  The world's leading directional dark matter experiments currently all utilize
low-pressure gas Time Projection Chamber (TPC) technologies. We discuss some of
the challenges for this technology, for which balancing the goal of achieving
the best sensitivity with that of cost effective scale-up requires optimization
over a large parameter space. Critical for this are the precision measurements
of the fundamental properties of both electron and nuclear recoil tracks down
to the lowest detectable energies. Such measurements are necessary to provide a
benchmark for background discrimination and directional sensitivity that could
be used for future optimization studies for directional dark matter
experiments. 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. The
performance of the detector was characterized using alpha particles, X-rays,
gamma-rays, and neutrons, enabling detailed measurements of electron and
nuclear recoil tracks. Stable effective gas gains of greater than $1 \times
10^5$ were obtained in 100 Torr of pure CF$_4$ by a cascade of three standard
CERN GEMs each with a 140 $\mu$m pitch. The high signal-to-noise and
sub-millimeter spatial resolution of the GEM amplification and CCD readout,
together with low diffusion, allow for excellent background discrimination
between electron and nuclear recoils down below $\sim$10 keVee ($\sim$23 keVr
fluorine recoil). Even lower thresholds, necessary for the detection of low
mass WIMPs for example, might be achieved by lowering the pressure and
utilizing full 3D track reconstruction. These and other paths for improvements
are discussed, as are possible fundamental limitations imposed by the physics
of energy loss.

Instrumentation and Detectors;High Energy Physics - Experiment