MUPALS -- Miami University Positron Annihilation Lifetime Spectrometer

A spectrometer for measuring positron lifetimes in solids is described. It consists of a pair of BaF$_{\mathrm{2}}$ scintillators coupled to photomultiplier tubes and a fast-slow delayed coincidence system composed of standard NIM modules. The instrumental resolution curve as determined with a $^{\mathrm{60}}$Co source is well-described by a Gaussian with a full-width at half-maximum of approx. 300~ps. We used this apparatus to determine the lifetimes of positrons in commercial grade Al and PTFE (Teflon) at room temperature with a $^{\mathrm{22}}$Na positron source enclosed in Kapton$^{\mathrm{\mbox{\textregistered }}}$ foil. Both samples show a lifetime component in the 350--450 ps range. While the PTFE sample has a long component near 2 ns, the Al sample has a short (approx. 200 ps) as well a long component (\textgreater 2 ns).