Direct Dark Matter Detection with the DAMIC Experiment at SNOLAB

There is strong cosmological and astrophysical evidence for the existence of non-baryonic, cold dark matter as a major constituent of the Universe. Its nature, so far elusive, constitutes one of the most fundamental questions in science. The existence of weakly-interacting massive particles (WIMPs) is a compelling explanation for dark matter, and a large experimental effort is ongoing to detect the rare interactions of WIMPs with dedicated detectors located underground to minimize backgrounds. DAMIC (Dark Matter in CCDs) is a novel experiment with unique sensitivity to dark matter particles of masses below 10 GeV/c$^2$. It employs the bulk silicon of scientific-grade charge-coupled devices (CCDs) as the target for coherent WIMP-nucleus elastic scattering; the CCD is then used to detect the ionization produced by the recoiling nucleus. These devices, which are widely used for light imaging in astronomical telescopes and for X-ray detection in a variety of applications, are used in DAMIC for the first time as dark matter detectors. DAMIC CCDs are fabricated from n-type, high-resistivity silicon wafers, and are fully depleted ({\it i.e.} fully sensitive) over a record thickness of 675~$\mu$m. With 16 million pixels per CCD arranged over an area of 6 cm $\times$ 6 cm, these detectors provide a uniquely detailed spatial reconstruction of the energy deposited by a particle interaction. Also, the extremely low noise of the CCD readout results in an unprecedentedly low energy threshold to ionization of a few tens of eV. DAMIC was installed at SNOLAB - a laboratory located 2 km underground in a mine in Sudbury, Canada - at the end of 2012, and is now completing the R\&D phase required for the construction of a 100~g detector, DAMIC100. The work presented in this thesis covers the progress of the DAMIC experiment during its R\&D phase. A significant contribution has been the development of data analysis and simulations tools, and their use in the characterization of DAMIC CCDs, including the measurement of charge diffusion and the response to electron-induced ionization. A major contribution of this thesis is the measurement of radioactive contaminants potentially present in the apparatus. Photons, $\alpha$ and $\beta$ particles - {\it e.g.} from isotopes of the uranium and thorium chains - can produce interactions in the CCD ultimately limiting the sensitivity of the experiment to dark matter searches. In addition to a careful screening of all materials located close to the CCDs, we have performed measurements of radioactive contaminants in the CCD itself. For this purpose we have exploited the unique spatial resolution of the detector, which allows for accurate identification of $\alpha$ and $\beta$ particles. Uranium and thorium contamination in the CCD bulk was measured through $\alpha$ spectroscopy, with an upper limit on the \ura\ (\thor\ ) 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 \sitwo\ $-$\ptwo\ or \pbten\ $-$ \biten\ sequences of $\beta$ decays. $^{32}$Si is a cosmogenic isotope which may be present in any silicon detector and constitute an irreducible background. The decay rate of \sitwo\ was found to be 80$^{+110}_{-65}$ kg$^{-1}$ d$^{-1}$ (95\% CI). An upper limit of $\sim$ 35 kg$^{-1}$ d$^{-1}$ on the \pbten\ decay rate was obtained independently by $\alpha$ spectroscopy and the $\beta$ decay sequence search. These levels of radioactive contamination are sufficiently low for the successful operation of the forthcoming DAMIC100. Another major result presented in this thesis is the measurement of the nuclear ionization efficiency in silicon, namely the ratio of the energy released in ionization by a recoiling nucleus to the nucleus kinetic energy. Since only a fraction of the kinetic energy is released in the form of ionization, the experimental determination of the nuclear ionization efficiency is required for any consistent interpretation of a search for dark matter particles. We have performed a measurement of the nuclear ionization efficiency by detecting nuclear recoils induced in a DAMIC CCD by a fairly monochromatic beam of neutrons from a $^{124}$Sb-Be source. This is the first measurement in the literature for kinetic energies of the recoiling silicon nucleus below 3~keV. A notable result is that the measured nuclear ionization efficiency is consistently lower than the prediction of the Lindhard theory in silicon, which is used as a reference in the field.

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