2.3 Prototype Tests
For the test, the same Hamamatsu R5900 U PMT was used, as it will be the case in the AMS experiment. By removing the aerogel from the box, we observed 1.7 photoelectrons coming from Cerenkov production in PMP. However, in the ATC since the transparency of the aerogel is not unity, we expect that this noise will drop to 0.8 photoelectrons. This is going to be an irreducible background. Furthermore, we expect that the level of signal of particles above the Cerenkov threshold will be 20% lower than in our tests, since the magnetic shield cover is forcing us to move the location of the PMT by approximately 2 cm. In any event, we estimate that we need at least 6 detected photoelectrons on ATC (i.e. sum of both layers) in order to get an electron rejection power higher than 1:2000, using a threshold of 1 photoelectron (Section 2.5).