Influence of the Chemical State on the Lifetime of a Nuclear Isomer, ${\mathrm{Tc}}^{99m}$

Since a nuclear isomeric transition may take place by emission of either a gamma-ray or an internal conversion electron, the total rate of decay may be expected to vary with any change in electronic environment, such as a difference in chemical state. Differences in the decay constant of the isomer ${\mathrm{Tc}}^{99m}$ (6 hours) in different states of chemical combination were measured by the double ionization chamber balance method. $\lambda$ for this isomer is determined mainly by the internal conversion probability of a 2-kev transition which is followed promptly by a conveniently measurable 140-kev gamma-ray. Two compounds in the +7 valence state were compared: $\lambda \left(\mathrm{KTc}{\mathrm{O}}_4\right)-\lambda \left({\mathrm{Tc}}_2{\mathrm{S}}_7\right)=27.0\mathrm{}\pm 1.0\times {10}^{-4\mathrm{}}\lambda \left({\mathrm{Tc}}_2{\mathrm{S}}_7\right)$. $\lambda$ for the metal electroplated on nickel and reduced in ${\mathrm{H}}_2$ at 1000°C for 1 hour $-\lambda \left({\mathrm{Tc}}_2{\mathrm{S}}_7\right)=3.1\mathrm{}\pm 1.2\times {10}^{-4\mathrm{}}\lambda \left({\mathrm{Tc}}_2{\mathrm{S}}_7\right)$. Errors are standard deviations. There is evidence for diffusion of the Tc into the nickel base and an accompanying decrease in $\lambda \left(\mathrm{Tc}\right)$ of ∼2 parts in ${10}^4$ compared to pure crystalline metal. $\lambda$ of the metal was measured directly as 0.1148±0.0005 ${\mathrm{hr}}^{-1\mathrm{}}$, corresponding to a half-life of 6.04±0.03 hours (limit of error). The methods used for internally checking the data and the operation of the apparatus are illustrated, and the statistics of the measurements are discussed.