The interaction of injected helium with lattice defects in a tungsten crystal

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Abstract
Helium injected into tungsten has been found to undergo rapid interstitial diffusion at room temperature unless it encounters lattice defects. This effect has been used in a study of the lattice damage produced near a (100) tungsten surface by the impact of small numbers of 5 keV heavy ions. The helium, injected as 250eV ions, does not itself produce any observable damage, but is trapped in that created by the prior heavy ion bombardment. Thermal desorption spectra obtained by raising the crystal temperature at 40°K/sec show that the helium is bound with several discrete energies. Attempts to correlate the binding states with particular lattice defects have been made by varying the damage anneal temperature, the helium dose, the helium injection temperature and the mass of the damaging ion. The results suggest that all of the entrapment occurs at defects of the vacancy type, and that the binding energy is modified by parameters that change the available vacancy volume. In particular there is evidence that at least two and possibly three helium atoms can be trapped in a single vacancy, that two types of divacancies exist, and that vacancies become trapped in the strain field of large impurity atoms. It also appears that at sufficiently high helium doses (> 1014 ions/ cm2), any of the trapping sites can act as nuclei for the formation of helium bubbles.