Prolate deformation in neutron-dificient lanthanum isotopes

Abstract The odd-A La isotopes from A = 125 to 137 have been studied by in-beam γ-ray spectroscopy of Sn ( 14 N , x n ) La reactions and from A = 131 to 139 by particle spectroscopy of the Ba(α, t) La and Ba(τ, d) La single-proton transfer reactions. In the latter experiments a low-lying 11 2 − level was systematically identified, and the rotational bands built on these levels were observed in the γ-ray work. The strong resemblance of the transition energies in these bands to the ground-band transitions in the neighbouring even Ba isotopes is very striking. These results can be understood as a decoupling of an h 11 2 single proton from the Ba core. This interpretation of the observed bands requires a prolate deformation for these nuclei, which is in contrast to previous conclusions.