First evidence of luminescence in a He/CF$_4$ gas mixture induced by non-ionizing electrons

Optical readout of Gas Electron Multipliers (GEM) provides very interestingperformances and has been proposed for different applications in particlephysics. In particular, thanks to its good efficiency in the keV energy range,it is being developed for low-energy and rare event studies, such as DarkMatter search. So far, the optical approach exploits the light produced duringthe avalanche processes in GEM channels. Further luminescence in the gas can beinduced by electrons accelerated by a suitable electric field. The CYGNOcollaboration studied this process with a combined use of a triple-GEMstructure and a grid in an He/CF$_4$ (60/40) gas mixture at atmosphericpressure. Results reported in this paper allow to conclude that with anelectric field of about 11~kV/cm a photon production mean free path of about1.0~cm was found.