Positron Mean Lives in Scandium, Yttrium, and the Rare-Earth Metals

Positron mean lives relative to that in aluminum are reported for scandium, yttrium, and all of the stable rare-earth metals. An automatic cycling procedure allows these relative measurements to be made with an accuracy of ±3×${10}^{-12\mathrm{}}$ sec. The average positron lifetime in the trivalent rare earths and in yttrium is 0.675×${10}^{-10\mathrm{}}$ sec longer than the positron lifetime in aluminum, with an average deviation of ±0.035×${10}^{-10\mathrm{}}$ sec. The positron lifetimes in the divalent rare earths (europium and ytterbium) are appreciably longer, while the lifetime in scandium is somewhat shorter. The experimental results indicate a correlation between the positron lifetime and the conduction electron density, and they show that the $4f$ electrons do not participate in the annihilation process.