The Alpha Magnetic Spectrometer (AMS) to be installed on the International Space Station (ISS), in 2006, will measure cosmic ray fluxes of elements up to Iron, in the rigidity range from 1 GV to 1 TV, for at least three years. Its aims are the search for cosmic antimatter, through the detection of antinuclei with |Z|>=2; the search of non-baryonic dark matter and the study of the origin and isotopic composition of cosmic rays. AMS is a large angular acceptance spectrometer composed of different subdetectors; in particular, there will be a proximity focusing Ring Imaging CHerenkov detector (RICH). This will be equipped with a mixed radiator of aerogel and a square of sodium fluoride in the center, a lateral conical mirror and a detection plane made of 680 photomultipliers and light guides. The RICH detector allows the measurement of particle electric charge, up to the iron, and velocity of the particle, the last one with a resolution of 0.1% for protons. When a charged particle traverses a dielectric medium (radiator), with a speed greater than the speed of light in that medium, emission of Cherenkov photons occurs. These photons are distributed in a conical surface with an aperture angle (Cherenkov angle) related to the particle's velocity.

  The cone hits the photomultipliers plane and draws a ring with a certain geometrical acceptance. This ring is used to reconstruct the Cherenkov angle and therefore the particle velocity. An implementation of a double radiator configuration, made of aerogel and sodium fluoride, on one hand allows to increase the RICH acceptance and on the other hand extends the linear momentum region covered by the detector to lower values. Therefore there are more constraints on the propagation models of cosmic rays, based on a measurement of the ratios 3He/4He and 10Be/9Be. The measurement of the 10Be/9Be ratio will provide a determination of confinement time of the cosmic rays in the galaxy. Since 3He is essentially a secondary product coming from the spallation of 4He, its measurement will allow the determination of the mean density of interstellar material crossed.