Persistence of cluster structure in the ground state of 11B

Background: The alpha particles emitted from excited states of 12C could be utilized for cancer treatment and the method is called proton boron capture therapy (PBCT). The alpha particles are created by a clinical proton beam on the 11B target. The 11B +p threshold corresponds to the excitation energy of 16.0 MeV in 12C, where the excited states decay by emitting alpha particles. Purpose: For the nuclear structure side, whether the ground state of 11B contains the seeds of the cluster states is a crucial question. It has been known that one of the noncentral interactions, the spin-orbit interaction, plays a role in nuclear systems of washing out the alpha clusters; it works as a driving force to stabilize the symmetry of the j j-coupling shell model. It is of special importance to show the persistence of alpha + alpha + t cluster configuration in the ground state of 11B. Methods: In addition to the basis states with cluster configurations, we include their breaking effects by employing the antisymmetrized quasicluster model. The cluster states and shell-model states are treated on the same footing and are coupled. Results: 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, and cluster structure is slightly broken. The third 3/2- state has been suggested as a cluster state both theoretically and experimentally, and we confirmed the well-developed clustering. Conclusions: The ground state of 11B can be considered as a seed of the cluster states; it still keeps enough component of alpha + alpha + t cluster. Assuming the typical three-alpha-cluster state of 12C as an equilateral triangular configuration with the relative distances of 3-4 fm, the ground state of 11B is found to have a certain squared overlap with such state when a proton approaches.

Automated summary

This research confirms the presence of α+α+t cluster states in the ground state of 11B and finds that the breaking effect of cluster structure contributes to a 2 MeV reduction in ground-state energy.