Level structure of light neutron-rich La isotopes beyond the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>82</mml:mn></mml:mrow></mml:math> shell closure
Аннотация
The high spin excited states of Lanthanum isotopes $^{140\text{--}143}\mathrm{La}$, above the $N=82$ closed shell, were populated in fission reactions. The prompt $\ensuremath{\gamma}$-ray transitions were measured using two complementary methods: (a) in coincidence with the isotopically identified fragments produced in the fission of the $^{238}\mathrm{U}+^{9}\mathrm{Be}$ system using the Variable Mode Spectrometer ($\mathrm{VAMOS}++$) and the Advanced Gamma Tracking Array (AGATA) spectrometer, and (b) high statistics threefold $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ and fourfold $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence data from the spontaneous fission of $^{252}\mathrm{Cf}$ using the Gammasphere. This work reports the first identification of a pair of parity doublet structures in $^{143}\mathrm{La}$ and the new high spin level structure in $^{140\text{--}142}\mathrm{La}$ from prompt $\ensuremath{\gamma}$-ray spectroscopy. The level structures are interpreted in terms of the systematics of neighboring odd-$Z$ nuclei above the $Z=50$ shell closure and large-scale shell model calculations. The present results indicate the presence of stable octupole deformation in $^{143}\mathrm{La}$. The excitation energy pattern and their comparison with neighboring isotones, moving away from the $N=82$ closed shell, point towards a transition from single-particle structures to an alternating parity rotational band structure in the La isotopic chain.