Role of the cluster structure of 7Li in the dynamics of fragment capture

Abstract

Exclusive measurements of prompt γ-rays from the heavy-residues with various light charged particles in the 7 Li + 198 Pt system, at an energy near the Coulomb barrier (E/V b ∼ 1.6) are reported. Recent dynamic classical trajectory calculations, constrained by the measured fusion, α and t capture cross-sections have been used to explain the excitation energy dependence of the residue cross-sections. These calculations distinctly illustrate a two step process, breakup followed by fusion in case of the capture of t and α clusters; whereas for 6 He + p and 5 He + d configurations, massive transfer is inferred to be the dominant mechanism. The present work clearly demonstrates the role played by the cluster structures of 7 Li in understanding the reaction dynamics at energies around the Coulomb barrier.Keywords: Particle gamma coincidence, Weakly bound nuclei, Breakup fusion, Nuclear cluster structure, Classical dynamical model In weakly bound nuclear systems, correlation among nucleons and pairing are manifested, among others, as an emergence of strong clustering and exotic shapes. This has renewed interest in the understanding of clusters based on concepts of molecular physics and the role of cluster states in nuclear synthesis [1,2]. Lithium isotopes present a unique example of nuclear clustering, with lighter isotopes ( 6,7 Li) having a well known α + x cluster structure and the heaviest bound isotope ( 11 Li) exhibiting a two neutron Borromean structure.9 Li has also been described as 6 He + t in a recent work [3]. 7 Li is an equally interesting case with its well known weakly bound α + t structure (S α/t = 2.47 MeV), as well as less studied more strongly bound clusters 6 He + p (S6 He/p = 9.98 MeV) and 5 He + d (S5 He/d = 9.52 MeV) [4,5].Recent studies with weakly bound nuclei have also focused on the understanding of the role of novel structures in the reaction dynamics [6]. Dominant reaction modes in nuclei with low binding energies, involve inelastic excitation to low lying states in the continuum or transfer/capture of one of the cluster fragments from their bound/unbound states to the colliding partner nucleus [6,7,8]. The role of inelastic excitation of low lying unbound states and transfer in the fusion hindrance, observed at energies well below the barrier, is also a topic of current interest [9,10]. When the capture occurs from unbound states of the projectile, the process could be looked upon as a two step process, breakup followed by fusion (breakup fusion) [11,12,13]. In case of well bound nuclei, nuclear reaction related to cap- * Corresponding author Email address: aradhana@barc.gov.in (A. Shrivastava ) ture of heavy fragments by the target has been identified as incomplete fusion or massive transfer [14] and occurs predominately at energies ≥ 10 MeV/A. For weakly bound cluster nuclei such as 6,7 Li, the former has been shown to be important both above and at energies much below the Coulomb barrier [10,15]. Earlier studies have found the process of breakup fusion to be more dominant ove...

Low-energy fusion dynamics of weakly bound nuclei: A time dependent perspective

“…For this system, the ICF has also been studied with the PLATYPUS model, as reported in Refs. [8,9]. PLATYPUS provides useful information (angular momentum and excitation energy distributions) on the primary ICF products along with differential cross sections of the surviving breakup fragment [9].…”

“…The interaction between the fusing fragment and the survival fragment is very important, as this interaction affects both the angular momentum and kinetic energy brought by the captured fragment into the target. These observables are useful for planning and interpreting particle-gamma coincidence measurements [8]. In Ref.…”

“…In Ref. [8], the spin and excitation energy distributions from PLATYPUS are used in combination with the evaporation code PACE for calculating the production of specific evaporation residues. Some evaporation residue cross sections can be explained by the breakup-fusion mechanism implemented in the PLATYPUS model.…”

Study of breakup and transfer of weakly bound nucleus 6Li to explore the low energy reaction dynamics

“…In the case of reactions with neutron-rich radioactive ion beams, the coupling to transfer is found to be important. Recently for the first time in 7 Li+ 198 Pt reaction [9] at energies near the Coulomb barrier the α-capture cross sections were measured by in-beam exclusive measurements of prompt γ -rays from the heavy-residues with various light charged particles (α, t, d and p). The cross sections of the residues resulting from the processes of fusion, t-capture and neutron transfer channels were measured by off-beam γ methods.…”

“…However, owing to the low intensities of the presently available radioactive ion beam (RIB), it is difficult to clearly explore the reaction mechanisms of nuclear systems with unstable nuclei. In comparison with RIB, the beam intensities of stable weakly bound nuclei such as 6,7 Li and 9 Be, which have significant breakup probability, are orders of magnitude higher. Precise fusion measurements have already been performed with those stable weakly bound nuclei, and the effect of breakup of those nuclei on the fusion process has been extensively studied.…”