  The search for a novel technology able to detect and reconstruct nuclear
recoil events in the keV energy range has become more and more important as
long as vast regions of high mass WIMP-like Dark Matter candidate have been
excluded. Gaseous Time Projection Chambers (TPC) with optical readout are very
promising candidate combining the complete event information provided by the
TPC technique to the high sensitivity and granularity of last generation
scientific light sensors. A TPC with an amplification at the anode obtained
with Gas Electron Multipliers (GEM) was tested at the Laboratori Nazionali di
Frascati. Photons and neutrons from radioactive sources were employed to induce
recoiling nuclei and electrons with kinetic energy in the range [1-100] keV. A
He-CF4 (60/40) gas mixture was used at atmospheric pressure and the light
produced during the multiplication in the GEM channels was acquired by a high
position resolution and low noise scientific CMOS camera and a photomultiplier.
A multi-stage pattern recognition algorithm based on an advanced clustering
technique is presented here. A number of cluster shape observables are used to
identify nuclear recoils induced by neutrons originated from a AmBe source
against X-ray 55Fe photo-electrons. An efficiency of 18% to detect nuclear
recoils with an energy of about 6 keV is reached obtaining at the same time a
96% 55Fe photo-electrons suppression. This makes this optically readout gas TPC
a very promising candidate for future investigations of ultra-rare events as
directional direct Dark Matter searches.
