GAS TRAPPING AND RELEASE IN POLYCRYSTALLINE NICKEL PREIMPLANTED WITH HELIUM

Hydrogen profiling using nuclear reaction analysis (NRA) and a thermal desorption technique coupled with scanning electron microscopy (SEM) observations have been used to study the gas trapping and release in h igh-purity polycrystalline nickel. The effect of the preimplanted heli um dose on both deuterium and helium desorption was investigated over a wide range of helium doses (1 x 10(21) to 4 x 10(21) ions/m2). A com puter code, DIFFER, was used to simulate the deuterium flux curves, an d the trapping characteristics were evaluated. The simulation results clearly show that a wide distribution of trapping energies exists. Thi s can be explained using a stress-field trapping model. The effective binding energy, E(b)eff was estimated to be in the range of 0.4 to 0.5 eV. For samples which were irradiated with helium ions to high doses, a massive helium release was also observed. Thermal charging with deu terium was found to reduce the helium self-trapping energy as was expr essed by lower temperature helium release. For the high dose samples, deuterium or hydrogen gas charging and thermal ramping were also found to induce blisters growth and surface exfoliation.