The Phonon Background from Gamma Rays in Sub-GeV Dark Matter Detectors
  High-energy photons with $\mathcal{O}$(MeV) energies from radioactive
contaminants can scatter in a solid-state target material and constitute an
important low-energy background for sub-GeV dark matter direct-detection
searches. This background is most noticeable for energy deposits in the $1 -
100$ meV range due to the partially coherent scattering enhancement in the
forward scattering direction. We comprehensively quantify the resulting single-
and multi-phonon background in Si, Ge, GaAs, SiC, and Al$_2$O$_3$ target
materials, which are representative of target materials of interest in low-mass
dark matter searches. We use a realistic representation of the high-energy
photon background, and contrast the expected background phonon spectrum with
the expected dark matter signal phonon spectrum. An active veto is needed to
suppress this background sufficiently in order to allow for the detection of a
dark matter signal, even in well-shielded environments. For comparison we also
show the expected single- and multi-phonon event rates from coherent
neutrino-nucleus scattering due to solar neutrinos, and find that they are
sub-dominant to the photon-induced phonon background.
