The AMS-02 RICH detector: Performance during ground-based data taking at CERN

The Alpha Magnetic Spectrometer (AMS), whose final version AMS-02 is to be installed on the International Space Station (ISS) in 2011, is a detector designed to measure charged cosmic ray spectra with energies up to the TeV region and with high energy photon detection capability up to a few hundred GeV, using state-of-the art particle identification techniques.Among several detector subsystems, AMS includes a proximity focusing RICH detector enabling precise measurements of particle electric charge (charge identification up to the iron region) and velocity (Δβ/β∼10−3Δβ/β∼10−3 for Z  =1, Δβ/β∼10−4Δβ/β∼10−4 for Z=10–20). The optimization of the RICH reconstruction efficiency imposed a dual radiator configuration with 16 NaF tiles (n=1.33) in the centre and 92 aerogel tiles (n=1.050) in the outer region, a pixelized detection matrix with 680 Hamamatsu R7600-M16 photomultipliers (each with 4×4 pixels) and a highly reflective conical mirror to increase photon collection.After its assembly at CIEMAT in Madrid, the RICH was taken to CERN in January 2008 and integrated into the full AMS-02 detector. AMS-02 underwent a pre-assembly in 2008 without magnet followed by a second detector assembly with a superconducting magnet in 2009 and the final assembly with a permanent magnet in mid-2010. Cosmic events were acquired in the context of the 2008 pre-assembly and in 2009, and two beam tests from CERN SPS took place in 2010. Results obtained with data from ground-based tests on the RICH performance are presented. A comparison with the aerogel light yield obtained on previous beam tests with a prototype detector is also discussed.