Character of particle-hole excitations in ${}^{94}$Ru deduced from $\gamma$-ray angular correlation and linear polarization measurements

Linear polarization and angular correlations of $\gamma$-rays depopulating excited states in the neutron-deficient nucleus ${}_{44}^{94}{\mathrm{Ru}}_{50}$ have been measured, enabling firm spin-parity assignments for several excited states in this nucleus. The deduced multipolarities of strong transitions in the yrast structure were found to be mostly of stretched $M1$, $E1$, and $E2$ types and, in most cases, in agreement with previous tentative assignments. The deduced multipolarity of the 1869 keV and the connecting 257 and 1641 keV transitions indicates that the state at 6358 keV excitation energy has spin parity ${12}_1^{-}$ rather than ${12}_3^{+}$ as proposed in previous works. The presence of a ${12}_1^{-}$ state is interpreted within the framework of large-scale shell-model calculations as a pure proton-hole state dominated by the $\pi (p_{1/2}^{-1}\otimes g_{9/2}^{-5})$ and $\pi (p_{3/2}^{-1}\otimes g_{9/2}^{-5})$ configurations. A new positive-parity state is observed at 6103 keV and is tentatively assigned as ${12}_2^{+}$. The ${14}_1^{-}$ state proposed earlier is reassigned as ${13}_4^{-}$ and is interpreted as being dominated by neutron particle-hole core excitations. The strengths of several $E1$ transitions have been measured and are found to provide a signature of core-excited configurations.

Figure 1

$\gamma$-ray-asymmetry spectrum for ${}^{94}\mathrm{Ru}$. The spectrum is obtained by subtracting the energy spectra corresponding to perpendicular and parallel scattering (see text) and is shown for illustrative purposes. The asymmetry values were extracted from Eq. (1) after fitting photopeak intensities in the individual spectra.

Figure 2

Partial level scheme of ${}^{94}\mathrm{Ru}$ showing spins and parities deduced in the present work and in Ref. [17]. The stars indicate spins and parities confirmed by our asymmetry-$R_{DCO}$ measurement. The relative intensities were taken from Ref. [17].

Figure 3

Deduced asymmetry parameter A vs $R_{DCO}$ values for $\gamma$-ray transitions in ${}^{94}\mathrm{Ru}$. The stretched $M1$, stretched $E2$, stretched $E1$, and nonstretched $E1$ data points are shown as circles, squares, triangles, and stars, respectively. The color code indicates the assigned multipolarity by Roth et al. [17], where blue is stretched $E1$, red is stretched $M1$, and green is stretched $E2$. Some transitions of special interest are labeled by their energy values in keV (see text). Transitions of new or reassigned multipolarity are marked by black circles around the symbols.

Figure 4

Comparison between observed excitation energies and shell-model calculations for ${}^{94}\mathrm{Ru}$. The experimental levels shown here are those for which the spins and parities were deduced in this work as well as some of the previously known levels from Ref. [17].