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 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 bulk α decays, we set limits of <11 μBq/kg (0.9 ppt) on 238U and of <7.3 μBq/kg (1.8 ppt) on 232Th. 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. This capability demonstrates the readiness of the CCD technology to achieve kg-scale dark matter sensitivity.
A. Aguilar-Arevalo et al., submitted to Phys. Rev. Lett. (Dec 2020), arXiv
Results on low-mass weakly interacting massive particles from a 11 kg-day target exposure of DAMIC at SNOLAB
We present constraints on the existence of weakly interacting massive particles (WIMPs) from a 11kg d target exposure of the DAMIC experiment at the SNOLAB underground laboratory. The observed energy spectrum and spatial distribution of ionization events with electron-equivalent en- ergies >200 eVee in the DAMIC CCDs are consistent with backgrounds from natural radioactivity. An excess of ionization events is observed above the analysis threshold of 50 eVee. While the origin of this low-energy excess requires further investigation, our data exclude spin-independent WIMP- nucleon scattering cross sections σχ−n as low as 3×10^{−41} cm^2 for WIMPs with masses mχ from 7 to 10 GeV c^{-2}. These results are the strongest constraints from a silicon target on the existence of WIMPs with mχ<9GeV/c^2 and are directly relevant to any dark matter interpretation of the excess of nuclear-recoil events observed by the CDMS silicon experiment in 2013.
A. Aguilar-Arevalo et al., accepted on Phys. Rev. Lett. (2020), arXiv
Constraints on Light Dark Matter Particles Interacting with Electrons from DAMIC at SNOLAB
We report direct-detection constraints on light dark matter particles interacting with electrons. The results are based on a method that exploits the extremely low levels of leakage current of the DAMIC detector at SNOLAB of 2–6×10^−22 Acm/2. We evaluate the charge distribution of pixels that collect < 10 e− for contributions beyond the leakage current that may be attributed to dark matter interactions. Constraints are placed on so-far unexplored parameter space for dark matter masses between 0.6 and 100 MeV/c2. We also present new constraints on hidden-photon dark matter with masses in the range 1.2–30 eV/c2
A. Aguilar-Arevalo et al., Phys. Rev. Lett. 123, 181802 (2019), arXiv
First Direct-Detection Constraints on eV-Scale Hidden-Photon Dark Matter with DAMIC at SNOLAB
We present direct detection constraints on the absorption of hidden-photon dark matter with particle masses in the range 1.2 – 30 eV/c^2 with the DAMIC experiment at SNOLAB. Under the assumption that the local dark matter is entirely constituted of hidden photons, the sensitivity to the kinetic mixing parameter κ is competitive with constraints from solar emission, reaching a minimum value of 2.2×10^{−14} at 17 eV/c^2. These results are the most stringent direct detection constraints on hidden-photon dark matter in the galactic halo with masses 3–12 eV/c^2 and the first demonstration of direct experimental sensitivity to ionization signals < 12 eV from dark matter interactions.
A. Aguilar-Arevalo et al., Phys. Rev. Lett. 118, 141803 (2017), link
Search for low-mass WIMPs in a 0.6 kg day exposure of the DAMIC experiment at SNOLAB
We present results of a dark matter search performed with a 0.6kgd exposure of the DAMIC experiment at the SNOLAB underground laboratory. We measure the energy spectrum of ionization events in the bulk silicon of charge-coupled devices down to a signal of 60 eV electron equivalent. The data are consistent with radiogenic backgrounds, and constraints on the spin-independent WIMP- nucleon elastic-scattering cross section are accordingly placed. A region of parameter space relevant to the potential signal from the CDMS-II Si experiment is excluded using the same target for the first time. This result obtained with a limited exposure demonstrates the potential to explore the low-mass WIMP region ( < 10GeV/c^2) with the upcoming DAMIC100, a 100g detector currently being installed in SNOLAB.
A. Aguilar-Arevalo et al., Phys. Rev. D 94, 082006 (2016), arXiv
Measurement of low energy ionization signals from Compton scattering in a CCD dark matter detector
An important source of background in direct searches for low-mass dark matter particles are the energy deposits by small-angle scattering of environmental γ rays. We report detailed measurements of low-energy spectra from Compton scattering of γ rays in the bulk silicon of a charge-coupled device (CCD). Electron recoils produced by γ rays from 57Co and 241Am radioactive sources are measured between 60 eV and 4 keV. The observed spectra agree qualitatively with theoretical predictions, and characteristic spectral features associated with the atomic structure of the silicon target are accurately measured for the first time. A theoretically-motivated parametrization of the data that describes the Compton spectrum at low energies for any incident γ-ray flux is derived. The result is directly applicable to background estimations for low-mass dark matter direct-detection experiments based on silicon detectors, in particular for the DAMIC experiment down to its current energy threshold.
A. Aguilar-Arevalo et al., Phys. Rev. D 96, 042002 (2017), arXiv
Measurement of the ionization produced by sub-keV silicon nuclear recoils in a CCD dark matter detector
We present results of a dark matter search performed with a 0.6kgd exposure of the DAMIC experiment at the SNOLAB underground laboratory. We measure the energy spectrum of ionization events in the bulk silicon of charge-coupled devices down to a signal of 60 eV electron equivalent. The data are consistent with radiogenic backgrounds, and constraints on the spin-independent WIMP- nucleon elastic-scattering cross section are accordingly placed. A region of parameter space relevant to the potential signal from the CDMS-II Si experiment is excluded using the same target for the first time. This result obtained with a limited exposure demonstrates the potential to explore the low-mass WIMP region ( < 10GeV/c^2) with the upcoming DAMIC100, a 100g detector currently being installed in SNOLAB.
A. Aguilar-Arevalo et al., Phys. Rev. D 94, 082006 (2016), arXiv
Measurement of radioactive contamination in the high resistivity silicon CCDs of the DAMIC experiment
We present measurements of radioactive contamination in the high-resistivity silicon charge-coupled devices (CCDs) used by the DAMIC experiment to search for dark matter particles. Novel analysis methods, which exploit the unique spatial resolution of CCDs, were developed to identify α and β particles. Uranium and thorium contamination in the CCD bulk was measured through α spectroscopy, with an upper limit on the 238U (232Th) decay rate of 5 (15) kg^−1 d^−1 at 95% CL. We also searched for pairs of spatially correlated electron tracks separated in time by up to tens of days, as expected from 32Si –32P or 210Pb –210Bi sequences of β decays. The decay rate of 32Si was found to be 80+110 kg−1 d−1 (95% CI). An upper limit of ∼35 kg−1 d−1 (95% CL) 210 −65Pb decay rate was obtained independently by α spectroscopy and the β decay sequence on the search. These levels of radioactive contamination are sufficiently low for the successful operation of CCDs in the forthcoming 100 g DAMIC detector.
A. Aguilar-Arevalo et al., JINST 10 (2015) P08014, arXiv