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.

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...

We report on the measurement of the ^{7}Be(n,p)^{7}Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a ^{7}Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal ^{7}Li(p,n)^{7}Be reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+^{7}Li reaction is also discussed.

The two-step process of transfer followed by breakup is explored by measuring a rather complete set of exclusive data for reaction channels populating states in the ejectile continua of the 7 Li+ 93 Nb system at energies close to the Coulomb barrier. The cross sections for α + α events from one proton pickup were found to be smaller than those for α + d events from one neutron stripping and α + t events from direct breakup of 7 Li. Coupled channels Born approximation and continuum discretized coupled channels calculations describe the data well and support the conclusion that the α + d and α + α events are produced by direct transfer to unbound states of the ejectile.PACS numbers: 25.70. Hi,25.70.Bc,24.10.Eq,25.70.Mn, Exploring the properties of weakly-bound stable/unstable nuclei via transfer reactions is a topic of current interest [1,2] and also a focus of the next generation of high-intensity isotope-separator on-line (ISOL) radioactive ion beam facilities. Due to the low breakup threshold of such nuclei, population of the continuum is probable and consequently a large coupling effect is expected at energies around the Coulomb barrier. This may take place directly through inelastic excitation of the projectile (prompt or resonant breakup) or by nucleon transfer leaving the ejectile in an unbound state (transferbreakup) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The large positive Q-values for the transfer of neutrons from light neutron-rich projectiles to heavy targets also emphasize the role of neutron evaporation following transfer [16,17]. Exclusive measurements are essential to disentangle these reaction channels. Also, complete measurements of different reaction channels as well as theoretical calculations are required to understand the interplay between them. Among the limited exclusive measurements aimed at studying different breakup processes, very few data on absolute cross sections are available for direct breakup [4][5][6][7][8]13], while for transfer-breakup absolute differential cross sections are only available for the neutron transfer channels at energies close to the Coulomb barrier [5,7].Among the processes discussed above, investigation of the two-step reaction mechanism, viz., one nucleon transfer followed by breakup, is of current interest for the weakly-bound stable nuclei 6,7 Li and 9 Be [5, 9-12, 14, 18]. This complex process needs the simultaneous understanding of both the breakup and transfer reactions. In an earlier measurement of the 7 Li+ 65 Cu system [5] it was observed that 1n-stripping leading to 6 Li in its unbound 3 + 1 excited state is more probable than inelastic excitation of 7 Li to its resonant states. In recent measurements with 7 Li [9, 10], the importance of * sanat@barc.gov.in 1p-pickup over the direct breakup of the projectile was highlighted while explaining the suppression of fusion at energies above the Coulomb barrier. Hence, understanding the mechanism of projectile breakup-whether direct or transfer breakup-is crucial while studying the reaction d...