Ring Imaging Cerenkov Counter


The Ring Imaging Cerenkov Detector, or RICH, is a clever way of measuring a particle's velocity close to the speed of light.

  • Why we need the RICH Extremely accurate velocity measurements are needed to distinguish light isotopes. Beryllium-9 and Beryllium-10, for example, are exactly identical except for a 10% mass difference. That's a very small difference, too small to be seen with the tracker and time-of-flight information. However, the RICH's velocity measurement can clearly tell them apart (when combined with the tracker's momentum measurement)

  • How the detector works Light moves slower in glass than in vacuum. Many particles that hit AMS, (while of course they are slower than the vacuum speed of light) may be traveling faster than the glass speed of light! This sort of particle emits a "sonic boom" of light when it passes through glass. This is in principle very similar to the sonic boom emitted by a supersonic jet. When the Concorde exceeds the speed of sound in air, it emits a conical shock wave of sound; similarly, when a particle exceeds the speed of light in glass, it emits a conical spray of light called Cerenkov radiation (pronounced "chair EN cough"). Measuring the opening angle of the cone tells you the speed!

  • How the detector is built The "sonic boom" is generated when the particle crosses a piece of ultralight glass ("aerogel") or salt (Sodium fluoride, NaF). There are a few hundred tiny, pixelated photon detectors (photomultiplier tubes or "PMT"s) looking up at the aerogel or salt; when the Cerenkov radiation occurs, it will make a circular pattern on the PMTs. We try to gather as much of the Cerenkov light as possible, so there is an ultra-thin plastic mirror surrounding the RICH, which reflects any escaping light back onto the PMT plane. (This can make some of the circles look ... well, very non-circular, but they can still be reconstructed and correctly interpreted)

    Despite wanting to catch all of the photons, we've left a gap in the PMT layer in the middle. PMTs are heavy and dense, and we don't want particles to crash into the PMT material above the calorimeter. This would confuse the calorimeter and make it less accurate.

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