Decay properties of high-spin isomers and other structures in Sb121 and Sb123

Abstract

High-spin states populated in the decay of microsecond isomers in the transitional nuclei 121 Sb and 123 Sb have been investigated in detail in several experiments using γ-ray and electron spectroscopy. The nuclei were formed using multinucleon transfer and fusion-fission reactions with 136 Xe beams and also using the 120 Sn( 7 Li,α2n) 121 Sb and 122 Sn( 7 Li,α2n) 123 Sb incomplete-fusion reactions. Isomeric half-lives ranging from several nanoseconds to a few hundred microseconds were determined by means of conventional decay curve analyses, whereas very short-lived isomers (T 1/2 ∼ 1 ns) were identified using the generalized centroid-shift method. A number of new transitions were observed, including a branch through spherical states from the 19/2 + member of the 9/2 + deformed band in 121 Sb, in competition with the main decay path through the rotational band. This is attributed to mixing between the 19/2 + band member and a 19/2 + spherical state. Both levels are predicted to coincide approximately in energy in 121 Sb. The fact that a 25/2 + isomer occurs for A = 121 and the lighter isotopes, while a 23/2 + isomer is observed for A = 123-131 is explained through a multistate mixing calculation, taking into account the gradual shift of the 2d 5/2 and 1g 7/2 proton orbitals and the change in proton-neutron effective interactions from an attractive particle-particle type in the lower part of the shell to a repulsive particle-hole type with increasing the neutron number toward the N = 82 shell closure. The observed enhancement of the B(E2;19/2 - → 15/2-) values in 121 Sb and 123 Sb over the B(E2; 7 - → 5 - ) values in the corresponding Sn cores is discussed in terms of configuration mixing between spherical and deformed states.