The time of flight (TOF) system measures the flight time of particles with high resolution. Combined with the momentum of the particle (derived from the curvature of its tracked in the magnetic field, measured with central detector tracking chamber), this gives a contstraint on the particle mass that aids in identifying the particle type. The barrel time of flight system consists of 64 counters just outside the central drift chamber and fastened by straps to the inside surface of the crystal calorimeter. The detector are 5 or 10 cm thick bars of Bicron BC-408 scintillator--a material that emits a flash of light when a charged particle passes through it--that run the length of the detector barrel. At both ends of each scintillator is a lucite light pipe that channels light to a phototube light detector that sits outside the magnetic field of the superconducting coil. This system allows measuring the flight time of particles produced at the interaction point to 150 picoseconds.
There are similar scintillators in the endcaps of the detector, providing a total solid angle coverage of 97%.