Anti-Coincidence Veto Counters (ACC)


The Anti-Councidence Counter system, or ACC, helps tell AMS which particles are worth analyzing.

  • Why we need the ACC The Anti-coincidence counters (ACC) make AMS more efficient and reliable. There will be 10000 particles per second passing through AMS in all directions; we don't want to record them all. We can only learn physics from the ~2000 particles per second that come in through the top and pass through several detectors. The ACC is a detector "barrel" surrounding the AMS tracker; by looking for signals (and lack of signals) in this barrel. A particle which hits both Time-Of-Flight planes, and does NOT hit the ACC barrel, is likely to be worth keeping.

  • The ACC is vital for the Anti-helium search The important thing about the AMS anti-helium search: if we see a single antihelium nucleus, we want to be able to announce it to the world. If there are particles passing through AMS's sides, as well as through the top, it can complicate things; it is possible that some combination of particles passing sideways through the detector (or even a shower of particles from a hard nuclear collision) would fake an antihelium signal. The ACC saves us the trouble of worrying about such effects, by detecting them and throwing out the data.

  • How the detector works The ACC surrounds the trackers. It is made of two very thin layers of plastic scintillator, a lot like the time-of-flight system. Flashes of light from the plastic are detected by photomultiplier tubes. Since these photomultipliers do not work in strong magnetic fields, we must mount them far away from the magnet, and use light-pipes to carry the photons to them. Since everything is doubled up (two layers, each read out by at least two PMTs), any one component can fail and the ACC will still work.
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    Technical details

  • Weight: ~50 kg
  • Electronics weight: ~60 kg (shared with Time-of-Flight)
  • Electronics power: ~190 W (shared with Time-of-Flight