Propagation speed of γ-radiation in air

To perform such measurements the availability of a gamma radiation source in which two γ-rays are emitted simultaneously in opposite directions -as already used(5,6) as well as applied in the present case- turns out to be essential to the feasibility of the experiment, as far as no reflection techniques could be used. Such suitable source was the positron emitter 22Na placed in a metal container in which the positrons are stopped and annihilated when reacting with the medium electrons, in such way originating -as it is very well established from momentum/energy conservation laws(7)- two γ-rays, energy 511 KeV each, both emitted simultaneously in opposite directions. In all these previous experiments were used photomultiplier detectors coupled to NaI(Tl) crystal scintillators, which have a good energy resolution but a deficient time resolution for such purposes. Presently, as an innovatively improvement, were used BaF2 and CsF crystal scintillators which display a much better time resolution. (author)