Surface backgrounds in the DEAP-3600 dark matter experiment

DEAP-3600 is a dark matter experiment using 3.6 tons of liquid argon to search for Weakly Interacting Massive Particles (WIMPs), with a target sensitivity to the spin-independent WIMP-nucleon cross-section of 10{sup -46} cm{sup 2}. The detector is designed to allow for a three year background-free run with a 1-ton fiducial volume. We identify in this paper the potential sources of surface contamination. We require {sup 238}U and {sup 232}Th contaminations on the order of 10{sup -12} g/g or less, a level achieved by the SNO experiment, and {sup 210}Pb not significantly out of equilibrium with {sup 238}U, i.e., 10{sup -20} g/g or less {sup 210}Pb in the acrylic vessel or TPB wavelength shifter, which should be achievable with appropriate control of exposure to radon.