Anomalous $L$-Subshell Conversion Coefficients of the Highly $K$-Forbidden $E1\mathrm{}$ Transition in ${\mathrm{Hf}}^{180m} \left(5.5\mathrm{} \mathrm{h}\right)$

The extremely $K$-forbidden $E1\mathrm{}$ transition (8- → 8+) of 57.5 keV in ${\mathrm{Hf}}^{180m}$ was long known to have an anomalously high ($L_{\mathrm{I}}+L_{\mathrm{II}}$) conversion coefficient. In order to resolve these two lines, the electrons emitted from an evaporated ${\mathrm{Hf}}_2$ ${\mathrm{O}}_3$ source of ∼50-μg/${\mathrm{cm}}^2$ thickness were analyzed in a double-focusing spectrometer. The values found for the conversion coefficients in the three different $L$ subshells are ${\epsilon }_{L_{\mathrm{I}}}=0.308\mathrm{}\pm 0.025$; ${\epsilon }_{L_{\mathrm{II}}}=0.067\mathrm{}\pm 0.010$; ${\epsilon }_{L_{\mathrm{III}}}=0.055\mathrm{}\pm 0.010$, as compared with Rose's theoretical values ${\alpha }_1\left(L_{\mathrm{I}}\right)=0.108\mathrm{}$; ${\alpha }_1\left(L_{\mathrm{II}}\right)=0.047\mathrm{}$; ${\alpha }_1\left(L_{\mathrm{III}}\right)=0.062\mathrm{}$. No admixture of $M2\mathrm{}$ or $E3\mathrm{}$ components fits the experimental results. Of the two explanations for the anomaly considered, namely penetration effects and parity mixing, the second was excluded by the lack of circular polarization of the 57.5-keV $\gamma$ rays (Bock, Jenschke, and Schopper; and Paul, McKeown, and Scharff-Goldhaber), while the first is compatible with our results. For the 8- → 6+ transition of 501.5 keV, more precise values of the branching ratio, energy, and $K$-conversion coefficient are obtained. Its $L$ conversion coefficients have been measured. The results are compatible with the mixing ratio reported by Bodenstedt et al.: $3.5\%M2+96.5\%E3$. The $\frac{(L_{\mathrm{I}}+L_{\mathrm{II}})}{L_{\mathrm{III}}}$ ratio of the 93.3-keV $E2\mathrm{}$ transition is found to be somwehat lower than the values reported by other authors, but still anomalously high (1.28±0.03, compared to the theoretical ratio of 1.11). A search for the 834-keV 8- → 4+ transition yielded an upper limit of 2×${10}^{-5\mathrm{}}$ per decay. The capture cross section of ${\mathrm{Hf}}^{179}$ for thermal neutrons leading to the activation of ${\mathrm{Hf}}^{180m}$ is found to be ${\sigma }_{\mathrm{act}}$, $\mathrm{Hf}_{}^{179}{}_{\mathrm{th}}{}^{}=0.34\mathrm{}\pm 0.03b$. The analysis of the resulting isomeric ratio $\frac{\sigma \left({\mathrm{Hf}}^{180m}\right)}{{\sigma }_{\mathrm{tot}}}=0.52\mathrm{}$% leads to the conclusion that the spin of the capturing state for thermal neutrons in ${\mathrm{Hf}}^{180}$ is predominantly 5.