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

TL;DR

This paper measures radioactive contamination in silicon CCDs used by DAMIC for dark matter detection, developing novel spatial analysis methods to quantify uranium, thorium, and other isotopes, demonstrating contamination levels low enough for future experiments.

Contribution

It introduces new analysis techniques exploiting CCD spatial resolution to identify and quantify radioactive isotopes in detector materials, improving contamination assessment for dark matter searches.

Findings

Uranium and thorium contamination limits established

Decay rates of $^{32}$Si and $^{210}$Pb measured or constrained

Contamination levels deemed acceptable for future DAMIC detector operation

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 $\alpha$ and $\beta$ particles. Uranium and thorium contamination in the CCD bulk was measured through $\alpha$ 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 $\beta$ decays. The decay rate of $^{32}$Si was found to be $80^{+110}_{-65}$ 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 $\alpha$ spectroscopy and the $\beta$ decay sequence search. These levels of radioactive contamination are sufficiently low for the successful operation of CCDs in the forthcoming 100 g DAMIC detector.