The nature of dark matter is one of the fundamental questions in physics today. Direct signals for dark matter have remained elusive, indicating that multi-tonne scale detectors are needed to measure large numbers of dark matter interactions, while current efforts are at the 100 kg scale. The foremost challenge is distinguishing dark matter signals from backgrounds, the most uncertain of which are from neutrons. The research objective of this proposal is a world-leading dark matter search with a novel liquid argon (LAr) detector and a new analysis approach to measuring neutron backgrounds in-situ.The DEAP/CLEAN program of single-phase LAr detectors is a new direction for dark matter searches. It draws on successful, proven approaches of solar neutrino physics to building low-background detectors that scale simply to multi-tonne target masses. Demonstration of this approach by the current 100 kg stage (MiniCLEAN) will break new ground for future experiments. At the 100 tonne scale, such a detector would be a new kind of observatory for fundamental physics at the low background frontier, testing predicted properties of dark matter, neutrinos, supernovae, and stellar evolution. Success depends critically on demonstrating the required background suppression.This proposal addresses the key challenges of dark matter detection in two new ways, with the novel single-phase effort for multi-tonne scalability, and by developing new methods to overcome neutron backgrounds. The tasks of this proposal are: (i) to develop a measurement of the in-situ neutron background in LAr; (ii) to develop an active neutron veto for in-situ measurement of the cosmogenic neutron background, beginning with a measurement of the flux and energy spectrum in an existing prototype; and, (iii) to lead the dark matter search, using the measured backgrounds. The MiniCLEAN dark matter sensitivity is a factor of 20 beyond current experimental results, with great potential for discovery.