The world's leading directional dark matter experiments currently all utilizelow-pressure gas Time Projection Chamber (TPC) technologies. We discuss some ofthe challenges for this technology, for which balancing the goal of achievingthe best sensitivity with that of cost effective scale-up requires optimizationover a large parameter space. Critical for this are the precision measurementsof the fundamental properties of both electron and nuclear recoil tracks downto the lowest detectable energies. Such measurements are necessary to provide abenchmark for background discrimination and directional sensitivity that couldbe used for future optimization studies for directional dark matterexperiments. In this paper we describe a small, high resolution, high signal-to-noise GEM-based TPC with a 2D CCD readout designed for this goal. Theperformance of the detector was characterized using alpha particles, X-rays,gamma-rays, and neutrons, enabling detailed measurements of electron andnuclear recoil tracks. Stable effective gas gains of greater than $1 \times10^5$ were obtained in 100 Torr of pure CF$_4$ by a cascade of three standardCERN GEMs each with a 140 $\mu$m pitch. The high signal-to-noise andsub-millimeter spatial resolution of the GEM amplification and CCD readout,together with low diffusion, allow for excellent background discriminationbetween electron and nuclear recoils down below $\sim$10 keVee ($\sim$23 keVrfluorine recoil). Even lower thresholds, necessary for the detection of lowmass WIMPs for example, might be achieved by lowering the pressure andutilizing full 3D track reconstruction. These and other paths for improvementsare discussed, as are possible fundamental limitations imposed by the physicsof energy loss.
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