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 j_pj_h ^-1, where j_p=ℓ_p+1/2 and j_h=ℓ_h+1/2 are the largest angular momenta found in the first open shell and the last filled shell, respectively. When the j_p and j_h couple to the angular momentum of the maximum value J_max=j_p+j_h=ℓ_p+ℓ_h+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.