Abstract
Thermal Helium Desorption Spectrometry (HDS) has been used to study the room temperature nucleation of helium precipitates at point defects in Ni(110), notably HeV defects at depth ~ 20 nm below the crystal surface. Helium is injected into the crystal by 50 eV He ion-irradiation which causes no atomic displacements. It has been observed that He nV defects with occupation from n = 2 He to n = 4 He bind helium equally strongly,but weaker than for HeV. For n⩾5 He the binding increases rapidly. The observed behaviour is attributed to helium induced trap mutation and agrees qualitatively with results of atomistic calculations in nickel for this case. Helium precipitation at near surface trapping sites is held responsible for the observed increase of helium release temperatures with helium dose when an undamaged crystal is irradiated. Preliminary TEM observations of Ni specimens irradiated with 50 times higher helium doses than the maximum dose used in the HDS experiments indicated planar clustering of the helium.
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