VACANCY RECOVERY IN HEAVY ION-IMPLANTED REFRACTORY BCC METALS STUDIED BY MOSSBAUER-SPECTROSCOPY

Mossbauer effect measurements are reported with sources of 133Xe implanted in Mo, W and Ta by means of the Leuven Isotope Separator. Information about the direct environment of an implanted radioactive probe atom is obtained through the hyperfine interaction of the daughter nucleus (133Cs) with its surroundings. Each of the sources is subjected to an isochronal annealing sequence in order to study the recovery of each metal. For Mo and W four visible components are detected in the Mossbauer spectra. Identification of the components on the basis of their Mossbauer parameters shows that the components represent substitutional xenon atoms and xenon atoms associated with one, two and three vacancies. The observed population of the visible sites leads to the necessity of the existence of at least one type of 'invisible' site with a recoilless fraction very close to zero. Vacancy trapping at xenon impurities occurs in Mo around 425K and in W around 550K. The authors show that the associated vacancy migration is largely a free migration, indicating that recovery stage III in Mo and W is due to vacancy migration. The free-vacancy migration in Mo and W is shown to be due to single vacancies. For Ta the results are less clear due to considerable overlap of lines in the Mossbauer spectra. However, the spectra suggest vacancy mobility around or below room temperature.