Lifetime of Positrons in Superconducting Lead

Comparison of the mean lifetime of positrons in lead at 4.2\ifmmode^\circ\else\textdegree\fi{}K with that at 10\ifmmode^\circ\else\textdegree\fi{}K indicates (a), no change greater than 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec in the short-lived component, and (b), no more than 0.3% abundance of a component with lifetime greater than 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec.