  Solid-state detectors with a low energy threshold have several applications,
including in direct-detection searches of non-relativistic halo dark-matter
particles with sub-GeV masses. Moreover, when searching for relativistic or
quasi-relativistic beyond-the-Standard-Model particles (i.e., $v/c\gtrsim
0.01$) that have an enhanced cross section for small energy transfers, a
comparatively small detector with a low energy threshold may have better
sensitivity than a larger detector with a higher energy threshold. In this
paper, we provide accurate calculations of the low-energy ionization spectrum
from high-velocity particles scattering in a dielectric material. We focus on
silicon, although our results can be easily applied to other materials. We
consider the full material response, in particular also the excitation of bulk
plasmons. We generalize the energy-loss function to relativistic kinematics,
and benchmark existing tools used for halo dark-matter scattering against
publicly available electron energy-loss spectroscopy data. Compared to
calculations of energy loss that are commonly used in the literature, such as
the Photo-Absorption-Ionization model or the free-electron model, the inclusion
of collective effects shifts the recoil ionization spectrum towards higher
energies, typically peaking around 4--6 electron-hole pairs. We apply our
results to the three benchmark examples: millicharged particles produced in a
beam, neutrinos with a magnetic dipole moment produced in a reactor, and
dark-matter particles that are upscattered by cosmic rays or in the Sun. Our
results show that the proper inclusion of collective effects typically enhances
a detector's sensitivity to these particles, since detector backgrounds, such
as dark counts, peak at lower energies.
