Low-temperature helium release in nickel

Measurements of ³He released by thermal desorption have been made in high-purity annealed, cold-worked, and single-crystal nickel samples. The ³He was introduced by natural tritium decay to eliminate implantation produced defects. Release was observed in the temperature region from ∼100 to 300 K and above 800 K; no desorption occurred at intermediate temperatures. The low-temperature release amounted to only ∼1% of the total ³He generated. Thus, even in the case of relatively defect-free materials, helium becomes strongly trapped with a binding energy of ∼2 eV. The low-temperature data can best be modeled with a bulk mobility of ∼0.35 eV and a pipe diffusion path of lower activation energy.