Persistence of cluster structure in the ground state of $^{11}$B

TL;DR

This study investigates the persistence of alpha cluster structures in the ground state of $^{11}$B, employing the antisymmetrized quasi cluster model to account for cluster breaking effects and exploring their impact on nuclear structure.

Contribution

The paper introduces the inclusion of cluster breaking effects via AQCM to analyze the ground state of $^{11}$B, revealing a significant energy lowering and the presence of cluster configurations.

Findings

Cluster breaking lowers ground-state energy by about 2 MeV.

Ground state shows overlap with three-alpha cluster configurations.

The third 3/2^{-} state exhibits well-developed clustering.

Abstract

The persistence of $\alpha+\alpha+t$ cluster configuration in the ground state of $^{11}$B is discussed. The $\alpha$ particles emitted from excited states of $^{12}$C could be utilized for cancer treatment, where the cluster states are created by a clinical proton beam on the $^{11}$B target. Although the cross section has been already available, for the nuclear structure side, whether the ground state of $^{11}$B contains the seeds of the cluster states is a crucial question, since the cluster structure may be washed out by the spin-orbit interaction, which has been known as a driving force to break the $\alpha$ clusters. For this purpose, in addition to the basis states with cluster configurations, we include their breaking effects by employing the antisymmetrized quasi cluster model (AQCM). The inclusion of the breaking effect of $\alpha+\alpha+t$ cluster structure is found to contribute to the lowering of the ground-state energy by about 2 MeV. Assuming the typical three-$\alpha$ cluster state of $^{12}$C as an equilateral triangular configuration with the relative distances of $\approx 4$ fm, the ground state of $^{11}$B is found to have a certain squared overlap with such state when a proton approaches. The third $3/2^{-}$ state has been suggested as a cluster state both theoretically and experimentally, and we confirmed the well-developed clustering.