Annihilation of Positrons in Flight

The weak continuous spectrum of hard $\gamma$ rays resulting from the stopping of positrons in various materials has been investigated with a NaI(Tl) scintillation spectrometer. Measurements were made chiefly in the $\gamma$-ray energy interval from 0.511 Mev to 1.3 Mev, for the continuous $\beta$ spectra of ${\mathrm{Ne}}^{19}$ and ${\mathrm{A}}^{35}$ (maximum positron energies of 2.2 Mev and 4.4 Mev, respectively). The experimental geometry was chosen to ensure averaging of the annihilation spectrum over all angles. Theoretical spectra were calculated for Lucite, brass, and lead as stopping materials. Comparison with experiment on an absolute basis was made possible by an experimental determination of the spectrometer efficiency as a function of $\gamma$-ray energy. The calculations included bremsstrahlung and single-quantum annihilation as well as two-quantum annihilation. In the case of ${\mathrm{Ne}}^{19}$ positrons annihilating in brass, the data are in satisfactory agreement with the calculations. However, the calculated spectra show a conspicuously stronger dependence on the atomic number of the stopping material than do the observed spectra. The experimental spectra from ${\mathrm{Ne}}^{19}$ positrons stopping in Lucite and brass are indistinguishable, while the calculated spectra show a brass to Lucite ratio of 1.3. In addition, the calculated intensities tend to be lower than the observed ones, particularly for ${\mathrm{A}}^{35}$ positrons and for stopping materials of low atomic number.