POSITRON-ANNIHILATION STUDY OF HELIUM CLUSTERING IN ALPHA-IRRADIATED COPPER

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of l00appm and 400appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ2 and an increase of its intensity /2, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ2 corresponding to helium bubbles increases and saturates while its intensity I2decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium atom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using 10 B(n, α)7 Li reaction. Copyright © 1991, Taylor & Francis Group, LLC. All rights reserved.