Diffusion of zirconium in niobium: The influence of fast diffusing impurities on the self-diffusion isotope effect

The diffusion of ${}_{}{}^{95}{}_{}{}^{}\mathrm{Zr}$ in niobium was measured over the temperature range from 1923 to 2523 K. The data fit the Arrhenius relation $D=0.85\mathrm{}\exp (-3.93\mathrm{} \mathrm{eV}/kT)$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$. The changes in the self-diffusion isotope effect caused by ultralow levels (less than 100 at. ppm) of a fast-diffusing radioactive impurity are analyzed. It is concluded that radioactive-impurity levels not easily observed by Ge(Li) detectors can introduce large errors into the measurement of the isotope effect.