  Due to their simplicity and comparatively low cost, Resistive Plate Chambers
are gaseous detectors widely used in high-energy and cosmic rays physics, when
large detection areas are needed. However, the best gaseous mixtures are
currently based on tetrafluoroethane, which has the undesirable characteristic
of a large Global Warming Potential (GWP) of about 1400 and, because of this,
it is currently being phased out from industrial use. Tetrafluoropropene (which
has a GWP close to 1) is being considered as a possible replacement. Since
tetrafluoropropene is more electronegative than tetrafluoroethane, it has to be
diluted with gases with a lower attachment coefficient in order to maintain the
operating voltage close to 10 kV. One of the main candidates for this role is
carbon dioxide. In order to ascertain the feasibility and the performance of
tetrafluoropropene-CO2 based mixtures, an R&D program is being carried out
within the ALICE collaboration, employing an array of 72 Bakelite RPCs (Muon
IDentifier, MID) in order to identify muons. Different proportions of
tetrafluoropropene and CO2, with the addition of small quantities of isobutane
and sulphur hexafluoride, have been tested with 50x50 cm 2 RPC prototypes with
2 mm wide gas gap and 2 mm thick Bakelite electrodes. In this contribution,
results from tests with cosmic rays will be presented, together with data
concerning the current drawn by a RPC exposed to the gamma-ray flux of the
Gamma Irradiation Facility (GIF) at CERN.
