Giant GT⁺ Excitations of N=82 Nuclei Populated in β ⁺-Decay

Abstract

Whereas the GT^- giant excitation is energetically not accessible in β^--decay, the GT⁺ giant state may be reached in β⁺-decay of very neutron-deficient nuclei where the proton single-particle states are raised through the Coulomb repulsion. This becomes possible whenever the proton and neutron Fermi-energies are between the spin-orbit partners of a high-ℓ orbit, i.e. such that the j_> state is partly filled by protons whereas the j_< state for neutrons is (partly) empty. This condition is fulfilled for the 1g shell in the N⩾50 nuclei below ¹⁰⁰Sn, and for the 1h orbitals in the N⩾82 nuclei above ¹⁴⁶Gd, and in both regions such πj_> → νj_< GT β⁺-decays have been identified. Since e.g. in nuclei with 82 neutrons all N=4 neutron states are filled, the Pauli principle only allows a στ₊ transition of the N=5 h_11/2 protons to the unoccupied h_9/2 neutron orbit. This is the only possible στ₊ transition which will exhaust the entire available strength and thus the (πj_>νj_<)1⁺ state in the daughter nucleus is identical with the GT⁺ giant state. The principal locations of the pertinent states for the N=82 region are shown in fig. 1, which refers to ¹⁴⁸Dy; this case has recently¹) been investigated in a high sensitivity measurement. The figure also includes the isobaric-analog state at its estimated energy, and the GT^- resonance based on the ¹⁴⁸Gd₈₄ ground state, which should lie about 2 MeV higher.