Coulomb Breakup and Soft E1 Excitation of Neutron Halo Nuclei

Abstract

What happens when a nucleus with a neutron halo is excited by an external field? This review discusses how a neutron-halo nucleus can be excited when it absorbs a “photon.” In particular, we focus on the characteristic excitation of a halo nucleus at low excitation energies around Ex ∼ 1 MeV due to the electric dipole component (E1) of a photon field, called “soft E1 excitation.” Experimentally, soft E1 excitation of halo nuclei has been investigated using the Coulomb breakup reaction. The soft E1 excitation using Coulomb breakup has become a powerful and useful method to probe a halo structure microscopically. The halo nuclei can be categorized into one-neutron halo nuclei (core+one halo neutron, such as 11Be, 19C, and 31Ne) and two-neutron halo nuclei (core+two halo neutrons, such as 6He, 11Li, and 19B). Accordingly, the soft E1 excitation has different characteristics between the one-neutron halo nuclei and two-neutron halo nuclei. For one-neutron halo nuclei, the soft E1 excitation manifests as a motion between the core and the halo neutron. We illustrate characteristic features of soft E1 excitation and Coulomb breakup as a spectroscopic tool of one-neutron halo nuclei, using the experimental results of the classical one-neutron halo nucleus 11Be. Recent experimental results on soft E1 excitation of neutron-rich nuclei in the island of inversion are also shown and discussed, particularly for 31Ne, a representative of the deformed halo nuclei. For two-neutron halo nuclei, the soft E1 excitation can be a probe of the neutron-neutron correlation of halo neutrons. We introduce and discuss the recent experimental results of the Coulomb breakup of 6He and 19B. Both cases show spatial neutron-neutron correlation, called dineutron. Finally, we show some perspectives on Coulomb breakup, soft E1 excitation, and related topics.