Quantum tunnelling through a potential barrier (such as occurs in nuclear fusion) is very sensitive to the detailed structure of the system and its intrinsic degrees of freedom. A strong increase of the fusion probability has been observed for heavy deformed nuclei. In light exotic nuclei such as 6He, 11Li and 11Be (termed 'halo' nuclei), the neutron matter extends much further than the usual nuclear interaction scale. However, understanding the effect of the neutron halo on fusion has been controversial--it could induce a large enhancement of fusion, but alternatively the weak binding energy of the nuclei could inhibit the process. Other reaction channels known as direct processes (usually negligible for ordinary nuclei) are also important: for example, a fragment of the halo nucleus could transfer to the target nucleus through a diminished potential barrier. Here we study the reactions of the halo nucleus 6He with a 238U target, at energies near the fusion barrier. Most of these reactions lead to fission of the system, which we use as an experimental signature to identify the contribution of the fusion and transfer channels to the total cross-section. At energies below the fusion barrier, we find no evidence for a substantial enhancement of fusion. Rather, the (large) fission yield is due to a two-neutron transfer reaction, with other direct processes possibly also involved.

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.

Isotopic effects in the fragmentation of excited target residues following collisions of 12C on (112,124)Sn at incident energies of 300 and 600 MeV per nucleon were studied with the INDRA 4pi detector. The measured yield ratios for light particles and fragments with atomic number Z < or = 5 obey the exponential law of isotopic scaling. The deduced scaling parameters decrease strongly with increasing centrality to values smaller than 50% of those obtained for the peripheral event groups. Symmetry-term coefficients, deduced from these data within the statistical description of isotopic scaling, are near gamma = 25 MeV for peripheral and gamma < 15 MeV for central collisions.

Single nucleon pickup reactions were performed with a 18:1 MeV=nucleon 14 O beam on a deuterium target. Within the coupled reaction channel framework, the measured cross sections were compared to theoretical predictions and analyzed using both phenomenological and microscopic overlap functions. The missing strength due to correlations does not show significant dependence on the nucleon separation energy asymmetry over a wide range of 37 MeV, in contrast with nucleon removal data analyzed within the sudden-eikonal formalism. DOI: 10.1103/PhysRevLett.110.122503 PACS numbers: 24.50.+g The existence of single-particle-like modes in nuclei, near the Fermi surface, is particularly important because these are at the basis of the nuclear shell model and thus govern the low energy nuclear dynamics. Yet, they result from nontrivial many-body correlations, which affect energy ordering and filling of active orbits. Spectroscopic factors (SFs) are a unique tool to address the question of correlations as they are strictly linked to the notion of shell occupancies and can be probed using direct reaction cross section measurements [1,2]. Information for stable nuclei was formerly provided by the electromagnetic probe (e, e 0 p) [3][4][5]. Even for closed shell nuclei like 16 O or 208 Pb, a cross section reduction by 30%-40% relative to an independent-particle-based model was observed. Different origins are now well established, like short range correlations [1] and couplings to collective modes at high excitation energy [6] or to the continuum [7]. Single nucleon pickup reactions were also used for stable nuclei yielding results consistent with (e, e 0 p) measurements [8,9].For nuclei away from the valley of stability, new approaches have been developed in inverse kinematics at various incident energies, knockout and transfer reactions. From knockout reactions at intermediate energy, a reduction factor R s was deduced as the ratio between the experimental cross section and a theoretical value obtained in a sudden-eikonal approach [10]. A strong dependence was claimed for R s versus the asymmetry (difference in separation energy) ÁS ¼ ðS p À S n Þ with ¼ þ1 (À1) for proton (neutron) removal reactions, with a reduction as high as 70% for large positive ÁS values. This reduction is still not understood and was first accounted for by possible missing correlations in shell-model calculations [10]. Different conclusions were drawn from (i) the possibility of dissipative processes beyond the sudden approximation [11,12], and (ii) transfer reactions at lower incident energies showing no ÁS dependence of R s [13]. From a theoretical point of view, ab initio calculations suggest only a mild dependence of SFs on ÁS [7,14], with equal SFs found for the nucleon removals from 56 Ni [6] despite significant ÁS values (AE 9:5 MeV). Coupled-cluster calculations [7] pointed out a further decrease of proton SFs for isotopes at the neutron dripline, due to coupling to the continuum. This has the substantial effect of enhancing the dependence on...

The N = 28 shell closure has been investigated via the 46Ar(d,p)47Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p(3/2), p(1/2), and f(5/2) states in 47Ar were determined and compared to those of the 49Ca isotone. We deduced a reduction of the N = 28 gap by 330(90) keV and spin-orbit weakenings of approximately 10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.

We report on the single neutron and proton removal reactions from unstable nuclei with large asymmetry ΔS = S(n)-S(p) at incident energies below 80 MeV/nucleon. Strong nonsudden effects are observed in the case of deeply-bound-nucleon removal. The corresponding parallel momentum distributions exhibit an abrupt cutoff at high momentum that corresponds to an energy threshold occurring when the incident energy per particle is of comparable magnitude to the nucleon separation energy. A large low-momentum tail is related to both dissipative processes and the dynamics of the nucleon removal process. New limits for the applicability of the sudden and eikonal approximations in nucleon knockout are given.

Expérience GANIL/SPIRAL/MUST2/E525SThe low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p(8He, t) with the 8He beam delivered by the SPIRAL facility at 15.4 A MeV. The light charged particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2+ state, two new resonances were observed: at E∗ = 2.6±0.3 MeV (width Γ = 1.6±0.4 MeV) and at 5.3±0.3 MeV with Γ = 2 ± 1 MeV. Through the analysis of the angular distributions, they correspond to a 2+ state and to an L = 1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects

The properties of fragments and light charged particles emitted in multifragmentation of single sources formed in central 36 A.MeV Gd+U collisions are reviewed. Most of the products are isotropically distributed in the reaction c.m. Fragment kinetic energies reveal the onset of radial collective energy. A bulk effect is experimentally evidenced from the similarity of the charge distribution with that from the lighter 32 A.MeV Xe+Sn system. Spinodal decomposition of finite nuclear matter exhibits the same property in simulated central collisions for the two systems, and appears therefore as a possible mechanism at the origin of multifragmentation in this incident energy domain.