Neutron Halo and Breakup Reactions

Abstract

Characteristic features of neutron halos in the context of breakup experiments at intermediate/high energies are discussed. Neutron halos have been found for light neutron rich nuclei along the neutron drip line, as intense radioactive nuclear beams have become available in recent years. A neutron halo nucleus is composed of a tightly bound core surrounded by one or two neutrons which extend outside of the mean field potential due to quantum tunneling. Coulomb breakup of halo nuclei shows extremely enhanced cross sections for such systems, originating from the characteristic electric dipole response of halo nuclei at low excitation energies, called soft $$E1$$ excitation. Such features are shown for one-neutron halo nuclei by Coulomb breakup experiments of $${}^{11}$$ Be on Pb at about 70 MeV/nucleon, performed at RIKEN, where it was found that the direct breakup mechanism is responsible for this excitation. We then show how the $$E1$$ excitation spectrum can be related to properties of the halo distribution, and hence that the method of Coulomb breakup is a powerful spectroscopic tool. As such, applications of the Coulomb breakup of $${}^{15}\hbox{C}$$ and $${}^{19}\hbox{C}$$ are shown. The $${}^{19}\hbox{C}$$ case is valuable for extracting the microscopic structure of $${}^{19}\hbox{C},$$ which has only recently been clarified. The $${}^{15}\hbox{C}$$ case can be used to extract the radiative capture cross section of the $${}^{14}\hbox{C}(n,\gamma)^{15}\hbox{C}$$ reaction. We then demonstrate recent applications of the “inclusive” Coulomb breakup method to a new-region of loosely bound nuclei near the island of inversion ( $$\hbox{N}\sim20$$ ). There, evidence obtained, of the 1n halo structure in $${}^{31}\hbox{Ne},$$ is presented. In the 2n halo case the Coulomb breakup of $${}^{11}\hbox{Li}$$ at 70 MeV/nucleon, measured at RIKEN, is shown. This reaction has provided evidence of dineutron-like structure, revealed by the strong enhancement of the soft $$E1$$ excitation. For nuclear dominated breakup, where a light target is used, the momentum distribution of the core fragment has key information on the halo distribution, and the single-particle properties of the valence neutron. Here we show an example of a spectroscopic study of $${}^{13}\hbox{Be},$$ populated by removing one neutron from the two neutron halo nucleus $${}^{14}\hbox{Be}.$$ These results show that the breakup reactions play significant roles in elucidating the structures along the neutron drip line. This feature will be very important for further investigations of the drip-line nuclei towards the heavier region, as will be produced using the new-generation RI (Rare-Isotope) beam facilities, as has just been completed in RIKEN (RIBF) in Japan. Such enhanced RI-beam facilities are soon to be commissioned in Europe (SPIRAL2, FAIR etc.), in Asia (KoRIA), and in the US (FRIB).