Trapping of hydrogen isotopes in molybdenum and niobium predamaged by ion implantation

Abstract

The trapping of hydrogen isotopes at defects in Mo and Nb have been studied. Ion beams of 11‐ and 18‐keV He⁺, 55‐keV O⁺ and Ne⁺, and 500‐keV Bi⁺ were used to create defects. Subsequently H or D was injected at room temperature by use of molecular beams of 16‐keV H⁺₂ and D⁺₂. Appreciable enhancements were observed in the amount of H and D retained within the near‐surface region of predamaged samples compared to samples with no prior damage. The total amount of D retained within the near‐surface region was measured by means of the nuclear reaction D(³He,p)⁴He, and H depth profiles were measured via a resonance in the nuclear reaction ¹H(¹⁹F,αγ)¹⁶O. The H profiles correlate with the predicted predamaging ion profiles; however, appreciable tails to deeper depths for the hydrogen profiles are observed for the heavier predamaging ions. For a given predamage ion fluence, the amount of trapped deuterium increases linearly with incident deuterium fluence until a saturation in the enhancement is reached. The amount of deuterium trapped when saturation occurs increases with increasing predamage fluence. The experiments indicate that lighter ions, which create fewer primary displacements, are more effective per displacement in trapping hydrogen. An appreciable release of hydrogen is obtained upon annealing at 200 and 300 °C, and a preannealing experiment indicates this is due to detrapping rather than to any loss of traps. These temperatures suggest a much higher binding energy for the trapped hydrogen isotopes (∼1.5 eV) than the available evidence gives for simple H‐defect binding energies (≲0.3 eV). The detailed trapping mechanism is not known. However, it is suggested on the basis of the high binding energies and the high concentrations of hydrogen which can be trapped that clusters of hydrogen may be formed.