Test of new eco-gas mixtures for the multigap resistive plate chambers of the EEE project

The Extreme Energy Events (EEE) experiment is a project by Centro Fermi (Museo Storico della Fisica e Centro Studi e Ricerche "Enrico Fermi") in collaboration with INFN, CERN and MIUR, designed to study cosmic rays via a network of muon telescopes, based on the Multigap Resistive Plate Chambers (MRPC) technology. Due to its wide coverage over the Italian territory (more than 10 degrees in latitude and longitude, covering more than 3 x 10(5) km(2)), the EEE network is the largest MRPC - based system for cosmic rays detection. Each MRPC has 6 gas gaps obtained by a stack of glass plate, spaced 250 mu m each, and is equipped with 24 copper strips. Since its beginning, the EEE MRPCs were filled with a gas mixture of 98% of tetrafluoroethane and 2% of sulfur hexafluoride, but recent restrictions on greenhouse gases have prompted the study of their performance with new gas mixtures. To this aim, extensive tests of tetrafluoropropene and carbon dioxide or sulfur hexafluoride gas mixtures have been carried out.