Observation of Isomeric Decays in the $r$-Process Waiting-Point Nucleus ${}^{130}\mathrm{Cd}_{82}$

The $\gamma$ decay of excited states in the waiting-point nucleus ${}^{130}\mathrm{Cd}_{82}$ has been observed for the first time. An $8^{+}$ two-quasiparticle isomer has been populated both in the fragmentation of a ${}^{136}\mathrm{Xe}$ beam as well as in projectile fission of ${}^{238}\mathrm{U}$, making ${}^{130}\mathrm{Cd}$ the most neutron-rich $N=82$ isotone for which information about excited states is available. The results, interpreted using state-of-the-art nuclear shell-model calculations, show no evidence of an $N=82$ shell quenching at $Z=48$. They allow us to follow nuclear isomerism throughout a full major neutron shell from ${}^{98}\mathrm{Cd}_{50}$ to ${}^{130}\mathrm{Cd}_{82}$ and reveal, in comparison with ${}^{76}\mathrm{Ni}_{48}$ one major proton shell below, an apparently abnormal scaling of nuclear two-body interactions.

Figure 1

Example of particle identification plots from the fragmentation of ${}^{136}\mathrm{Xe}$. Left: $Z$ identification from the energy losses measured in two multiple sampling ionization chamber ionization chambers. Right: Isotope identification from the positions of the Cd ions in the final focal plane S4 of the FRS shown as a function of $A/Q$.

Figure 2

Delayed $\gamma$-ray spectrum ($75\mathrm{ns}–1\mu \mathrm{s}$) in coincidence with identified ${}^{130}\mathrm{Cd}$ ions implanted in the stopper. The inset shows the time distribution between the ion implantation and the detection of one of the four $\gamma$ rays. The lower panels show the $\gamma$ spectra observed in coincidence with the 128, 138, 539, and 1325 keV transitions, respectively.

Figure 3

Proposed level scheme of ${}^{130}\mathrm{Cd}$ compared to two different shell-model calculations (see text for details). The isomeric $8^{+}$ state [$T_{1/2}=220(30)\mathrm{ns}$] at an excitation energy of 2130 keV is connected to the ground state via a cascade of four $E2$ transitions.

Figure 4

Proposed level scheme of ${}^{130}\mathrm{Cd}$ compared to the known level schemes of ${}^{76}\mathrm{Ni}$ [29] and ${}^{98}\mathrm{Cd}$ [13]. The solid arrows indicate the observed $A^{-1}$ scaling, whereas the dashed arrows correspond to an $A^{-1/3}$ scaling of the $2^{+}-8^{+}$ energy spread.