Non-trivial effect for the large radius at the dripline of Oxygen isotopes

TL;DR

This paper microscopically investigates the large matter radius of $^{24}$O at the neutron dripline, revealing that expansion of alpha clusters, rather than increased clustering, explains the observed radius increase.

Contribution

It introduces a cluster model analysis showing that alpha cluster expansion accounts for the large radius of $^{24}$O, despite its relatively high neutron separation energies.

Findings

Calculated radii match the experimental jump at $^{24}$O.

Alpha cluster expansion is energetically favored over increased clustering.

The model reproduces the radius increase despite slight underestimation.

Abstract

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 neutron halo structure. Although the nucleus $^{24}$O 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 enough binding, the enhancement of the matter radius is observed. In this study, we microscopically describe the structure change of $^{22}$O core in $^{24}$O 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 size of each $\alpha$ cluster are examined, where Tohsaki interaction, which has finite range three-body terms is employed. The nucleus $^{24}$O has the almost the same amount of the clusterization compared with $^{22}$O, but the expansion of each $\alpha$ cluster plays an important role. When two neutrons are added to $^{22}$O at the center, the expansion of each $\alpha$ clusters is energetically more favored than enhancing the clustering for reducing the kinetic energy of the neutrons. The calculated rms matter radius of $^{22}$O and $^{24}$O are 2.75 fm and 2.92 fm, respectively. Although these are slightly smaller than the experimental values, the jump at $^{24}$O from $^{22}$O is reproduced.