  The SuperCDMS experiment in the Soudan Underground Laboratory searches for
dark matter with a 9-kg array of cryogenic germanium detectors. Symmetric
sensors on opposite sides measure both charge and phonons from each particle
interaction, providing excellent discrimination between electron and nuclear
recoils, and between surface and interior events. Surface event rejection
capabilities were tested with two $^{210}$Pb sources producing $\sim$130 beta
decays/hr. In $\sim$800 live hours, no events leaked into the 8--115 keV signal
region, giving upper limit leakage fraction $1.7 \times 10^{-5}$ at 90% C.L.,
corresponding to $< 0.6$ surface event background in the future 200-kg
SuperCDMS SNOLAB experiment.
