First spectroscopic information from even-even nuclei in the region “southeast” of ${}^{132}\mathrm{Sn}$: Neutron-excitation dominance of the $2_1^{+}$ state in ${}^{132}\mathrm{Cd}$

The neutron-rich nucleus ${}^{132}\mathrm{Cd}$ has been studied at the RIKEN Radioactive Isotope Beam Factory using in-beam $\gamma$-ray spectroscopy with two-proton removal reactions from ${}^{134}\mathrm{Sn}$. A $\gamma$-ray transition was observed at 618(8) keV and was assigned to the $2_1^{+}\to 0_{\mathrm{g}.\mathrm{s}.}^{+}$ decay. The $2_1^{+}$ state provides the first spectroscopic information from the even-even nuclei located in the region “southeast” of the doubly magic nucleus ${}^{132}\mathrm{Sn}$. By comparing with the $2_1^{+}$ excitation energies in the semimagic nuclei ${}^{134}\mathrm{Sn}$ and ${}^{130}\mathrm{Cd}$, it is found that neutron excitations dominate the $2_1^{+}$ state in ${}^{132}\mathrm{Cd}$, in a similar manner to ${}^{136}\mathrm{Te}$. The results are discussed in terms of proton-neutron configuration mixing.

Figure 1

Part of the nuclear chart showing the exotic isotopes identified experimentally so far in the vicinity of ${}^{132}\mathrm{Sn}$. Proton and neutron magic numbers are highlighted by the thick blue lines. Highlighted by the green color are the even-even nuclei which have available spectroscopic information. The present work reports on the excited $2_1^{+}$ state in ${}^{132}\mathrm{Cd}$ (red).

Figure 2

Particle identification plots for the ZeroDegree spectrometer. (a) Two-dimensional plot of $Z$ vs $A/Q$ for the reaction residues produced from ${}^{134}\mathrm{Sn}$. The horizontal dashed lines show the $Z$ gate that was adopted for selecting the Cd isotopes. The red circle indicates the mixture of ${}^{132}\mathrm{Cd}^{48+}$ and ${}^{129}\mathrm{Cd}^{47+}$. (b) Two-dimensional plot of $A$ vs $A/Q$ for the Cd isotopes. The fully stripped ${}^{132}\mathrm{Cd}^{48+}$ ions could be separated from the ${}^{129}\mathrm{Cd}^{47+}$ fragments using this technique.

Figure 3

Doppler-shift corrected $\gamma$-ray energy spectrum deduced from the ${}^9\mathrm{Be}({}^{134}\mathrm{Sn},{}^{132}\mathrm{Cd}$) reaction. The solid line represents the total fit resulting from a sum of the DALI2 simulated response function (red dotted line) and an exponential background (blue dashed line).

Figure 4

Systematics of $2_1^{+}$ excitation energies for (a) the even-even $N=84$ isotones, and (b) the Cd and Te isotopic chains. The result of the present work is indicated by the square in both panels. Other experimental values are taken from Ref. [17]. In (b), mean-field calculations using a mapped collective Hamiltonian and the D1S Gogny interaction [18] (solid line), QRPA [19] (dashed line), and SHF method with pairing forces DDDI [20] (dot-dashed line) are displayed for comparison. It is noted that the error bars are smaller than the symbol sizes, and the vertical dotted lines represent the magic numbers in both panels.