Study of excited halo states in ¹⁰Be and ¹²Be using one-neutron transfer reactions from ¹¹Be on ⁹Be at TRIUMF ISAC-II

Abstract

The structures of beryllium isotopes display a complex interplay between Shell Model and cluster configurations. From ${}^{8}\mathrm{Be}$ on, the structures of Be isotopes is expected to be built on the $\alpha-\alpha$ cluster configuration with an increasing number of valence neutrons. Interestingly, the ground state of ${}^{11}\mathrm{Be}$ and ${}^{14}\mathrm{Be}$ are also well-known halo nuclei, hence the apparent competition between cluster configurations and more traditional shell-model like structures. For ${}^{10}\mathrm{Be}$, the ground state may retain some cluster properties, but the most developed cluster states are expected to be found near 6 MeV very close to the ${}^{9}\mathrm{Be}+n$ threshold. Those states are therefore good candidates to study the competition of cluster and single particle configurations. To this end, the ${}^{11}\mathrm{Be}$(${}^{9}\mathrm{Be}$,${}^{10}\mathrm{Be}$)${}^{10}\mathrm{Be}$ transfer reaction was studied at 30.14 MeV at TRIUMF's ISAC-II facility utilizing a combination of charged particles ((PCB)$^2$) and $\gamma$-ray (TIGRESS) detectors.This Thesis seeks to answer if the resulting ${}^{10}\mathrm{Be}$ excited states are molecular-like, shell-model-like, or some exotic combination. The $\gamma$-tagged angular distributions for the 2$^+_2$, $2^-$, and $1^-$ states in ${}^{10}\mathrm{Be}$ were successfully extracted and normalized to the ${}^{11}\mathrm{Be}$(${}^{9}\mathrm{Be}$,${}^{9}\mathrm{Be}$)${}^{11}\mathrm{Be}$ elastic scattering. The transfer reaction code FRESCO was utilized to model and fit angular distributions considering the transfer of the valence $2s_{1/2}$ halo neutron in ${}^{11}\mathrm{Be}$, which couples to the unpaired $1p_{3/2}$ in the ${}^{9}\mathrm{Be}$ ground state. This transfer was $\ell = 0$ for the negative parity states, and $\ell = 1$ for the $2^+_2$. The spectroscopic factors were found to be 0.63 $\pm$ 0.33 for the 2$^-$, 0.52 $\pm$ 0.27 for the 1$^-$, and 1.4 $\pm$ 0.7 for the 2$^+_2$ states. Given the clustered nature of ${}^{9}\mathrm{Be}$(3/2$^-$, gs) and the predicted clustered nature of the 6 MeV states in ${}^{10}\mathrm{Be}$, we conclude that the $1^-$ and $2^-$ states have a ${}^{9}\mathrm{Be}$(gs) $\otimes$ s$_{1/2}$ structure. This makes these states potential hybrid states where clustered and halo structures (especially for the $2^-$, which lies closer to the one-neutron separation energy) appear to coexist. The 2$^+_2$ state may also display a molecular configuration with the s$_{1/2}$ halo neutron transferring as a $\ell=1$ neutron, likely filling a p$_{1/2}$ orbit in the final state. This work is partially supported by the US Department of Energy through Grant/Contract No. DE-FG03-93ER40789 (CSM).