Identification of low energy nuclear recoils in a gas time projection chamber with optical readout

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 aslong as vast regions of high mass WIMP-like Dark Matter candidate have beenexcluded. Gaseous Time Projection Chambers (TPC) with optical readout are verypromising candidate combining the complete event information provided by theTPC technique to the high sensitivity and granularity of last generationscientific light sensors. A TPC with an amplification at the anode obtainedwith Gas Electron Multipliers (GEM) was tested at the Laboratori Nazionali diFrascati. Photons and neutrons from radioactive sources were employed to inducerecoiling nuclei and electrons with kinetic energy in the range [1-100] keV. AHe-CF4 (60/40) gas mixture was used at atmospheric pressure and the lightproduced during the multiplication in the GEM channels was acquired by a highposition resolution and low noise scientific CMOS camera and a photomultiplier.A multi-stage pattern recognition algorithm based on an advanced clusteringtechnique is presented here. A number of cluster shape observables are used toidentify nuclear recoils induced by neutrons originated from a AmBe sourceagainst X-ray 55Fe photo-electrons. An efficiency of 18% to detect nuclearrecoils with an energy of about 6 keV is reached obtaining at the same time a96% 55Fe photo-electrons suppression. This makes this optically readout gas TPCa very promising candidate for future investigations of ultra-rare events asdirectional direct Dark Matter searches.