Positron annihilation studies of hydrogen in nickel

Positron lifetime measurements have been carried out in 99.98% pure polycrystalline nickel samples that were cold worked and charged with hydrogen. The charging of hydrogen was done cathodically in a bath of 1N H2SO4 at 100degC with a current density of 25mA/cm2. The results of the positron lifetime measurements are as follows: (i) Ni sample, annealed at 800degC for 1 hour, tau = (171 +- 2) psec, (ii) sample reduced in thickness by 30% by cold-work, tau = (225 +- 3) psec, and (iii) sample cold-worked as above and charged with hydrogen for 3 hours, tau = (237 +- 2) psec. The lifetime in the annealed sample is in agreement with previous measurements. Trapping of positrons at dislocations generated during plastic deformation explains the increase in lifetime in the cold-worked sample. Further increase in the lifetime on hydrogen charging of the sample suggests that new dislocations are produced due to the presence of hydrogen in nickel. This result is consistent with the earlier observation of the narrowing of the Doppler broadened annihilation line on hydrogen charging of the sample. (author)