The CYGNO experiment aims at the development of a large gaseous TPC with
GEM-based amplification and an optical readout by means of PMTs and scientific
CMOS cameras for 3D tracking down to O(keV) energies, for the directional
detection of rare events such as low mass Dark Matter and solar neutrino
interactions. The largest prototype built so far towards the realisation of the
CYGNO experiment demonstrator is the 50 L active volume LIME, with 4 PMTs and a
single sCMOS imaging a 33×33 cm\textsuperscript{2} area for 50 cm drift,
that has been installed in underground Laboratori Nazionali del Gran Sasso in
February 2022. We will illustrate LIME performances as evaluated overground in
Laboratori Nazionali di Frascati by means of radioactive X-ray sources, and in
particular the detector stability, energy response and energy resolution. We
will discuss the MC simulation developed to reproduce the detector response and
show the comparison with actual data. We will furthermore examine the
background simulation worked out for LIME underground data taking and
illustrate the foreseen expected measurement and results in terms of natural
and materials intrinsic radioactivity characterisation and measurement of the
LNGS underground natural neutron flux. The results that will be obtained by
underground LIME installation will be paramount in the optimisation of the
CYGNO demonstrator, since this is foreseen to be composed by multiple modules
with the same LIME dimensions and characteristics