Time-Of-Flight


The Time-Of-Flight system, or TOF, uses accurate timing to measure a particle's velocity and direction.

  • Why we need the TOF Zoom! There goes a particle! How fast was it going? In the case of an electron at typical cosmic ray energies (billions of electron volts), it was probably going very close to the speed of light. A proton or nucleus (He, Be, Fe, Si ...) at the same momentum would be going a lot slower. The Time-Of-Flight (TOF) system is a speed trap for particles; it sees each particle when it enters the magnet barrel, and again when it leaves. The TOF measures the time elapsed between these two events, and this tells you the speed. We expect to get about 2% precision; that is, we can tell 96% from 99% of the speed of light, but we can't tell 99.5% from 99.8%.

    The TOF also tells you whether the particle was going upwards or downwards. Upgoing particles are dangerous; if you see one going through the tracker, and mistake it for a downgoing particle, you will derive the wrong value for the charge. You'll mistake matter for antimatter, which we very much do not want to do!

    In addition to these timing functions, the TOF gives an independent measurement of the particle's charge.

  • How the detector works The particles are seen by a "scintillator" bar. This is a specially formulated plastic which emits a flash of light when a particle passes through. The light (a few hundred photons) bounces down the bar, through a twisty light pipe, and is detected by a photomultiplier tube. This all happens very fast and reproducibly, so we have timing accuracy down to 120 trillionths of a second (picoseconds). We detect the flash at both ends of the bar; by careful timing, we can figure out exactly where (and when) the particle crossed. Also, the phototubes don't work well if there is a magnetic field sideways across them, so the light pipes are "twisted" to position the phototubes along the fields.

  • The AMS Trigger AMS cannot read out all of its detectors all of the time; it must wait until an interesting particle passes through, and then collect the data. (In this way, it is like a photographic camera, not like a video camera) The TOF, being a very fast detector, alerts the other detectors that a particle is passing. This is called the "Level 0 Trigger".
    Click here to return to the AMS tour Lower plane of scintillators twisted light pipes Photomultiplier tubes the silicon tracker goes here in the middle support structure ("M structure") support structure ("M- structure') support structure ("M-structure") Upper plane of scintillators Photomultiplier tubes photomultiplier tubes