Influence of Defects on the Angular Correlation of Positron-Annihilation Photons in Irradiated and in Deformed Iron

Measurements of two-photon angular correlations following positron annihilation in both electron-irradiated and plastically deformed iron have been made at room temperature. The changes in the angular correlation observed for both treatments are similar and are attributed to the presence of vacancy-type defects. The changes due to the presence of these defects are analyzed in terms of the enhancement of the probability of annihilation of a positron with conduction electrons relative to annihilation with core electrons. The observed effects saturate with increasing defect concentration for both treatments, the total change being larger at saturation for the deformed specimen than for the irradiated one. This is interpreted as being due to less overlap of the positron wave function with core-electron wave functions at vacancy-agglomerate trapping sites in the deformed specimen than at the single-vacancy sites present in the irradiated sample. Stepwise annealing of the specimens firmly established a vacancy-positron interaction, which tends to localize positrons in the vicinity of vacancies, as responsible for the angular correlation changes observed. Arrival of carbon interstitial impurities at single-vacancy sites completely nullified the vacancy-positron interaction. The capture cross section of a vacancy for a positron in iron is estimated to be about 10 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. A method for deducing this cross section from a combination of lifetime and angular correlation results is presented in the Appendices. Recovery of the single vacancies present in the deformed specimen reduced the effect of the deformation by approximately one-half. Additional annealing to 400°C reduced the effect further, but complete recovery was not achieved for the deformed sample up to this temperature.