Three-Quantum Annihilation of Positrons in Metals and Insulators

The three-quantum decay of positrons annihilating in some metals and in some insulators was investigated by using a triple coincidence method. Relative measurements were made by comparing the three-quantum yield in each substance with that of aluminum. Small differences between the three-quantum yields of various metals were found: in Pb, where this effect appears to be the largest, the relative yield is $\frac{P}{P_0}=1.16\mathrm{}\pm 0.03$. At present any simple attempt to explain this result is unsuccessful. From the relative three-quantum rates measured in insulators, and on the basis of a simple model, the following information was obtained: The contribution to three-quantum production in insulators due to the positronium atom arises only from the orthostate decaying with a partial lifetime ${\tau }_3$ equal to that in free space (${\tau }_3=1.39\mathrm{}\times {10}^{-7\mathrm{}}$ sec). No evidence was found of three-quantum production arising from interaction of the positronium with matter. The remaining three-quantum annihilations occur with the same yield characteristic of positrons in Al ($\frac{P_f}{P_0}=1.00\mathrm{}\pm 0.05$). From the same measurements in insulators, the yield in Al is $P_0=(2.64\mathrm{}\pm 0.08){10}^{-3\mathrm{}}$, in good agreement with the value predicted by Ore and Powell (2.70×${10}^{-3\mathrm{}}$).