High-spin structures of ${}_{57}^{138}{\mathrm{La}}_{81}$

Odd-odd ${}^{138}$La nuclei have been produced in the ${}^{12}$C+${}^{238}$U fusion-fission reaction and their $\gamma$ rays studied with the Euroball array. The high-spin level scheme has been built up to $\sim$4.5 MeV excitation energy and spin $I\sim 16\hslash$ from the triple $\gamma$-ray coincidence data. An isomeric state lying at 739-keV excitation energy has been identified. The configurations of the structures are first discussed by analogy with the proton excitations identified in the semimagic ${}^{139}$La${}_{82}$ nucleus, which involve the three high-$j$ orbits lying above the $Z=50$ gap, $\pi g_{7/2}$, $\pi d_{5/2}$, and $\pi h_{11/2}$. The competition of these three orbits to produce the yrast line is confirmed by the results of shell-model calculations performed in this work.

Figure 1

Low-energy part of coincidence spectra double-gated on the 1165-keV transition and on one transition emitted by a complementary fragment of ${}^{138}$La: (a) 158-keV transition from ${}^{103}$Nb and (b) 162-keV transition from ${}^{102}$Nb. The 98-, 123-, and 351-keV $\gamma$ lines (in red) are assigned to ${}^{138}$La. The peak marked with a star is a contaminant.

Figure 2

Intensity ratios of the two transitions 119 keV in ${}^{101}$Nb and 126 keV in ${}^{103}$Nb, measured in each doubly gated La spectra (782–824 keV for ${}^{137}$La [14], 1165–351 keV for ${}^{138}$La, and 1215–166 keV for ${}^{139}$La [1]).

Figure 3

Level scheme of ${}^{138}$La established in this work. The width of the arrows is proportional to the measured $\gamma$-ray intensity. The low intensity of the 739-keV transition measured in our spectra is due to the half-life of the emitting state (see text).

Figure 4

(a) Spectrum doubly gated on the 123- and 351-keV transitions of ${}^{138}$La. (b) Spectrum doubly gated on the 351- and 1165-keV transitions of ${}^{138}$La. Transitions emitted by the Nb complementary fragments are labeled. The peaks marked with a star are contaminants.

Figure 5

Comparison of the high-spin states of ${}^{138}$La, experimental results (this work) and states predicted by the SM calculations which are sorted as a function of their main component (see text), the two orbits occupied by an odd number of nucleons are written at the top of each set of states. All the excited states are drawn relative to the $7_1^{-}$ state. The SM yrast states with negative parity are drawn with a thick line.

Figure 6

Comparison of the positive-parity low-energy states of ${}^{138}$La, experimental results [5], and states predicted by the SM calculations, which are sorted as a function of their main component on the first four seniority-2 basis states (see text).

Figure 7

Comparison of the negative-parity low-energy states populated in the ${}^{139}$La$(d,t)$${}^{138}$La reaction with a $\Delta L_n=5$ neutron pick-up [5], (the three colored states have been also observed in the present work) and states predicted by the SM calculations which are sorted as a function of their main component (see text). All the excited states are drawn relative to the $7_1^{-}$ state.