Tests of the AMS RICH prototype with secondary ions at CERN

The AMS experiment will be installed on the International Space Station in 2008. It will include a proximity focusing Cherenkov imager (RICH) to perform isotope separation for light nuclei between 1 and about 10 GeV/c and element identification up to beyond Fe (Z=26) over a momentum range extending from threshold up to the upper limit of the spectrometer capability (TeV/c per nucleon). The architecture of the counter consists of a solid state radiator plane at the top, combining silica aerogel and sodium Fluoride (NaF) Cherenkov radiators, separated from the photodetector plane by a drift space for Cherenkov ring expansion. The detector plane includes 680 16-anode PMTs, corresponding to 10880 readout channels. A prototype of the counter including 96 PMTs has been built and tested at the CERN SPS, using secondary beams of ion fragments obtained from primary beams of Lead (Pb) and Indium (In) ions at 20 and 158 GeV/c per nucleon respectively. The response of the prototype to these ions has been studied for various radiators (silica aerogels with refraction index 1.03 to 1.05 and NaF). Cherenkov rings associated to ions over the covered range of mass have been observed, the charge and mass resolutions of the prototype have been evaluated from the analysis of the 5 million and 11 million events recorded during the two experimental runs respectively. The results obtained with the prototype will be presented together with the design and the expected performances of the final detector