A study of hydrogen charging of nickel by positron Doppler broadening

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Abstract
The cathodic charging of hydrogen into polycrystalline nickel has been studied by measurements of the Doppler energy shifts of gamma-rays emitted during positron annihilation in the samples. In general, the annihilation photopeak initially narrows with hydrogen charging, indicating an increase in defect density. The extent of this photopeak narrowing increases with the amount of cold work prior to hydrogen charging. A mechanism is proposed in which dissolved hydrogen (protons) migrate to dislocations introduced by cold work and subsequently form gaseous hydrogen molecules and/or nickel hydride particles which have enough pressure and/or lattice misfit, respectively, to produce structural damage in the form of new dislocations at those locations. The broadening of the Doppler peak following the initial narrowing is attributed to protons reducing the attractive potential between positrons and dislocations. The results show that positrons can be an effective tool with which to study hydrogen in solids.