The time a particle takes to travel a known distance, used to identify it.Time-of-flight is a measurement technique that identifies a particle by how long it takes to travel a known distance. In particle physics, when a particle passes through a detector, its speed can be deduced from the time it takes to move between two points. Combined with its momentum (measured by how much it bends in a magnetic field), the speed reveals the particle's mass, and therefore its identity. This is a fundamental tool in experiments that study cosmic rays, nuclear reactions, and particles produced in accelerators.
The technique became practical with the development of fast photomultiplier tubes and electronics in the mid-20th century, and it has been a standard part of particle detectors since the 1960s.
Two detector layers are placed a known distance apart. When a particle hits the first layer, a clock starts; when it hits the second, the clock stops. The time difference, divided by the distance, gives the particle's velocity. Since particles of different masses but the same momentum travel at different speeds (lighter ones are faster), the velocity measurement distinguishes them. The technique is especially effective for particles moving at a significant fraction of the speed of light, where time differences of nanoseconds or picoseconds must be resolved.
Time-of-flight is used in particle physics detectors (e.g., at CERN), in mass spectrometry for chemical analysis, and in medical imaging (PET scanners) to improve image resolution.
In a typical particle physics detector, time-of-flight systems measure arrival times with a precision of about 100 picoseconds — the time it takes light to travel just 3 centimeters.
