Excitation of Stretched Particle-Hole States in Closed Shell Nuclei

Abstract

High-spin states of unnatural parity have recently been observed in nuclei over a wide range of the nuclear charts by using various hadronic and leptonic reactions [1]. The high-spin states are expected to be generated by a simple particle-hole (p-h) configuration jpjh -1, where jp=lp+1/2 and jh=lh+1/2 are the largest angular momenta found in the first open shell and the last filled shell, respectively. When the jp and jh couple to the angular momentum of the maximum value Jmax=jp+jh=lp+lh+1, the high-spin states are often referred to as stretched p-h states. A number of experimental data indicate that the excitation strengths of the stretched and nearly stretched p-h states are suppressed by a factor of 1/5–1/2 compared to those predicted by the single-particle shell model. Subnucleonic effects such as the isobar-hole excitations and the mesonic exchange currents are expected to be rather weak in the stretched transitions due to higher multipole transitions[2]. The observed quenching of the strengths might be therefore attributed mainly to nuclear structure effects, such as ground state correlations and higher-shell configuration mixing effects. Since the stretched state in a closed shell nucleus is a unique p-h state with the lℏω excitation, and since the number of configurations is severely limited due to high angular momentum of the state, a perturbative method would be applicable in order to estimate the nuclear structure effects. We attempt to make a systematic configuration mixing calculation of the transition strengths for the high-spin p-h states in the closed shell nuclei from 12C to 208Pb. Several calculations[3,4] have so far been made to interpret inelastic electron scattering form factors of the 14- and 12- states in 208Pb.