High-spin structures of57138La81

Abstract

Odd-odd ${}^{138}$La nuclei have been produced in the ${}^{12}$C+${}^{238}$U fusion-fission reaction and their $\ensuremath{\gamma}$ rays studied with the Euroball array. The high-spin level scheme has been built up to $\ensuremath{\sim}$4.5 MeV excitation energy and spin $I\ensuremath{\sim}16\ensuremath{\hbar}$ from the triple $\ensuremath{\gamma}$-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, $\ensuremath{\pi}{g}_{7/2}$, $\ensuremath{\pi}{d}_{5/2}$, and $\ensuremath{\pi}{h}_{11/2}$. The competition of these three orbits to produce the yrast line is confirmed by the results of shell-model calculations performed in this work.