  We report a measurement of the ionization efficiency of silicon nuclei
recoiling with sub-keV kinetic energy in the bulk silicon of a charge-coupled
device (CCD). Nuclear recoils are produced by low-energy neutrons ($<$24 keV)
from a $^{124}$Sb-$^{9}$Be photoneutron source, and their ionization signal is
measured down to 60 eV electron equivalent. This energy range, previously
unexplored, is relevant for the detection of low-mass dark matter particles.
The measured efficiency is found to deviate from the extrapolation to low
energies of the Lindhard model. This measurement also demonstrates the
sensitivity to nuclear recoils of CCDs employed by DAMIC, a dark matter direct
detection experiment located in the SNOLAB underground laboratory.
