Energetic photons through water: an undergraduate experiment with correlated gamma-rays from Na-22

Measurements of the speed of light have been carried out in the past by the correlation between gamma rays detected in two scintillation detectors. To demonstrate that the speed of gamma rays in a transparent medium is very close to that observed in the vacuum, a time-of-flight experiment was organized in an educational context with undergraduate students, making use of two BaF2 detectors and a small intensity Na-22 source. Measurements of time-of-flight spectra were carried out for several values of the distance between the two detectors, allowing us to extract the speed of photons in air. A measurement was also done with an amount of water along the photon path and the result compared with that obtained in air. Various experimental aspects of the interaction of gammas with the detectors were finally addressed through GEANT simulations of the setup to interpret the results.