Searching for Light Dark Matter with DAMIC at SNOLAB and DAMIC-M: Investigations into Radioactive Backgrounds and Silicon Skipper Charge-Coupled Devices

The dark matter community has reached an inflection point. Strong limits have been placed on O(100 GeV) WIMPs, and attempts to produce dark matter at accelerators have not yet been successful. As such, there has been a shift towards exploring new parameter space, specifically that of light dark matter, and looking for hidden photons or dark matter-electron interactions. The DAMIC (Dark Matter in CCDs) program employs the bulk silicon of scientific CCDs to search for ionization signals produced by interactions of particle dark matter from the galactic halo of the Milky Way. By virtue of the low noise (~2 electrons) and small pixel size (15 μm) of conventional DAMIC CCDs, as well as the relatively low mass of the silicon nucleus, DAMIC is sensitive to small ionization signals from recoiling nuclei or electrons following the scattering of dark matter particles, and especially to low-mass WIMPs with mχ in the 1−10 Gev/c^2 range. A decade-long deployment of such detectors at the SNOLAB underground laboratory has demonstrated CCDs as successful dark matter detectors. There are two major themes of this thesis: first, understanding backgrounds that limit the sensitivity of silicon CCD direct-detection searches, and second, improving the resolution of CCDs using Skipper technology in order to drive next-generation searches for dark matter. A major contribution of this thesis is the development of a powerful technique to distinguish and reject background events in the DAMIC at SNOLAB detector. Utilizing the exquisite spatial resolution of CCDs, discriminating between α and β particles, we identify spatially-correlated decay sequences over long periods. We report measurements of radiocontaminants in the high resistivity CCDs from the DAMIC at SNOLAB experiment, including bulk 32Si and surface 210Pb; we also set limits for radiocontaminants along the 238U and 232Th chains. This technique will enable future silicon-based dark matter programs to optimize silicon ingot selection in order to minimize what may otherwise become a dominant and irreducible background. We also present a direct experimental measurement of the cosmogenic activation of silicon, following the irradiation of DAMIC CCDs at the LANSCE beam of the Los Alamos National Laboratory. Beyond measurements of key problematic backgrounds, we present the first-ever radioactive background model constructed for a dark matter CCD detector, which revealed the existence of a partial charge collection region in DAMIC CCDs. We outline fabrication efforts of the core element of CCD devices, with steps that can address this problematic partial charge collection layer. Finally, this work presents results from the successful deployment of novel Skipper CCDs. These CCDs are able to reach sub-electron resolution by performing non-destructive, multiple measurements of pixel charge. DAMIC-M, a record mass, kg-size CCD experiment under development, will feature such devices. A resolution of 0.07 electrons has been demonstrated in a DAMIC-M prototype CCD. We present the work to fully characterize and integrate these detectors via the construction of automated test chambers, and outline the projected sensitivity both of DAMIC-M and its prototype, the Low Background Chamber.