Probing the sustainability of the $N=82$ and $Z=50$ shell closures for neutron-rich nuclides: Decay of ${}^{120}\mathrm{Rh}_{75}$ to levels of ${}^{120}\mathrm{Pd}_{74}$

The low-energy levels of ${}^{120}\mathrm{Pd}_{74}$ were populated by $\beta$ decay of ${}^{120}\mathrm{Rh}_{75}$, which was produced via projectile fragmentation of a ${}^{136}\mathrm{Xe}_{82}$ beam at $120\mathrm{MeV}∕\text{nucleon}$. Delayed $\beta$-gated $\gamma$ rays with energies of 438 and $618\mathrm{keV}$ were observed in coincidence with ${}^{120}\mathrm{Rh}_{75}$ fragments and assigned to the $2_1^{+}\to 0^{+}$ and $4_1^{+}\to 2_1^{+}$ transitions, respectively, in ${}^{120}\mathrm{Pd}_{74}$. Isomeric $\gamma$-ray transitions are also reported for ${}^{120}\mathrm{Rh}_{75}$ and ${}^{126}\mathrm{Cd}_{78}$. The low-energy structure of ${}^{120}\mathrm{Pd}_{74}$ shows remarkable similarity to those of the isotopic ${}^{108}\mathrm{Pd}_{62}$ and isotonic ${}^{128}\mathrm{Xe}_{74}$ suggesting that these nuclides share the same $Z=50$ and $N=82$ closed shell structures with neutron-rich ${}^{120}\mathrm{Pd}_{74}$.

Figure 1

(Color online) Particle identification spectrum for (a) all fragments incident on the first $\mathrm{Si}$ PIN detector and (b) fragments reaching the DSSD. The intensity scaling in the two-dimensional plot is identical for both (a) and (b). The different isotope patterns reflect the difference in ranges for the implanted isotopes.

Figure 2

(a) Total kinetic energy spectrum derived by gating on the fragment group representing fully stripped ${}^{120}\mathrm{Rh}_{75}$ ions in the particle identification spectrum that includes all fragments incident on the first $\mathrm{Si}$ PIN detector. The prominent low-energy peak corresponds to hydrogen-like ${}^{117}\mathrm{Rh}_{72}^{44+}$, and the higher-energy tail to fully-stripped ${}^{120}\mathrm{Rh}_{75}^{45+}$. (b) Same as above but with an additional requirement that the fragments reached the DSSD. Only the fully stripped ${}^{120}\mathrm{Rh}_{75}^{45+}$ ions are present due to stopping of the hydrogen-like contaminant in the upstream $\mathrm{Si}$ PIN detectors.

Figure 3

(a)$\gamma$-ray spectrum correlated with the $\beta$ decay of implanted ${}^{120}\mathrm{Rh}_{75}$ in the range $0–2000\mathrm{keV}$. The derived decay curve in shown in the inset, where the data were fitted with a function including both parent (${}^{120}\mathrm{Rh}_{75}$) and daughter (${}^{120}\mathrm{Pd}_{74}$) decays with a linear background. (b) $\gamma$-ray spectrum correlated with the $\beta$ decay of implanted ${}^{118}\mathrm{Rh}_{73}$ in the range $0–1200\mathrm{keV}$.

Figure 4

Isomeric $\gamma$ rays occuring within a 20 $\mu$s time window after implantation of (a) ${}^{120}\mathrm{Rh}_{75}$, and (b) ${}^{126}\mathrm{Cd}_{78}$.

Figure 5

(Color online) Energies for the first $2^{+}$ levels in the 2-hole (particle) nuclei ${}_{52}\mathrm{Te}$ (open black squares), ${}_{48}\mathrm{Cd}$ (open red circles), and ${}_{80}\mathrm{Hg}$ (open blue diamonds), and the 4-hole (particle) nuclei ${}_{54}\mathrm{Xe}$ (filled black squares), ${}_{46}\mathrm{Pd}$ (filled red circles), and ${}_{78}\mathrm{Pt}$ (filled blue diamonds). The data for $\mathrm{Pd}$, $\mathrm{Cd}$, $\mathrm{Te}$, and $\mathrm{Xe}$ make use of the neutron range $50<N<82$, while the data for $\mathrm{Hg}$ and $\mathrm{Pt}$ are referenced to $114<N<126$.