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 the charged particle ionization in a gaseous medium. Large volume TPCs can be readout with a suitable number of channels offering a complete 3D reconstruction of a 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 the high granularity and very good sensitivity of 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 clusters of ionizations corresponding to a few keV energy release each. Their study leads to predict a keV energy threshold and 1–10 mm longitudinal and 0.1–0.3 mm transverse position resolution (sigma) for nuclear recoils, very promising for the application of optically read out TPC to DM searches and SN measurements.