Measurement of low-energy Compton and neutron scattering in Si CCDs for dark matter searches

Abstract

For optimal sensitivity to low-mass dark matter candidates experiments like DAMIC-M employ skipper charged-coupled devices (CCDs) with detection threshold of just a few ionization charges. Ionization signals from small-angle Compton scatters of environmental \gammaγ-rays, an important component of the background in dark matter searches, must thus be characterized down to OO(10 eV) energy. Using a ^{241}241Am \gammaγ-ray source, we report a precise measurement of scattering on Si atomic shell electrons in a skipper CCD with single-electron resolution. Notable differences are observed between data and theoretical expectations in the L-shell energy region (<150 eV). We also present preliminary data from a skipper CCD exposed to low-energy neutrons (<24 keV) from a ^{124}124Sb^99Be photoneutron source, demonstrating a measurement of the nuclear recoil ionization efficiency in Si down to few ionization charges.