Trapping of deuterium by helium bubbles and defects in ion-implanted tantalum

Nuclear reaction analysis and ion implantation techniques were used to investigate the trapping of deuterium (D) by He bubbles and implantation defects in Ta. A series of four complementary temperature ramp experiments have enabled us to determine that: (1) the binding enthalpy of D at trap sites associated with He bubbles in Ta is 0.53±0.05 eV relative to solution sites; (2) the binding enthalpies of D at implantation defects in Ta are 0.53±0.10, 0.42±0.10, and 0.33±0.10 eV; and (3) the approximate number of He bubble trap sites produced per He atom implanted into Ta is 0.45 for mean He atomic fractions ranging from 1.5% to 2.5%. The measured binding enthalpies are compared with effective-medium theory calculations of the binding enthalpies of both hydrogen chemisorption sites and monovacancy sites containing from one to six trapped hydrogen atoms. This comparison suggests that the strongest 0.53-eV traps associated with implanted He are bubbles which trap hydrogen on their interior surface via a chemisorptionlike mechanism. The strong 0.53-eV defect trap sites are probably vacancy clusters, whereas the 0.42-eV defect traps appear to be monovacancy sites containing one to two hydrogen atoms, and the 0.33-eV sites appear to be monovacancy sites containing three to six hydrogen atoms.