Results of measurement of residues formed in fusion of (6)Li with (198)Pt in the energy range of 0.68 < E/V(b) < 1.3 using a new sensitive off-beam technique are reported. The fusion excitation function and the derived average angular momenta do not indicate a change of slope at deep sub-barrier energies, contrary to recent observations. The present results for a system with weakly bound projectile confront the current understanding of the fusion hindrance at these low energies, underlying the role of internal reorganization on the dynamical path towards fusion.

The observation of a large cross-section for the α + d channel compared to breakup into the α + t channel from an exclusive measurement for the 7 Li+ 65 Cu system at 25 MeV is presented. A detailed analysis of the angular distribution using coupled channels Born approximation calculations has provided clear evidence that the observed α + d events arise from a two step process, i.e. direct transfer to the 2.186 MeV (3 + ) resonance in the α + d continuum of 6 Li followed by breakup, and are not due to final state interaction effects.

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...

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