High-spin structures of ${}_{55}^{136}$Cs${}_{81}$

Odd-odd ${}^{136}$Cs nuclei have been produced in the ${}^{18}\mathrm{O}+{}^{208}$Pb and ${}^{12}\mathrm{C}+{}^{238}$U fusion-fission reactions and their $\gamma$ rays studied with the Euroball array. The high-spin level scheme was built up to $\sim$4.7 MeV excitation energy and spin $I\sim 16\hslash$ from the triple $\gamma$-ray coincidence data. The configurations of the three structures observed above $\sim$2 MeV excitation energy are first discussed by analogy with the proton excitations identified in the semimagic ${}^{137}$Cs${}_{82}$ nucleus, which involve the three high-$j$ orbits lying above the $Z=50$ gap, $\pi g_{7/2}$, $\pi d_{5/2}$, and $\pi h_{11/2}$. This is confirmed by the results of shell-model calculations performed in this work.

Figure 1

High-energy part of coincidence spectra gated on transitions belonging to the complementary fragments of ${}^{136}$Cs: (a) spectrum double-gated on the 345- and 1227-keV transitions of ${}^{85}$Br [16], built from the ${}^{18}\mathrm{O}+{}^{208}$Pb data set, (b) spectrum double-gated on the 190- and 331-keV transitions of ${}^{86}$Br [17], built from the ${}^{18}\mathrm{O}+{}^{208}$Pb data set, (c) spectrum double-gated on the 63- and 84-keV transitions of ${}^{105}$Tc [18], built from the ${}^{12}\mathrm{C}+{}^{238}$U data set. The 1184-keV transition (in blue) belongs to ${}^{137}$Cs [6] and the new 1398-keV one (in red) is assigned to ${}^{136}$Cs.

Figure 2

Level scheme of ${}^{136}$Cs established in this work. The excitation energies of the long-lived 8${}^{-}$ state and the 4${}^{+}$ state are from Ref. [3]. The width of the arrows is proportional to the $\gamma$-ray intensity. The colored states are new. The color code is the same as that of Fig. 4.

Figure 3

Coincidence spectra gated on transitions belonging to ${}^{136}$Cs: (a) spectrum double-gated on the 1398- and 262-keV transitions, built from the ${}^{18}\mathrm{O}+{}^{208}$Pb data set [transitions emitted by ${}^{85,86}$Br, the complementary fragments of ${}^{136}$Cs, are labeled by (Br)] and (b) spectrum double-gated on the 1398- and 945-keV transitions, built from the ${}^{12}\mathrm{C}+{}^{238}$U data set.

Figure 4

(a) Excitation energy as a function of angular momentum of the ${}^{136}$Cs states having $I⩾8\hslash$, predicted by the SM calculations to lie above the 5${}^{+}$ ground state (see text). The first two or three states for each spin value are drawn. The states having the same main configuration are linked by a solid line. (b) Experimental states of ${}^{136}$Cs drawn using the same symbols as in (a).