  Low-energy nuclear recoils (NRs) are hard to measure because they produce few
e$^{-}$/h$^+$ pairs in solids -- i.e. they have low "ionization yield". A
silicon detector was exposed to thermal neutrons over 2.5\,live-days, probing
NRs down to 450\,eV. The observation of a neutron capture-induced component of
NRs at low energies is supported by the much-improved fit upon inclusion of a
capture NR model. This result shows that thermal neutron calibration of very
low recoil energy NRs is promising for dark matter searches, coherent neutrino
experiments, and improving understanding of ionization dynamics in solids.
