The Anti-Coincidence Counter
The Anti-Coincidence Counter (ACC) is the AMS Cerberus. Ten thousand particles per second pass through AMS. ACC discards 8/10 of these particles and save the ones useful for the physics analysis.
Why do we need the ACC?
Cosmic-rays come from all directions. The AMS has the maximum analyzing power for particles traversing the instrument from top to bottom. Particles with a high incident angle can not be measured well, ACC is meant to reject them.
ACC is also important for the rejection of events with bad topology. Indeed high-energy particles incident on AMS materials (magnet, aluminium honeycomb, etcetera) could interact inelastically. The result of such interaction is the production of a lot of particles, that will confuse the Tracker pattern recognition. These events could be a significant background for the search of faint antimatter signals. ACC is designed also to reject these kind of events.
How does the ACC work?
ACC is a barrel of scintillation counters around the Tracker. In such a geometry a vertical particle will give a signal in ToF and not on ACC. Conversely a horizontal particle may give a signal on ACC and not in ToF. Then an event should be recorded following the logic TOF AND NOT ACC. This restrictive condition can be released in 2 particular condition:
1) an incoming ion (high-Z particles): when an ion traverse matter it is accompanied by electron production, the so called δ-rays (central case in the figure). These electrons can easily fire the ACC. In order to keep ion events the ACC veto is disabled.
2) an electron/positron converting on the ECAL: when an electon/positron passes through the ECAL backsplash particles are produced (rightmost case in the figure). These particles may exit from the calorimeter surface and hit the ACC. In this case the trigger condition should be TOF AND NOT MORE THAN 4 ACC PADDLES FIRED.
How is the ACC is built?
Sixteen paddles arranged on a cylinder surrounding the Tracker constitute the ACC. Wavelength shifter fibers of 1 mm of diameter, embedded in grooves milled into the scintillating material, collect the light coming from the scintillating paddles. At both ends of the paddle, fibers are routed on 2 bunches of 37 fibers each to connectors located on the conical flanges of the magnet vacuum case. From these connectors, the light is routed through clear fibers to 8 PMTs mounted on the rim of the vacuum case. The ACC PMTs, similar to the ToF ones, are placed approximately 40 cm from the racetrack coils and are oriented with their axes parallel to the stray field in order to minimize the magnetic field effect.
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.