  The Alpha Magnetic Spectrometer (AMS) to be installed on the International
Space Station (ISS) will measure charged cosmic ray spectra of elements up to
iron, in the rigidity range from 1 GV to 1 TV, for at least three years. AMS is
a large angular spectrometer composed of different subdetectors, including a
proximity focusing Ring Imaging CHerenkov (RICH) detector. This will be
equipped with a mixed radiator made of aerogel and sodium fluoride (NaF), a
lateral conical mirror and a detection plane made of 680 photomultipliers
coupled to light guides. The RICH detector allows measurements of particle's
electric charge up to iron, and particle's velocity. Two possible methods for
reconstructing the Cherenkov angle and the electric charge with the RICH will
be discussed.
  A RICH prototype consisting of a detection matrix with 96 photomultipliers, a
segment of a conical mirror and samples of the radiator materials was built and
its performance was evaluated using ion beam data. Results from the last test
beam performed with ion fragments resulting from the collision of a 158
GeV/c/nucleon primary beam of indium ions (CERN SPS) on a lead target are
reported. The large amount of collected data allowed to test and characterize
different aerogel samples and the NaF radiator. In addition, the reflectivity
of the mirror was evaluated. The data analysis confirms the design goals.
