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 PP₀=1.16±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 τ₃ equal to that in free space (τ₃=1.39×10^-7 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 (PfP₀=1.00±0.05). From the same measurements in insulators, the yield in Al is P₀=(2.64±0.08)10^-3, in good agreement with the value predicted by Ore and Powell (2.70×{}10^-3).