TDPAC measurements on the reference and untreated sample indicate a loss in
anisotropy which is attributed mainly to the association of probe atoms wi
th defects produced by (n, gamma) reactions with isotopes of Ni and experie
ncing combined magnetic and quadrupole interactions of comparable strengths
. Evolution of defect free and substitutional fraction of probe atoms exper
iencing Larmor frequency characteristic of Ni matrix has been studied as a
function of isochronal annealing temperature in helium free alpha -irradiat
ed and homogeneously helium-implanted samples. No defect associated Larmor
precession frequency and/or quadrupole frequency could be deduced in these
uncorrelated damage studies. Comparison of recovery stages in alpha -irradi
ated and helium-implanted samples indicates the binding of helium associate
d defects by Hf impurities. Segregation of Hf atoms is observed in the heli
um free alpha -irradiated sample for annealing treatment at 973 K, while no
such effect is observed in the helium-implanted sample for isochronal anne
aling treatments up to 1273 K. (C) 2001 Elsevier Science B.V. All rights re
served.