The Angular Distribution of Positron Annihilation Radiation

The angular distribution of the two quanta which are emitted in the annihilation of a positron and an electron was observed with coincidence counters of high resolving powers both in time and angle. The direct observations are consistent with the hypothesis that the two quanta are emitted in exactly opposite directions to within at least one degree. A comparison of the counter efficiency obtained from this angular distribution curve with a direct and independent measurement of the counter efficiency for annihilation radiation shows that the half-width of a possible distribution of deviations from strict collinearity is zero to within 15 minutes of angle. It may be concluded that the momentum balance is accurately maintained by the quanta alone, and that, in general, a positron stops before it is annihilated in accord with the theoretical estimates.