Performance of an optically read out time projection chamber with ultra-relativistic electrons

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