Energy Levels ofRu100

The nuclear-level structure of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Ru}$ has been investigated through study of the radioactive decay of 16-sec ${}_{}{}^{100}{}_{}{}^{}\mathrm{Tc}$ and 20-h ${}_{}{}^{100}{}_{}{}^{}\mathrm{Rh}$. The experimental techniques employed include two-parameter Ge(Li)-Ge(Li) coincidence spectroscopy and NaI(Tl) $\gamma -\gamma$ angular-correlation measurements. Altogether, 24 excited states of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Ru}$ have been observed below 3.5 MeV. Examination of previous $\mathrm{Ru}(n, \gamma )$ data strongly suggests that at least seven of the observed levels are populated by primary ${}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}(n, \gamma )$ transitions. The present results, combined with previous internal-conversion and angular-correlation data, yield unique spin-parity assignments for nine levels, four of which are $0^{+}$ (at 1130.4, 1740.6, 2051.5, and 2387 keV). Also, through selection-rule arguments, narrow limits have been placed on the spin-parity values for the remaining states, almost all of which are of low spin (≤2). The assignments deduced for the parent nuclei are $1^{+}$ for ${}_{}{}^{100}{}_{}{}^{}\mathrm{Tc}$ and $1^{-}$ for ${}_{}{}^{100}{}_{}{}^{}\mathrm{Rh}$. The $\beta$ branching of ${}_{}{}^{100}{}_{}{}^{}\mathrm{Tc}$ is found to be anomalous in that the transitions to the lowest two $2^{+}$ levels are extremely hindered compared to those that proceed to the lowest two $0^{+}$ levels.