Positron annihilation study of tungsten exposed to low-energy deuterium plasma

Positron annihilation lifetime spectroscopy (PALS) measurements by use of a positron source of 22Na were performed for the polycrystalline tungsten samples that were exposed to low-energy deuterium (D) plasma. The energy of D plasma was low and then it was expected that it would affect just the near-surface region. However, we obtained the longer mean positron annihilation lifetime in the tungsten samples exposed to the low-energy D plasma than the virgin tungsten sample. Moreover, the same longer values were obtained even on the other (no exposed) side of the samples, although the thickness of the samples was 1 mm or 2 mm. The elongation of the positron lifetime probably indicates defects formation in W. Ohsawa, et al. predicted that the high concentration hydrogen creates high concentration vacancy-hydrogen clusters by the first-principles calculations. We also performed positron annihilation age-momentum correlation (AMOC) measurements to know if there are some D’s at the positron trapping sites such as vacancies and found that some of the positrons annihilate with electrons in D.