  The Alpha Magnetic Spectrometer (AMS) to be installed on the International
Space Station (ISS) will be equipped with a proximity Ring Imaging Cherenkov
(RICH) detector for measuring the velocity and electric charge of the charged
cosmic particles. This detector will contribute to the high level of redundancy
required for AMS as well as to the rejection of albedo particles. Charge
separation up to iron and a velocity resolution of the order of 0.1% for singly
charged particles are expected. A RICH protoptype consisting of a detection
matrix with 96 photomultiplier units, a segment of a conical mirror and samples
of the radiator materials was built and its performance was evaluated. 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 sodium fluoride radiator. In
addition, the reflectivity of the mirror was evaluated. The data analysis
confirms the design goals.
