Unexpectedly Small Effect of Isotopic Mass on Diffusion of Silver in Lead

The effect of isotopic mass for diffusion of silver in lead was measured to test the proposal that silver diffuses primarily by a simple interstitial mechanism. The relative diffusivity of the radioisotopes ${}_{}{}^{105}{}_{}{}^{}\mathrm{Ag}$ and ${}_{}{}^{110m}{}_{}{}^{}\mathrm{Ag}$ was accurately measured by codiffusion into single crystals of high-purity lead, sectioning, and analyzing the $\gamma$ radiation from each section. The value of the effect $E$ determined from the results of three separate experiments close to 300 °C is 0.25 ± 0.05. Alloying of lead with 0.08-at.% Ag did not change the value of the effect within experimental error. A further experiment performed at 150 °C yielded approximately the same result. Since the anticipated value of $E$ for a simple interstitial mechanism is close to unity, our results indicate that the mechanism for diffusion of silver in lead is more complex than has been thought.