Isomers and shell evolution in neutron-rich nuclei below the doubly magic nucleus ¹³²Sn

¹ IRCNPC, School of Physics and Nuclear Energy Engineering, Beihang University, Beijing, 100191, China
² RIKEN Nishina Center, 2-1 Hirosawa, Saitama, Wako, 351-0198, Japan

^* e-mail: hiroshi@ribf.riken.jp

Published online: 16 May 2018

Abstract

The level structures of the very neutron-rich nuclei ¹²⁸Pd₈₂ and ¹²⁶Pd₈₀ have been investigated for the first time. A new isomer with a half-life of 5.8(8) μs in ¹²⁸Pd is proposed to have a spin and parity of 8⁺ and is associated with a maximally aligned configuration arising from the g_9/2Pd proton subshell with seniority υ = 2. The level sequence below the 8⁺ isomer is similar to that in the N = 82 isotone ¹³⁰Cd, but the electric quadrupole transition that depopulates the 8⁺ isomer is more hindered in ¹²⁸Pd than in ¹³⁰Cd, as expected in the seniority scheme for a semi-magic, spherical nucleus. For ¹²⁶Pd, three new isomers with J^π = (5^-), (7^-), and (10⁺) have been identified with half-lives of 0.33(4) μs, 0.44(3) μs, and 23.0(8) ms, respectively. The smaller energy difference between the 10⁺ and 7^- isomers in ¹²⁶Pd than in the heavier N = 80 isotones can be interpreted as being ascribed to the monopole shift of the h_11/2 neutron orbit. The nature of the N = 82 shell closure scrutinized with these characteristic isomers is discussed.