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Time of flight

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time of flight

The time for a photon created by annihilation of a positron-electron pair to reach a detector; since annihilation photons are created in pairs and travel in opposite directions at about 3 × 1010 cm/sec, measurement of the difference in arrival time at detectors with sub-nanosecond resolution allows calculation of the location of the event; the basic physics of positron emission tomography.