Cluster structure of neutron-rich beryllium isotopes investigated via (p. p[alpha]) reaction in inverse kinematics at 150 MeV/u

Abstract

Alpha clustering structure in nuclei, which is beyond the description of modern shell models, challenges the understanding of fundamental interactions and reveals new aspects of nuclear forces. The 8Be (𝑁 = 4, 𝑍 = 4) nucleus is famous for its developed 𝛼-𝛼 structure. Neutron-rich Beryllium isotopes 10,12,14Be predicted as di-cluster structure with valence neutrons surrounding 𝛼-𝛼 core are the very appealing candidates of clustering studies. We aim at directly and quantitatively probing the cluster structure in the ground state of 10,12,14Be isotopes via quasifree (𝑝,𝑝𝛼) reaction in inverse kinematics. The experiment was performed at RIKEN with the world’s highest intensity of neutron- rich 10,12,14Be beams at 150 MeV/u. The reactions of interest were induced by beams of exotic Be isotopes impinging on a pure solid hydrogen target. The SAMURAI spectrometer was employed to achieve large acceptance and high resolution charac- teristics. Combined with charge-particle detection systems surrounding the target, exclusive measurements were achieved for the physics goals. The triple differential cross-sections (TDX) for the (𝑝,𝑝𝛼) reactions are ex- tracted at quasifree angle pairs (𝜃𝑝,𝜃𝛼) and compared to the distorted-wave im- pulse approximation (DWIA) reaction calculation with Tohsaki-Horiuchi-Schuck- Röpke (THSR) structure model. The extracted TDXs in 10Be(𝑝,𝑝𝛼)6He(g.s.) and 12Be(𝑝,𝑝𝛼)8He(g.s.) reactions are very close to each other. For the former reaction, both the shape and the magnitude of the TDX are very well reproduced by the DWIA calculation. This result clearly validates the molecular cluster structure 2𝛼 + 2𝑛(𝜋) in the ground state of 10Be nucleus described by the THSR wave-function. For the later reaction, the shape of the experimental result is in moderate agreement with the DWIA calculation, while the magnitude of the experimental results is greatly overestimated, suggesting a more compact structure in the ground state of 12Be. Further improvement in the cluster wave function is needed. The populations of the ground state in He residues are comparable between 10Be(𝑝,𝑝𝛼) and 12Be(𝑝,𝑝𝛼) while the populations of the excited states are radically different. The excited-state transition is as important as the ground-state transition for the former, whereas the ground-state transition strongly dominates over the other excited-state transitions for the latter. Cross-sections for the population of 2+ excited state in He residues have been extracted and compared to the result of the ground- state transition. The ratio of the 2+ state transition to the ground-state transition for 10Be(𝑝,𝑝𝛼) reaction is nearly a half at proton scattering angle 65◦; while the ratio for 12Be(𝑝,𝑝𝛼) reaction is less than 1%. Such a significant difference of 2+ state component in He-core excited states unambiguously indicates completely different cluster structures in the ground state of 10Be and 12Be.