Monopole-Driven Shell Evolution below the Doubly Magic Nucleus ${}^{132}\mathrm{Sn}$ Explored with the Long-Lived Isomer in ${}^{126}\mathrm{Pd}$

A new isomer with a half-life of 23.0(8) ms has been identified at 2406 keV in ${}^{126}\mathrm{Pd}$ and is proposed to have a spin and parity of ${10}^{+}$ with a maximally aligned configuration comprising two neutron holes in the $1h_{11/2}$ orbit. In addition to an internal-decay branch through a hindered electric octupole transition, $\beta$ decay from the long-lived isomer was observed to populate excited states at high spins in ${}^{126}\mathrm{Ag}$. The smaller energy difference between the ${10}^{+}$ and $7^{-}$ isomers in ${}^{126}\mathrm{Pd}$ than in the heavier $N=80$ isotones can be interpreted as being ascribed to the monopole shift of the $1h_{11/2}$ neutron orbit. The effects of the monopole interaction on the evolution of single-neutron energies below ${}^{132}\mathrm{Sn}$ are discussed in terms of the central and tensor forces.

Figure 1

Decay schemes of the $J^{\pi }=({10}^{+})$ isomer in ${}^{126}\mathrm{Pd}$. The widths of arrows represent relative intensities of $\gamma$ rays summarized in Table 1, except for the 86 keV transition in ${}^{126}\mathrm{Pd}$. In ${}^{126}\mathrm{Ag}$, the order of transitions between the levels at $2121+x$ and $194+x\mathrm{keV}$, which are populated via the $\beta$ decay of the $({10}^{+})$ isomer, is ambiguous. ($x$ denotes the unknown excitation energy of the lowest-lying state.)

Figure 2

(a) $\gamma$-ray spectrum measured within 50 ms after the ${}^{126}\mathrm{Pd}$ implantation with a gate on an electron-$\gamma$ time difference of $-0.5\le \mathrm{\Delta }{t}_{e\gamma }\le 0.5\mu \mathrm{s}$. Contaminants from the granddaughter ${}^{126}\mathrm{Cd}$ are marked with filled circles. The inset magnifies the energy region from 230 to 310 keV measured with $-4.0\le \mathrm{\Delta }{t}_{e\gamma }\le 0.5\mu \mathrm{s}$ (red) and $-0.5\le \mathrm{\Delta }{t}_{e\gamma }\le 50\mu \mathrm{s}$ (blue). (b) $\gamma$ rays measured with $-4.0\le \mathrm{\Delta }{t}_{e\gamma }\le 0.5\mu \mathrm{s}$, and additionally, with a gate on the electron peak marked with “I” in the inset, where the spectrum depicted with the red line (multiplied by a factor of 10) is obtained with a sum of gates on the $\gamma$ rays below the ($5^{-}$) isomer in ${}^{126}\mathrm{Pd}$, while the blue line represents an electron spectrum without $\gamma$-ray gates. (c) and (d) $\gamma$-ray coincidence spectra measured by gating on the 693 and 598 keV $\gamma$ rays, respectively. The insets show the time distributions of $\gamma$ rays as indicated.

Figure 3

(a) Energy differences between the ${10}^{+}$ and $7^{-}$ states, $\mathrm{\Delta }{E}_{7^{-}}^{{10}^{+}}$, in $N=80$ (filled circles) and between the $11/2^{-}$ and $3/2^{+}$ states in $N=81$ (filled squares). The data are taken from Ref. [25] for $Z\ge 50$, Ref. [20] for ${}^{128}\mathrm{Cd}$, and the present work for ${}^{126}\mathrm{Pd}$. For $Z\le 50$, the differences in energy between the $1h_{11/2}$ and $2d_{3/2}$ neutron orbits calculated by $V_{\text{MU}}$ interaction [6] and $\mathrm{\Delta }{E}_{7^{-}}^{{10}^{+}}$ by SM calculations with monopole corrections (MC) are also depicted. (b) $B(E3;{10}^{+}\to 7^{-})$ values in ${}_{46}{}^{126}\mathrm{Pd}$, ${}_{52}{}^{132}\mathrm{Te}$ [21], and ${}_{56}{}^{132}\mathrm{Xe}$ [22].