coherent elastic neutrino-nucleus scattering and low-mass Dark Matter
detectors rely crucially on the understanding of their response to nuclear
recoils. We report the first observation of a nuclear recoil peak at around 112
eV induced by neutron capture. The measurement was performed with a CaWO$_4$
cryogenic detector from the NUCLEUS experiment exposed to a $^{252}$Cf source
placed in a compact moderator. The measured spectrum is found in agreement with
simulations and the expected peak structure from the single-$\gamma$
de-excitation of $^{183}$W is identified with 3 $\sigma$ significance. This
result demonstrates a new method for precise, in-situ, and non-intrusive
calibration of low-threshold experiments.
