By exploiting Coulomb dissociation of high-energy radioactive beams of the neutron-rich nuclei [129][130][131][132]134 Sb, their dipole-strength distributions have been measured. A sizable fraction of "pygmy" dipole strength, energetically located below the giant dipole resonance, is observed in all of these nuclei. A comparison with available pygmy resonance data in stable nuclei ( 208 Pb and N = 82 isotones) indicates a trend of strength increasing with the proton-to-neutron asymmetry. On theoretical grounds, employing the RQRPA approach, a one-to-one correlation is found between the pygmy strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skins. On this basis, by using the experimental pygmy strength, parameters of the nuclear symmetry energy (a 4 = 32.0 ± 1.8 MeV and p o = 2.3 ± 0.8 MeV/fm 3 ) are deduced as well as neutron-skin thicknesses R n − R p of 0.24 ± 0.04 fm for 132 Sn and of 0.18 ± 0.035 fm for 208 Pb, both doubly magic nuclei. Astrophysical implications with regard to neutron stars are briefly addressed.The neutron root-mean-square (rms) radii of nuclei are fundamental quantities which are difficult to measure in a model-free way [1] and are, therefore, known only for few cases and with relatively poor accuracy [2][3][4]. This fact is particularly cumbersome since neutron rms radii belong to the few laboratory data that can be used to constrain the isospin-asymmetric part of the equation of state of nuclear matter [5][6][7], which in turn is closely related, e.g., to the radii of such exotic objects as neutron stars. Neutron skins in heavy nuclei and the crust of neutron stars are both built from neutron-rich nuclear matter and one-to-one correlations were drawn between neutron-skin thicknesses in nuclei [8][9][10] and specific properties of neutron stars. In a recent paper, Piekarewicz [11] pointed out that the experimentally observed "pygmy" dipole (E1) strength [12] might play an equivalent role as the neutron rms radius in constraining the nuclear symmetry energy. Excess neutrons forming the skin give rise to pygmy dipole transitions at excitation energies below the giant dipole resonance; to which extent such transitions represent a collective vibration of excess neutrons against an isospinsymmetric core is theoretically under discussion yet [13][14][15][16].Experimental evidence for pygmy dipole resonances (PDR) is still rather scarce. In an earlier paper [12], we reported on low-lying E1 strength observed in the exotic nuclei 130,132 Sn exhausting a few percent of the energy-weighted ThomasReiche-Kuhn (TRK) sum rule. Stable N = 82 isotones and 208 Pb investigated in (γ, γ ) reactions [17-19] display a concentration of dipole strength below the neutron-separation threshold, absorbing, however, a much smaller fraction of the TRK sum rule.In the first part of this Rapid Communication we present new experimental data for the unstable isotopes 129,131 Sn and 133,134 Sb obtained from the same measurement as in...
The E1 strength distribution in 68Ni has been investigated using Coulomb excitation in inverse kinematics at the R3B-LAND setup and by measuring the invariant mass in the one- and two-neutron decay channels. The giant dipole resonance and a low-lying peak (pygmy dipole resonance) have been observed at 17.1(2) and 9.55(17) MeV, respectively. The measured dipole polarizability is compared to relativistic random phase approximation calculations yielding a neutron-skin thickness of 0.17(2) fm. A method and analysis applicable to neutron-rich nuclei has been developed, allowing for a precise determination of neutron skins in nuclei as a function of neutron excess.
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Complete fusion excitation function for the 6 Li + 144 Sm reaction has been measured at near barrier energies by the activation technique. Coupled-channel calculations show an enhancement in fusion cross section at energies below the barrier compared to the one-dimensional barrier penetration model calculation, but they overpredict it in the entire energy range compared to the experimental data. Reduced fusion cross sections for the present system at energies normalized to the Coulomb barrier were also found to be systematically lower than those with strongly bound projectiles forming a similar compound nucleus. These two observations conclusively show that the complete fusion cross section, at above barrier energies, is suppressed by ∼32% in the 6 Li + 144 Sm reaction. Reanalyses of existing fusion data for 7 Li + 165 Ho and 7 Li + 159 Tb also show a suppression compared to those with strongly bound projectiles, which contradicts earlier conclusions. The fusion suppression factor seems to exhibit a systematic behavior with respect to the breakup threshold of the projectile and the atomic number of the target nucleus.
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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