Measurement of radioactive contamination in the high-resistivity silicon CCDs of the DAMIC experiment

Abstract

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 $\hat{I}\pm$ and ${\hat{I}}^2$ particles. Uranium and thorium contamination in the CCD bulk was measured through $\hat{I}\pm$ spectroscopy, with an upper limit on the ${}^{238}$U (${}^{232}$Th) 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 ${}^{32}$Si-${}^{32}$P or ${}^{210}$Pb-${}^{210}$Bi sequences of ${\hat{I}}^2$ decays. The decay rate of ${}^{32}$Si was found to be $80_{-65}^{+110}$ kg${}^{-1}$ d${}^{-1}$ (95% CI). An upper limit of $\sim$35 kg${}^{-1}$ d${}^{-1}$ (95% CL) on the ${}^{210}$Pb decay rate was obtained independently by $\hat{I}\pm$ spectroscopy and the ${\hat{I}}^2$ decay sequence search. These levels of radioactive contamination are sufficiently low for the successful operation of CCDs in the forthcoming 100 g DAMIC detector.

18 pages, 20 figures