Evidence of Partial Seniority Conservation in the $\pi g_{9/2}$ Shell for the $N=50$ Isotones

The reduced transition probabilities for the $4_1^{+}\to 2_1^{+}$ and $2_1^{+}\to 0_1^{+}$ transitions in ${}^{92}\mathrm{Mo}$ and ${}^{94}\mathrm{Ru}$ and for the $4_1^{+}\to 2_1^{+}$ and $6_1^{+}\to 4_1^{+}$ transitions in ${}^{90}\mathrm{Zr}$ have been determined in this experiment making use of a multinucleon transfer reaction. These results have been interpreted on the basis of realistic shell-model calculations in the $f_{5/2}$, $p_{3/2}$, $p_{1/2}$, and $g_{9/2}$ proton valence space. Only the combination of extensive lifetime information and large scale shell-model calculations allowed the extent of the seniority conservation in the $N=50$ $g_{9/2}$ orbital to be understood. The conclusion is that seniority is largely conserved in the first $\pi g_{9/2}$ orbital.

Figure 1

Left: experimental Doppler corrected $\gamma$-ray spectra for ${}^{92}\mathrm{Mo}$ for three target-to-degrader distances. The solid lines indicate the fit of the two components before and after the degrader. Right: lifetime determination of the $4_1^{+}$ state in ${}^{92}\mathrm{Mo}$. See text for more details.

Figure 2

Same as Fig. 1, but for the $4_1^{+}$ state in ${}^{94}\mathrm{Ru}$.

Figure 3

Top: experimental and theoretical excitation energies of the $2^{+}$, $4^{+}$, $6^{+}$, and $8^{+}$ yrast states for $N=50$ isotones with $A⩾90$. The black circles are the experimental energy values [19]. The green solid lines are the shell-model predictions made for this Letter. Bottom: experimental and theoretical $B(E2;2_1^{+}\to 0_1^{+})$, $B(E2;4_1^{+}\to 2_1^{+})$, $B(E2;6_1^{+}\to 4_1^{+})$, and $B(E2;8_1^{+}\to 6_1^{+})$ values for $N=50$ isotones with $A⩾90$. The red circles are the experimental results of this Letter. The black circles are the experimental values from previous experiments, $B(E2;2_1^{+}\to 0_1^{+})$ [19], $B(E2;4_1^{+}\to 2_1^{+})$ [17], $B(E2;6_1^{+}\to 4_1^{+})$ [17, 19, 40], and $B(E2;8_1^{+}\to 6_1^{+})$ [19, 41]. The green dashed (solid) lines show the shell-model calculations performed using empirical effective charges (microscopic effective charges). See text for details.