MIGRATION AND RELEASE OF HELIUM-3 IMPLANTED IN SINGLE CRYSTAL NICKEL

Several nickel single crystal samples have been implanted with 150 keV helium-3 ions to doses between 9.2 x 10('18) and 2.0 x 10('20) m('-2). After implantation, the samples were isochronally annealed at various temperatures between 300(DEGREES)C and 1300(DEGREES)C. Following each annealing step, helium distributions were measured by a nuclear reaction technique called neutron depth profiling. The technique determines a helium profile from the measured energy distribution of emitted protons produced by the ('3)He(n,p)('3)H reaction when a sample is placed in a thermal neutron beam. The measured profiles were then deconvoluted by an iterative chi-square minimization method. The helium depth profiles can be compared to illustrate helium migration or loss as a function of annealing temperature.;Two new results were determined by this experiment and may be summarized as follows. The first result was the demonstration of a helium concentration dependent release pattern where one sample with an initial helium dose of about 6 x 10('19) m('-2) was the most stable and displayed the least amount of helium release up to annealing temperatures as high as 1300(DEGREES)C. Other samples displayed increasing amounts of helium release at a given annealing temperature with increasing deviations in dose, both above and below, from the most stable dose. The second result was the observation that profile peaking (a narrowing of the helium distribution with an increase in peak concentration) occurred for samples with initial doses within a certain range when annealed at 1000(DEGREES)C. Profile peaking is evidence that helium coalescence occurred during the annealing stage. During heat treatment, helium-defect complexes are able to grow or coalesce to increasing dimensions with increasing helium concentration. For low helium concentrations, helium complexes tend to be more stable and display less helium release at a given annealing temperature with increasing dimensions. For sufficiently high helium concentrations, stable helium complexes may interact to form channels within a sample. With increasing helium concentration, channels have a greater probability of intersecting a sample surface which results in increasing amounts of helium release from the channels. Hence, the results from this experiment may be explained by the presence of varying concentrations of different helium-defect complexes.