High-energy photons with 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 multiphonon background in Si, Ge, GaAs, SiC, and Al2O3 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 multiphonon event rates from coherent neutrino-nucleus scattering due to solar neutrinos, and find that they are subdominant to the photon-induced phonon background.
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We thank Cyrus Dreyer, Marivi Fernández-Serra, Sinéad Griffin, Matt Pyle, Bjoern Penning, Tongyan Lin, Alan Robinson, and Tanner Trickle for useful discussions. We also thank Matt Pyle for comments on the manuscript and for the suggestion to show the single- and multiphonon backgrounds from coherent neutrino-nucleus scattering due to solar neutrinos. We are also grateful to Lin-Fan Zhu and Michael Walter for their comments on the relevance of Raman-activity for MeV-photons. K. B. acknowledges the support of NSF grant No. PHYS-1915093. R. E. acknowledges support from DoE Grant No. DE-SC0009854, Simons Investigator in Physics Grant No. 623940, and the US-Israel Binational Science Foundation Grant No. 2020220. The work of Y. H. is supported by the Israel Science Foundation (Grant No. 1112/17), by the Binational Science Foundation (Grant No. 2016155), by the I-CORE Program of the Planning Budgeting Committee (Grant No. 1937/12), and by the Azrieli Foundation. The work of Y. S. is supported by the I-CORE Program of the Planning Budgeting Committee (Grant No. 1937/12). M. S. acknowledges support from DoE Grants No. DE-SC0009854, No. DE-SC0009919, and No. DE-SC0022104.
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