  Directional detection of nuclear recoils is appealing because it can confirm
the cosmological origin of a dark matter signal and distinguish between
different neutrino sources. Gas Time Projection Chambers (TPCs) enable
directional recoil detection due to the high spatial granularity with which
they can image a recoil's ionization track, especially if micro-pattern gaseous
detectors (MPGDs) are utilized. A key challenge in these detectors at low
energies is identifying and rejecting background electron recoil events cause
by gamma rays from radioactive contaminants in the detector materials and the
environment. For gas TPCs with high readout segmentation, we can define
observables that can distinguish electron and nuclear recoils, even at
keV-scale energies, based on the measured ionization's topology. We define such
observables and show that they outperform the traditionally used discriminant,
dE/dx, by up to three orders of magnitude. Furthermore, these new observables
work well even at ionization energies below 10 keV and remain robust even in
the regime where directionality fails.
