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

The level structures of the very neutron-rich nuclei Pd-128(82) and Pd-126(80) have been investigated for the first time. A new isomer with a half-life of 5.8(8) mu s in Pd-128 is proposed to have a spin and parity of 8(+) and is associated with a maximally aligned configuration arising from the g(9/2) proton subshell with seniority v = 2. The level sequence below the 8(+) isomer is similar to that in the N = 82 isotone Cd-130, but the electric quadrupole transition that depopulates the 8(+) isomer is more hindered in Pd-128 than in Cd-130, as expected in the seniority scheme for a semi-magic, spherical nucleus. For Pd-126, three new isomers with J(pi) = (5(-)), (7(-)), and (10(+)) have been identified with half-lives of 0.33(4) mu s, 0.44(3) mu s, and 23.0(8) ms, respectively. The smaller energy difference between the 10(+) and 7(-) isomers in Pd-126 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.