Breakup of the proton halo nucleus 8B near barrier energies

Abstract

The dynamics of a nuclear open quantum system could be revealed in the correlations between the breakup fragments of halo nuclei. The breakup mechanism of a proton halo nuclear system is of particular interest as the Coulomb polarization may play an important role, which, however, remains an open question. Here we use a highly efficient silicon detector array and measure the correlations between the breakup fragments of ⁸B incident on ¹²⁰Sn at near-barrier energies. The energy and angular correlations can be explained by a fully quantum mechanical method based on the state-of-the-art continuum discretized coupled channel calculations. The results indicate that, compared to the neutron halo nucleus ⁶He, ⁸B presents distinctive reaction dynamics: the dominance of the elastic breakup. This breakup occurs mainly via the short-lived continuum states, almost exhausts the ⁷Be yield, indicating the effect of Coulomb polarization on the proton halo state. The correlation information reveals that the prompt breakup mechanism dominates, occurring predominantly on the outgoing trajectory. We also show that, as a large environment, the continuum of ⁸B breakup may not significantly influence elastic scattering and complete fusion.

Fig. 1. Illustration of ⁸B breaking up into ⁷Be and proton.

In the initial state, the nucleons in ⁸B occupy bound single particle orbits in the potential well. Although located in the potential well, the halo proton is extremely weakly-bound in an extended distribution around the centrum ⁷Be core. It hence can be regarded as a semi-open quantum system. During the collision, ⁸B is excited to the unbound states above the breakup threshold (either resonant or non-resonant), forming an open quantum system.

Fig. 2. Angular distributions of elastic scattering and breakup reactions.

Squares, diamonds and stars denote the experimental data of elastic scattering, inclusive and exclusive breakup at a 38.7 and b 46.1 MeV, respectively. The elastic scattering and breakup data are respectively related to the left and right axes. The error bars indicate the statistical uncertainties. CDCC results for elastic scattering and elastic breakup (EBU) are shown by the blue and magenta solid curves, respectively. As a comparison, the one-channel calculations for the elastic scattering are represented by the dashed curves. The dotted lines correspond to the non-elastic breakup (NEB) contributions, which are derived from the IAV model calculations. The dash-dotted lines stand for the sum of EBU and NEB.

Fig. 3. Measured E_rel distributions for breakup fragments ⁷Be and proton.

The experimental data (circles) at a 38.7 and b 46.1 MeV are compared with the simulated distributions of E_rel (solid curves). The error bars show the associated statistical uncertainties. The dashed curves denote simulation results of p-wave 1⁺ state. The raw theoretical E_rel distributions from CDCC are shown in the insets, where the solid and dashed curves represent the calculations with the orbital angular momentum up to l = 3 (total) and the contributions from the p-wave (l = 1) 1⁺ state. The vertical line indicates the expected location of the peak from the first 1⁺ resonance of ⁸B.

Fig. 4. Angular correlations of breakup fragments ⁷Be and proton.

Comparison of experimental data (circles, with error bars indicating the associated statistical uncertainties) with simulations (squares) for correlations of β and θ₁₂ at a 38.7 and d 46.1 MeV. The dashed curves show the expected β-θ₁₂ correlation assuming asymptotic breakup from the 1⁺ resonance of ⁸B. The projections of β and θ₁₂ at the corresponding energies are shown in b, c, and e, f, respectively, where the solid curves represent the simulations based on the CDCC calculations. The inset in a illustrates the definitions of β and θ₁₂. The error bars show the associated statistical uncertainties.

Fig. 5. Schematic view of the experimental setup.

The upstream and downstream parallel plate avalanche counters (PPACs) are denoted as the PPACa and PPACb, respectively. Ten silicon detector telescopes surround the ¹²⁰Sn target, arranged in a spherical shape with a radius around 70 mm. To demonstrate clearly, only the first layers of the silicon detector array are displayed.

Fig. 6. Particle identification.

a Typical ΔE − E_r spectrum at 46.1 MeV, taken by one strip of the forward-angle telescope, covering the polar angular range of 30.4^∘–50.9^∘. b ΔE-TOF spectrum of two radio-frequency cycles illustrating the separation between the ⁷Be from ⁸B reactions (⁷Be_reac.) and elastically scattered ⁷Be in the secondary beam (⁷Be_beam), as surrounded by the dashed and solid circles, respectively.