Prototype study of the Cherenkov imager of the AMS experiment

The AMS experiment includes a Cherenkov imager for mass and charge identification of charged cosmic rays. A second generation prototype has been constructed and its performances evaluated both with cosmic ray particles and with beam ions. In-beam tests have been performed using secondary nuclei from the fragmentation of 20GeV/c per nucleon Pb ions and 158GeV/c per nucleon In from the CERN SPS in 2002 and 2003. Partial results are reported. The performances of the prototype for the velocity and the charge measurements have been studied over the range of ion charge Z≲30Z≲30. A sample of candidate silica aerogel radiators for the flight model of the detector has been tested. The measured velocity resolution of the detector was found to scale with Z-1Z-1 as expected, with a value σ(β)/β≈0.7σ(β)/β≈0.7–110-3110-3 for singly charged particles and an asymptotic limit in Z   of 0.4–0.6×10-40.6×10-4. The measured charge resolution obtained for the n=1.05n=1.05 aerogel radiator material selected for the flight model of the detector is σ(Z)=0.18σ(Z)=0.18 (statistical) ⊕⊕ 0.015 (systematic), ensuring a good charge separation up to the iron element, for the prototype in the reported experimental conditions.