Measurement of the bulk radioactive contamination of detector-grade silicon with DAMIC at SNOLAB

We present measurements of bulk radiocontaminants in the high-resistivity silicon CCDs from the DAMIC at SNOLAB experiment. We utilize the exquisite spatial resolution of CCDs to discriminate between $\alpha$ and $\beta$ decays, and to search with high efficiency for the spatially-correlated decays of various radioisotope sequences. Using spatially-correlated $\beta$ decays, we measure a bulk radioactive contamination of ${}^{32}$Si in the CCDs of $140 \pm 30$ $\mu$Bq/kg, and place an upper limit on bulk ${}^{210}$Pb of $< 160~\mu$Bq\mathrm{/}kg\mathrm{.}Usingsimilaranalysesofspatially-correlatedbulk$\alpha$decays,wesetlimitsof$< 11$ $\mu$Bq/kg (0.9 ppt) on ${}^{238}$U and of $< 7.3$ $\mu$Bq/kg (1.8 ppt) on ${}^{232}$Th. The ability of DAMIC CCDs to identify and reject spatially-coincident backgrounds, particularly from ${}^{32}$Si, has significant implications for the next generation of silicon-based dark matter experiments, where $\beta$’s from ${}^{32}$Si decay will likely be a dominant background. This capability demonstrates the readiness of the CCD technology to achieve kg-scale dark matter sensitivity.