Conservation of longitudinal spin polarization of positrons emitted from a thin Ni(100) foil

Abstract

We have discovered that 5 keV bursts of $5\ifmmode\times\else\texttimes\fi{}10{}^{7}$ positrons with an initial longitudinal spin polarization of $(28.8\ifmmode\pm\else\textpm\fi{}0.7)%$, when implanted into a thin Ni(100) crystal, are emitted with 20% efficiency at thermal energies from its surface with $(30.9\ifmmode\pm\else\textpm\fi{}0.5)%$ polarization. We conclude that the positron spin polarization is preserved while interacting with the Ni, despite the 0.61 T average transverse magnetization of the Ni at room temperature. The resulting polarized beam has been focused to a 0.025-mm mean-diameter spot when accelerated to 5 keV and will be uniquely suited for experiments on a neutral spin aligned ${e}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$ plasma, spin- and angle-resolved positronium emission spectroscopy, and critical for producing a triplet positronium Bose-Einstein condensate.