Positron Lifetime Spectroscopy

We have assembled a positron lifetime spectrometer using two scintillation detectors in a slow-fast coincidence configuration. The decay of $^{22}$Na serves as a convenient source of positrons. Positron energies are sufficiently high that positrons penetrate into the bulk of the surrounding material under study. A 1275 keV gamma ray emitted less than 10 ps following the positron decay serves as the lifetime start signal, while the detection of a 511 keV annihilation gamma signals the end of life of the positron. The first version of our spectrometer employs NaI scintillators that have good detection efficiencies but with time resolution of several nanoseconds are not particularly suitable for measuring sub-nanosecond lifetimes of positrons in metals. Recently we have replaced the NaI detectors with ones employing plastic scintillators, which offer a time resolution of better than 1 ns. First results of measurements performed with this apparatus will be discussed.