High-spin states and isomers in 136, 137, 138Ce

Abstract

High-spin states in 136, 137, 138Ce have been populated by ( α, xn) reactions and studied using in-beam γ-ray spectroscopic methods. New features result from this investigation. Levels up to I = 15, 33 2 and 16 have been identified in 136, 137, 138Ce, respectively. In 136, 138Ce a 10 + isomer has been found which has probably a h 11 2 two-neutron-hole configuration. The level scheme for 137Ce shows a decoupled band structure different from that of the lighter odd-mass Ce nuclei, where perturbed rotational bands, based on the h 11 2 neutron-hole state were reported. The energies of the 15 2 − → 11 2 −, 19 2 − → 15 2 − and 21 2 + → 19 2 − transitions in 137Ce are similar to the 2 + → 0 +, 4 + → 2 + and 5 − → 4 + transition energies in 138Ce, and those of 12 + → 10 +, 14 + → 12 + and 15 → 14 + in 136Ce. The level structure of these nuclei can be understood if a transition from prolate to oblate shape through a triaxial region occurs between 135Ce and 137Ce. Furthermore, in 136Ce a negative-parity band with even- and odd-spin members was established. In 138Ce the negative-parity states 7 − and 5 − were observed.