Evidence for shallow positron traps in a neutron-irradiated A1 single crystal as studied with variable-energy positrons

A monoenergetic (±1 eV), variable-energy (0.5-4 keV) beam of positrons has been used to study the dependence on temperature (40-350 K) of positron diffusion out of a neutron-irradiated single crystal of A1. The results are interpreted in the context of a one-dimensional diffusion model which encompasses annihilations as well as trapping at voids and other microstructural defects in the bulk material by way of a removal rate ${\kappa }_{\mathrm{eff}}$ of positrons from freely diffusing states. ${\kappa }_{\mathrm{eff}}$ is found to have a slightly positive dependence on temperature above 125 K, as would be expected from previous measurements of the positron trapping rate into large voids. The data suggest a strongly negative dependence on temperatures below 125 K for ${\kappa }_{\mathrm{eff}}$, indicating the presence of some additional phenomenon which we attribute to positron localization in shallow, presumably radiation-induced, traps in the crystal. The results of our analyses provide support for conclusions reached previously for neutron-irradiated Mo. During preliminary annealing treatments of the sample we found, by Auger-electron spectroscopy, that Si produced by neutron-induced transmutation migrated to the crystal surface. It is conjectured that redistribution of Si within the sample is responsible for a recovery stage previously observed in neutron-irradiated A1 between 400 and 470 K.