No abstract

We present results from a 3.1 kg-day target exposure of two charge-coupled devices (CCDs), each with 24 megapixels and skipper readout, deployed in the DAMIC (DArk Matter In CCDs) setup at SNOLAB. With a reduction in pixel readout noise of a factor of 10 relative to the previous detector, we investigate the excess population of low-energy bulk events previously observed above expected backgrounds. We address the dominant systematic uncertainty of the previous analysis through a depth fiducialization designed to reject surface backgrounds on the CCDs. The measured bulk ionization spectrum confirms with higher significance the presence of an excess population of low-energy events in the CCD target with characteristic rate of ∼7 events per kg-day and electronequivalent energies of ∼80 eV, whose origin remains unknown.

We present measurements of bulk radiocontaminants in the high-resistivity silicon CCDs from the DAMIC experiment at SNOLAB. We utilize the exquisite spatial resolution of CCDs to discriminate between α and β decays, and to search with high efficiency for the spatially-correlated decays of various radioisotope sequences. Using spatially-correlated β decays, we measure a bulk radioactive contamination of 32Si in the CCDs of 140 ± 30 μBq/kg, and place an upper limit on bulk 210Pb of < 160 μBq/kg. Using similar analyses of spatially-correlated α and β decays, we set upper limits of < 11 μBq/kg (0.9 ppt) on 238U and < 7.3 μBq/kg (1.8 ppt) on 232Th in the bulk silicon. The ability of DAMIC CCDs to identify and reject spatially-coincident backgrounds, particularly from 32Si, has significant implications for the next generation of silicon-based dark matter experiments, where β's from 32Si decay will likely be a dominant background.