Ultrafast-timing lifetime measurements in ${}^{94}\mathrm{Ru}$ and ${}^{96}\mathrm{Pd}$: Breakdown of the seniority scheme in $N=50$ isotones

The advanced time-delayed $\gamma \gamma \left(t\right)$ method has been applied to determine half-lives of low-lying states in the $N=50$ isotones ${}^{94}\mathrm{Ru}$ and ${}^{96}\mathrm{Pd}$. The inferred experimental $E2$ strengths for the $4^{+}\to 2^{+}$ transitions in the two nuclei show a dramatic deviation with respect to the shell model predictions in the $(f_{5/2},p,g_{9/2})$ proton hole space in ${}^{100}\mathrm{Sn}$. The anomalous behavior can be ascribed to a breakdown of the seniority quantum number in the $\pi g_{9/2}^n$ configuration due to particle-hole excitations across the $N=Z=50$ shell as confirmed by large-scale shell model calculations.

Figure 1

Total projections of the energy spectra in the Ge (a) and ${\mathrm{BaF}}_2$ (b) detectors and partial decay scheme of ${}^{96}\mathrm{Pd}$ (c).

Figure 2

Background-subtracted time-delayed $\gamma \gamma \left(t\right)$ spectra for ${}^{96}\mathrm{Pd}$ with various start-stop conditions for the $6^{+}$ (a), $4^{+}$ (b), and $2^{+}$ (c) levels and exponential fit of the sum of time spectra to the $4^{+}\to 2^{+}$ decay (d). The second spectrum (on the right-hand side) in (c) was flipped around time = 0 before summing.

Figure 3

Background-subtracted time spectra and centroid shift for the $4^{+}$ (a) and $2^{+}$ (b) levels in ${}^{94}\mathrm{Ru}$ with various start-stop conditions and partial decay scheme (c). In order to select a particular $\gamma$-cascade decay, an additional coincidence in the HPGe detectors with the appropriate $\gamma$ transition was required. A visible shift in the centroids of the time spectra (a) from the prompt position (marked by a dotted line), is due to the mean life of the $I^{\pi }=4^{+}$ state in ${}^{94}\mathrm{Ru}$.

Figure 4

Experimental $E2$ strengths $B^{1/2}\left(E2\right)$ for yrast transitions in even $N=50$ isotones in comparison to shell model predictions in the proton $(f_{5/2},p,g_{9/2})$ model space for the approaches SMLB (full line) and SMCC (short dashed). The calculation including excitations of the ${}^{100}\mathrm{Sn}$ core is shown as long dashed line. Experimental values, error bars, and limits are shown in black dots and lines (a). For odd-$A$ nuclei experimental values for transitions to the $v=3$ and $v=5 I^{\pi }=17/2^{+}$ daughter states are connected by full and short dashed black lines and SMLB results by full and dotted blue lines (b). SDGN results are indicated by open red circles. Data are from present work and [36].