Isn - 99 . 98 Performance Results of the Ams - 01 Aerogel Threshold

XIth Rencontres de Blois (Frontiers of Matter) proceedings. The Alpha Magnetic Spectrometer (AMS) was flown in june 1998 on board of the space shuttle Discovery (flight STS-91) at an altitude ranging between 320 and 390 km. This preliminary version of AMS included an Aerogel Threshold ˘ Cerenkov detector (ATC) to separate ¯ p from e − background, for momenta less than 3.5 GeV/c. In this paper, the design and physical principles of ATC will be discussed briefly, then the performance results of ATC will be presented. One of the purposes of the AMS Shuttle flight was to search for cosmic ¯ p and to measure ¯ p/p ratio for momenta(P) below 3.5 GeV/c. The major background component to the ¯ p sample is expected to be e − (¯ p/e − ∼ 10 −2 for the considered P range). Using AMS-01 momentum resolution 1 (∆P /P = 7.%) and β resolution 2 (∆β/β ≃ 3.3%), it can be shown 3 that AMS electron rejection falls sharply above 1.5-2 GeV/c. ATC is used to extend ¯ p/e − discrimination range up to 3.5 GeV/c. As a secondary result, p/e + separation can also be improved using appropriate ATC selections. Many balloon-borne experiments (CAPRICE, BESS,...) have included a ˘ Cerenkov counter and a Ring Imaging ˘ Cerenkov counter is under study for the next phase of the AMS experiment 4,5. Using an aerogel material with a low refractive index (n=1.035), the ATC counter profits from ˘ Cerenkov effect to separate ¯ p from e − at low energy. Indeed, the momentum threshold is 1.9 MeV/c for e ± and 3.5 GeV/c for p(¯ p). Hence, in the GeV range and up to this momentum, p(¯ p) are not expected to produce any signal, while e ± will give a full signal. The design of ATC will be described in detail elsewhere 3. The elementary component of the ATC detector is the aerogel cell (11 × 11 × 8.8 cm 3 , see figure 1), filled with eight 1.1cm thick aerogel blocks. The emitted photons are reflected by three 250µm teflon layers surrounding the blocks, until they reach the photomultiplier's window (Hamamatsu R-5900).