Helium-vacancy interaction in tungsten

The helium-decorated In-vacancy complex InV₂{He}ₙ$ in tungsten was studied with the aid of the perturbed-angular-correlation technique, using $¹¹¹In$ as a probe. The release steps In${V}₂Heₙ{→}InV₂{He}_{n{{-}}1}$+He were identified for n=1--5. The corresponding dissociation energies are in the range of 2.9--4.5 eV, and agree perfectly well with the values determined by means of thermal-helium-desorption spectrometry. The quadrupole frequency is about 122 Mrad/s for n=1,2 and about 101 Mrad/s for n=3,4 as compared to 133 Mrad/s for the undecorated In-vacancy complex (n=0). Trap mutation occurs when the vacancy is filled to an occupation number n{}10, and gives rise to a quadrupole frequency of 218 Mrad/s. Trap mutation ultimately leads to bubble formation. At least three different bubble-associated quadrupole interactions were observed.