Identification and shell model calculation of high spin states in ${}^{137,138}\mathrm{Cs}$ nuclei

High spin states of ${}^{137,138}\mathrm{Cs}$ have been studied by measuring $\gamma \text{−}\gamma \text{−}\gamma$ coincidences from the spontaneous fission of ${}^{252}\mathrm{Cf}$ with the LBNL Gammasphere detector array. The high spin level scheme of the $N=83$ neutron-rich Cs ($Z=55$) isotope, ${}^{138}\mathrm{Cs}$, built on the $6^{-}$ isomeric state, has been established for the first time up to a 4626 keV level, assigned (${16}^{+}$). The level scheme of ${}^{137}\mathrm{Cs}$ has been expanded up to a 5495 keV level, assigned ($31/2^{-}$). Spins, parities, and configurations are assigned based on the agreement between experimental level energies and shell model calculations and level systematics. Similarities are observed in the $N=82$ isotones, ${}^{137}\mathrm{Cs}$ and ${}^{135}\mathrm{I}$, up to $17/2^{+}$ as well as in the $N=83$ isotones, ${}^{138}\mathrm{Cs}$ and ${}^{136}\mathrm{I}$, up to ${12}^{-}$. The shell model calculations indicate the important role played by interactions between the excitation of the valence protons outside the $Z=50$ major shell and the $f_{7/2}$ valence neutron outside the $N=82$ major shell.

Figure 1

Fission yield ratios of ${}^{138}\mathrm{Cs}$ and ${}^{137}\mathrm{Cs}$ in ${}^{109,110,111}\mathrm{Tc}$ gates, ${}^{144}\mathrm{Ba}$ and ${}^{143}\mathrm{Ba}$ in ${}^{103,104,105}\mathrm{Mo}$ gates [20].

Figure 2

(a) Spectrum gating on 1156.9 keV transition in ${}^{138}\mathrm{Cs}$ and 137.3 keV transition in ${}^{109}\mathrm{Tc}$, (b) spectrum gating on 1156.9 and 174.5 keV transitions in ${}^{138}\mathrm{Cs}$.

Figure 3

Partial level scheme of ${}^{138}\mathrm{Cs}$. Energies are in keV.

Figure 4

Spectra gating on 1184.7 and 487.1 keV transitions in ${}^{137}\mathrm{Cs}$, showing (a) the low energy region and (b) the high energy region.

Figure 5

Partial level scheme of ${}^{137}\mathrm{Cs}$. Energies are in keV.

Figure 6

Yrast states of Cs isotopes. Data from Refs. [28, 29] and this work.

Figure 7

Yrast states of $N=82$ and $N=83$ isotones.

Figure 8

Variations of excitation energies of the $11/2^{+}$ states in the even-$a$N Cs isotopes vs excitations of the first $2^{+}$ state in the corresponding even-even Xe core. $9^{+}$ state energy in ${}^{138}\mathrm{Cs}$ is also shown. Data are from Refs. [19, 28, 29] and from the present work.