  The nature of dark matter is still unknown and an experimental program to
look for dark matter particles in our Galaxy should extend its sensitivity to
light particles in the GeV mass range and exploit the directional information
of the DM particle motion. The CYGNO project is studying a gaseous time
projection chamber operated at atmospheric pressure with a Gas Electron
Multiplier amplification and with an optical readout as a promising technology
for light dark matter and directional searches.
  In this paper we describe the operation of a 50 liter prototype named LIME
(Long Imaging ModulE) in an overground location at Laboratori Nazionali di
Frascati of INFN. This prototype employs the technology under study for the 1
cubic meter CYGNO demonstrator to be installed at the Laboratori Nazionali del
Gran Sasso. We report the characterization of LIME with photon sources in the
energy range from few keV to several tens of keV to understand the performance
of the energy reconstruction of the emitted electron. We achieved a low energy
threshold of few keV and an energy resolution over the whole energy range of
10-20%, while operating the detector for several weeks continuously with very
high operational efficiency. The energy spectrum of the reconstructed electrons
is then reported and will be the basis to identify radio-contaminants of the
LIME materials to be removed for future CYGNO detectors.
