Isotope Effect in Self‐Diffusion of Cs in Single Crystals of CsI

The simultaneous diffusion of Cs‐131 and Cs‐137 has been measured over the temperature range 470 to 600°C. The isotope effect was measured by the method of separation of the radiations of the two isotopes. The strength of the isotope effect is E = ∫ δK = (0.54 ± 0.02) and is independent of temperature within the experimental error. This indicates that the contribution of divacancies (cation‐anion vacancy pairs) to the selfdiffusion in CsI is small and the predominant mechanism of self‐diffusion in these crystals s the diffusion via free monovacancies.