A time-of-flight (TOF) detector is a particle detector which can discriminate between a lighter and a heavier elementary particle of same momentum using their time of flight between two scintillators. The first of the scintillators activates a clock upon being hit while the other stops the clock upon being hit. If the two masses are denoted by and and have velocities and then the time of flight difference is given by where is the distance between the scintillators. The approximation is in the relativistic limit at momentum and denotes the speed of light in vacuum. See also Time-of-flight mass spectrometry Particle detectors