Positron implantation-profile effects in solids

A method is developed to measure positron implantation profiles in solids and the changes induced by anisotropies or externally applied electric fields through the dependence on depth, $x$, of positron-electron annihilations. The implantation profiles, ${P}_{+}(x)$, of positrons from $^{64}\mathrm{Cu}$ sources, with maximum kinetic energy 0.65 MeV, in 14 different solids, ranging in density $d$(g/${\mathrm{cm}}^{3}$) from 0.9 to 9, can be described by ${P}_{+}(x)=\mathrm{exp}(\ensuremath{-}\frac{x}{{R}_{+}})$, with a mean implantation range ${R}_{+}(0.65 \mathrm{MeV})=(345\ifmmode\pm\else\textpm\fi{}25){d}^{\ensuremath{-}1}(\ensuremath{\mu}\mathrm{m})$. Compared to polycrystalline samples, ${R}_{+}$ increases by 10% in Cu single crystals under channeling conditions along the 110g direction. The measured displacements of ${P}_{+}(x)$ in $C$ (diamond) and Si crystals induced by electric fields up to 5 kV/cm give positron mobilities 200 to 400 ${\mathrm{cm}}^{2}$/Vsec, which are 5 to 10 times smaller than the corresponding electron mobilities. Profiles of lattice defects induced in NaCl crystals by x rays of various energies are measured through positron trapping, as signified by $x$-dependent changes of the angular correlation between the annihilation $\ensuremath{\gamma}$ rays.