Direct Observation of Isotopic Exchange in the Solid State

Abstract

THE first report of isotopic exchange in a solid system was given by Kaucic and Vlatkovic¹. They heated Ca(IO₃)₂ doped with ¹³¹I⁻ at temperatures higher than 170° C and observed, by chemical analysis, incorporation of activity into the iodate. Apers et al.² found that thermal treatment of K₂CrO₄ doped with ⁵¹Cr³⁺ results in the incorporation of chromium-51 activity into the chromate form. This overall process, which results in the incorporation of radio-species having a non-parent oxidation state into the parent chemical form, has been termed “transfer annealing”²—a process which may involve either an oxidation or a reduction of the tracer species and is not necessarily an isotopic exchange. In a subsequent article³, the same group of researchers considered several possible mechanisms for oxidation of the dopant ⁵¹O³⁺, resulting in the formation of species (in the solid state) which would form the parent chromate during subsequent dissolution. Incorporation of activity into the chromate by isotopic exchange was not considered. Later, Nath et al.^4–6 showed that isotopic exchange exists in complex molecules; in this case, cobalt chelates doped with radio-Co²⁺. They considered that it was not possible to interpret the incorporation of cobalt activity in the chelate form by the oxidative mechanisms suggested for the K₂CrO₄–⁵¹Cr³⁺ system^2,3, and they regarded “transfer annealing” as a genuine case of isotopic exchange. In some systems, for example tris-dipyridy Co(III) perchlorate trihydrate doped with ⁵⁷Co⁺⁺, the exchang occurs quite rapidly even at room temperatures. Nath et al. have suggested that the energy required to overcome the large potential barriers encountered in isotopic exchange in the solid state is derived from electrons detrapped from donors during thermal treatment. A free electron is captured by ⁵⁷Co⁺⁺ ion to form a highly excited ⁵⁷Co⁺ ion which, in turn, interacts with a neighbouring chelate molecule and replaces the inactive cobalt atom. where ^*Co⁺ represents the highly excited ion.