  Directional detection of nuclear recoils is broadly desirable in nuclear and
particle physics. At low recoil energies, this capability may be used to
confirm the cosmological origin of a dark matter signal, to penetrate the
so-called neutrino floor, or to distinguish between different neutrino sources.
Gas Time Projection Chambers (TPCs) can enable directional recoil detection if
the readout granularity is sufficiently high, as is the case when micro-pattern
gaseous detectors (MPGDs) are utilized. A key challenge in such detectors is
identifying and rejecting background electron recoil events caused by gamma
rays from radioactive contaminants in the detector materials and the
environment. We define new observables that can distinguish electron and
nuclear recoils, even at keV-scale energies, based on the simulated
ionization's topology. We perform a simulation study that shows these
observables outperform the traditionally used discriminant, dE/dx, by up to
three orders of magnitude. Furthermore, these new observables work well even at
ionization energies well below 10 keV and remain robust even in the regime
where directionality fails.
