Nontrivial origin for the large nuclear radii of dripline oxygen isotopes

摘要

The anomalous large radii are exotic phenomena observed around the neutron dripline. Around the neutron dripline, the weak binding of the last bound neutron(s) causes the drastic increase of the radius, which is called the neutron halo structure. Although the nucleus O-24 is located at the dripline of oxygen isotopes, the separation energies of one and two neutron(s) are 4.19 MeV and 6.92 MeV, respectively. In spite of this sufficient binding, the enhancement of the matter radius is observed. In this study, we microscopically describe the structure change of the O-22 core in O-24 and explain the observed large radius based on the cluster model. Two degrees of freedom for the large radius, the relative distances among four alpha clusters and the size of each alpha cluster, are examined, where the Tohsaki interaction, which has finite range three-body terms, is employed. The nucleus O-22 has smaller alpha-alpha distance compared with O-16 owing to the gluelike effect of six valence neutrons around four alpha clusters. When two neutrons are added at the center of O-22, the expansion of each alpha cluster is energetically more favored than increasing the alpha-alpha distances for reducing the kinetic energy of the neutrons. The calculated rms matter radii of O-22 and O-24 are 2.75 fm and 2.92 fm, respectively. Although these are slightly smaller than the experimental values, the jump at O-24 from O-22 is reproduced.