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, $P_{+}\mathrm{}\left(x\right)\mathrm{}$, 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 $P_{+}\mathrm{}\left(x\right)=\exp (-\frac{x}{R_{+}})$, with a mean implantation range $R_{+}\left(0.65\mathrm{} \mathrm{MeV}\right)=(345\mathrm{}\pm 25)d^{-1\mathrm{}}\left(\mu \mathrm{m}\right)$. Compared to polycrystalline samples, $R_{+}$ increases by 10% in Cu single crystals under channeling conditions along the direction. The measured displacements of $P_{+}\mathrm{}\left(x\right)\mathrm{}$ 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 $\gamma$ rays.