THE ANGULAR CORRELATION OF ANNIHILATION RADIATION

The angular distribution of annihilation radiation from copper 64 has been measured with a pair of scintillation counters in coincidence. Coincidences were found when the counters and source were not quite collinear. The relation between the noncollinear coincidence rate and angle of deviation λ could be closely approximated to by an exponential of the form [Formula: see text], where λ0 = (3.60 ± 0.04) 10−3 radian. Interpreting this departure from antiparallelism of the two photons as arising from the residual motion of the annihilating pair, the average momentum of their mass center in copper is found to be (7.20 mc.) × 10−3 which corresponds to an electron energy of 13.2 ev., if the positron is assumed to be thermalized before annihilation.