  The Time Projection Chamber (TPC) is an ideal candidate to finely study the
charged particle ionization in a gaseous medium. Large volumes TPCs can be read
out with a suitable number of channels offering a complete 3D reconstruction of
an ultra-relativistic charged particle track, that is the sequence of its
energy releases in the TPC gas volume. Moreover, He-based TPCs are very
promising to study keV energy particles as nuclear recoils, opening the
possibility for directional searches of Dark Matter (DM) and the study of Solar
Neutrinos (SN).
  In this paper, we report the analysis of the data acquired with a small TPC
prototype (named LEMOn) built by the CYGNO collaboration that was exposed to a
beam of 450 MeV electrons at the Beam Test Facility of National Laboratories of
Frascati. LEMOn is operated with a He-CF4 mixture at atmospheric pressure and
is based on a Gas Electron Multipliers amplification stage that produces
visible light collected by a sub-millimeter position resolution scientific CMOS
camera. This type of readout - in conjunction with a fast light detection -
allows a 3D reconstruction of the electrons' tracks. The electrons are leaving
a trail of segments of ionizations corresponding to a few keV energy releases
each. Their study leads to predict a keV energy threshold and 1-10 mm
longitudinal and 0.1-0.3 mm transverse position resolution for nuclear recoils,
very promising for the application of optically readout TPC to DM searches and
SN measurements.
