Studies on the near-surface trapping of deuterium in implantation experiments

Abstract

Surface-shifted deuterium profiles are re-examined in deuterium-ion irradiation experiments by using a combined experimental and modelling approach. Recrystallized tungsten foil samples were irradiated with energetic deuterium ions and the defect and deuterium depth profiles were studied using positron annihilation spectroscopy and secondary ion mass spectroscopy. We report direct experimental evidence of trapping of deuterium at the vacancies created by the deuterium ions themselves during the implantation by using positron annihilation studies. The deuterium profile is simulated using a Monte-Carlo diffusion model by taking into account the defect-aided diffusion of deuterium due to the local strain field created by the vacancies. The simulations also elucidate the role of the anisotropy in the diffusion and trapping of deuterium in ion-implantation experiments in metals.

Studies on the role of ion mass and energy in the defect production in irradiation experiments in tungsten

“…The samples irradiated with 250 keV He and 80 MeV Au have a higher dpa and subsequently have a smaller L eff . For comparison we have also shown the diffusion length of a 50 keV D-irradiated sample taken from [18] where, for an average dpa of 0.65, the diffusion length was further smaller as expected. However, the S-parameter was much smaller compared to Au-/He-irradiated W. In the 50 keV D-sample we have reported extensive deuterium trapping in the defects, which might have reduced the S-parameter to a lower value, despite having a higher dpa.…”

“…Cold-rolled W foils of 99.96 wt% purity were procured from Princeton Sci. Corp., USA with the major impurities being Mo (∼0.01), C (∼0.002), O (∼0.0025), N (∼0.0015) and Fe (∼0.0045) [18]. Mechanically polished samples of 8 × 8 × 0.1 mm 3 size were annealed under two different conditions.…”

“…Details of the experimental set up can be found in [22]. The experiment was similar to that described in [18]. The depth distribution analysis of the samples was carried out by SIMS using a magnetic-sectorbased Cameca IMS-7f instrument equipped with both oxygen (O 2+ and O − ) and cesium (Cs + ) primary ion beams [23][24][25].…”

“…The defect (vacancy) density of the recrystallized samples was calculated using the defect model of positrons [28] with a specific trapping rate of ∼4 × 10 14 s −1 [29]. (For the RC1 samples, the vacancy cluster density was found to be about 2.4 × 10 23 m −3 [18] whereas, for the RC2, it was about 5.03 × 10 23 m −3 ). It appears that the samples annealed in the Ar + H environment have a lower defect concentration, which could be due to the reduced surface oxidation in the Ar + H environment.…”

“…It appears that, independently of the ion energy or mass, at this defect concentration and the dpa, the positron trapping saturates in W. At higher dpa, the bulk S-parameter becomes higher than the surface values. However, if there is significant gas trapping in the defects, such as deuterium, even a higher dpa can lead to lower saturation of the S-parameter, as shown in the 50 keV D-ions in [18] where, for an average dpa of 0.65, the S-parameter was low. The gas trapping reduces the available open volume defects, which might lead to a lower S-parameter, while keeping the effective diffusion length still proportional to the total defects produced (figure 5(b)).…”