Experimental Study for Breakup Reactions in ⁷Li+²³⁸U System at Near-barrier Energy

Weakly bound nuclei in the lighter mass region often have cluster structures like α+x, and are prone to breakup or transfer in nuclear reactions. The coupling effects of different reaction channels can significantly alter the reaction process. Therefore, the reaction mechanisms for these systems have attracted much attention over recent decades. Regarding the origin of the numerous α particles produced in weakly bound nuclear reactions, previous experimental measurements have indicated that α particles are not the product of a single reaction mechanism. Disentangling their specific reaction channels and quantitatively distinguishing the contributions of different reaction mechanisms are key to an in-depth understanding of the reaction dynamics of weakly bound nuclei. In this work, the ⁷Li+²³⁸U system at the laboratory frame energy E_lab=36.8MeV was selected to study the reaction mechanisms. Four sets of ΔE-E silicon detector telescopes, each consisting of three layers, were employed to perform coincidence measurements for the charged fragments produced by inelastic excitation breakup of the projectile or transfer-induced breakup. The first layer of each telescope measured the energy loss of incident particles, while the second and third layers measured their residual energy, thus enabling the kinetic energy determination and identification for the lighter charged particles. The detectors were calibrated with elastically scattered particles to ensure energy accuracy. Based on the physical characteristics of asymptotic breakup at large distances from the target for long-lived resonant states, the relative energy E_rel, opening angle θ₁₂, and breakup orientation angle β of the two breakup fragments of α+t and α+d were obtained via kinematic reconstruction of their energy and angular information. Combined with Monte Carlo simulations, a comprehensive analysis of the E_rel distribution and the angular correlation characteristics of the fragments was carried out. The first above-threshold resonance state of ⁷Li at the 4.630 MeV (7/2⁻) and the 2.186 MeV (3⁺) resonance state of ⁶Li populated by the direct neutron stripping process were reconstructed. The kinematic reconstruction analysis confirmed that the observed α+d events originate from a two-step process of direct neutron stripping to populate the 2.186 MeV (3⁺) resonance state in ⁶Li, followed by its breakup. Furthermore, the experimental results show that breakup via resonant states is the dominant process for the ⁷Li+²³⁸U system at E_lab=36.8 MeV, with a negligible contribution from near-target direct breakup. This study provides experimental evidence for disentangling the origin of α particles of weakly bound nuclear reactions at near-barrier energies, and also offers data support for further investigating the coupling effects in weakly bound nuclear reactions.