First multi-year muography of Etna Volcano

The muon was discovered in 1936 by American physicists C. D. Anderson and S. Neddermeyer during their extensive cloud-chamber experiments at the California Institute of Technology for the purpose of studying the energy lost by cosmic rays in matter. From their results, they concluded that ``there exist particles of unit charge, but with a mass larger than that of a normal free electron and much smaller than that of a proton''. For this reason, Anderson initially gave it the name of mesotron. In 1937 J. C. Street and E.C. Stevenson, while working at Harvard University, established beyond any doubt the existence of this new particle and provided a fairly estimate of about 200 electron masses for its mass. Since that moment, the history of muon science began. The idea of using muons as a probe to inspect the internal structure of large size objects dates back to 1955 with the pioneer work of E.P. George. He measured the muon flux inside and outside a tunnel to calculate the transmission rate and measured an areal density of overburden rock consistent with the result of drilling and sampling at the same site. He took this measurement with a Geiger counter and, therefore, he wasn't able to image the density structure. A milestone in the development of muography - or muon radiography - is the famous experiment of L. Alvarez in 1970. He attempted to use spark chambers with digital readout units inside the Chephren's pyramid in Egypt to search for hidden chambers. Although the collaboration did not succeeded in this goal, they demonstrated that the areal density of the pyramid was measured with a precision of 2% over a muon path of 100 m of limestone rock. The high visibility and historical importance of the target, had given a big resonance to the work of Alvarez. The same thing happened almost fifty years later, in 2017, after that a secret chamber was discovered inside the 4500 years old Great Pyramid in Giza, Egypt, by means of muon radiography. But in the last fifteen years, we are witnessing the return of muon-based imaging and the plethora of application fields requiring penetrating probes is continuously expanding. Archaeology, geology, mining, civil engineering and civil protection are all fields where muon imaging is applied as an inspection method complementary to traditional techniques, with the aim to overcome their limitations or to access information otherwise unachievable. Here, the results of the first multi-year muography experiment at Etna Volcano is presented. The Muography of Etna Volcano (MEV) project started in 2016 and the candidate took part in all activities since the beginning, as will be explained in the following. The collaboration involves physicists and engineers belonging to Department of Physics and Astronomy ``E. Majorana'', University of Catania, National Institute for Nuclear Physics (INFN) - Catania division and Southern National Laboratory (LNS), and National Institute for Astrophysics (INAF) - Catania Astrophysical Observatory, geophysicists and volcanologists from both Department of Biological, Geological and Environmental Sciences, University of Catania, and from National Institute for Geophysics and Volcanology (INGV) - Etna Observatory.