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 10-46 cm2. 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 238U and 232Th contaminations on the order of 10-12 g/g or less, a level achieved by the SNO experiment, and 210Pb not significantly out of equilibrium with 238U, i.e., 10-20 g/g or less 210Pb in the acrylic vessel or TPB wavelength shifter, which should be achievable with appropriate control of exposure to radon.