The ACC

ACC_WhiteBkg-M.jpgCosmic-rays reach AMS-02 from all directions. However, the maximal analyzing power of the instrument is for particles traversing from top to bottom. Therefore, AMS-02 contains a anti-coincidence counter (ACC), in order to reject particles with a high incident angle.

The ACC is also important for the rejection of events with bad topology. High-energy particles passing through AMS materials (magnet, aluminium honeycomb, etcetera) could interact inelastically. The result of such an interaction is the production of many secondary particles, that will confuse the tracker pattern recognition. These events represent a significant background in the search for faint antimatter signals. ACC is designed to reject these kind of events as well.

The ACC is composed of a barrel of scintillation paddles arranged around the tracker. In this geometry, a particle travelling vertically will produce a signal in the ToF but not in the ACC. Conversly, a particle travelling horizontally will produce a signal in the ACC but not in the ToF. The restrictive condition of discarding particles which pass through the ACC is relaxed in two specific cases, which are shown on the right:

  • ACC_Trigger1.jpgincoming ions (high-Z particles), which produce secondary electrons (so called delta rays) when transversing matter. These secondary particles may result in a signal in the ACC. Therefore, the ACC veto is disabled in order to keep these ion events.
  • an electron/positron converting in the ECAL: in this case, the electron/positron passes through the ECAL, where backsplash particles are produced. These particles may leave the ECAL and pass though the ACC, creating a signal. In this case, the trigger condition is modified to "ToF signal and signal in less than 4 ACC paddles".

The ACC consists of 16 paddles arranged on a cylinder around the tracker. Embedded into grooves in the scintillating material are wavelength-shifting fibers with a diameter of 1 mm, which collect the scintillation light produced in the scintillators. These fibers exit the paddles at each end, where they are then routed (in two bunches of 37 fibers) to connectors located on the conical flanges of the magnet vacuum case. From there, the light is routed to 8 photomultipliers (PMTs) mounted on the rim of the vacuum case.

The very high efficiency and a high degree of homogeneity of the scintillating fibers will ensure a reliable and fast ACC veto trigger signal for high inclination particles.