Sub and near barrier fusion of neutron rich heavy nuclei – studies with radioactive ion beams

The availability of accelerated fission fragments at HRIBF allows us to study fusion reactions where one of the reactants is a short-lived exotic nucleus. Most interesting in this respect are entrance channels involving neutron-rich target and projectile – where enhanced survival probability of the compound system may allow the synthesis of heavier system. Much depends though on the dynamic evolution of the captured nuclei into a compound nucleus and the ensuing competition between fission and evaporation residue decay modes. Our studies of fusion between heavy neutron-rich nuclei are aimed at acquiring data that will lead to the understanding and eventually the ability to predict the probabilities for these different processes.     

Experimental nuclear astrophysics with radioactive ion beams

Reactions involving unstable nuclei play an important role in many astrophysical sites. Radioactive ion beams provide a unique tool to investigate the structure of such unstable nuclei as well as the cross sections for many reactions of astrophysical relevance. This paper provides a brief survey of some recent results in experimental nuclear astrophysics with Radioactive Ion Beams, particularly for processes which take place during explosive hydrogen burning in novae and X-ray bursts. Some prospects for future studies at next generation facilities are also briefly discussed.     

Nucleon transfer reactions with exotic beams at ATLAS

We present some recent results from studies of light nuclei using exotic beams from ATLAS at Argonne National Laboratory. Light nuclei far from stability provide ideal testing grounds for modern theoretical methods, and may provide information about astrophysical environments. We focus on the nuclei ⁹Li and ⁷He, populated with the (d,p) and (d,³He) reactions.     

Low-lying dipole strength for weakly bound systems: A simple analytic estimate

A simple analytic expression is proposed for the total low-lying dipole strength observed in the break-up of weakly bound systems. The model assumes pure single-particle transitions from the halo weakly bound state to continuum states treated as plane waves (neutrons) or regular Coulomb functions (protons). The analytic expression for the total strength approximately depends on the inverse of the binding energy and the reduced mass of the halo, with correction factors accounting for the charge of the halo and the finite size of the potential.     

Excitation functions of fusion reactions and neutron transfer in the interaction of 6He with 197Au and 206Pb

Excitation functions for evaporation residues in the reactions ¹⁹⁷Au(⁶He, xn)^203-xnTl, x = 2-7, and ²⁰⁶Pb(⁶He, 2n)²¹⁰Po, as well as for neutron transfer reactions for the production of ¹⁹⁶Au and ¹⁹⁸Au in the interaction of ⁶He with ¹⁹⁷Au were measured. The ⁶He beam was obtained from the accelerator complex for radioactive beams DRIBs (JINR). The maximum energy of the beam was about 10AMeV and the intensity reached 2×10⁷pps. The stacked-foil activation technique was used directly in the beam extracted from the cyclotron or in the focal plane of the magnetic spectrometer MSP-144. The identification of the reaction products was done by their radioactive γ- or α-decay. The fusion reaction with the evaporation of two neutrons was characterized by an increase in the cross-section compared to statistical model calculations. The analysis of the data in the framework of the statistical model for the decay of excited nuclei, which took into account the sequential fusion of ⁶He has shown good agreement between the experimental and the calculated values of the cross-sections in the case of sub-Coulomb-barrier fusion in the ²⁰⁶Pb + ⁶He reaction. An unusually large cross-section was observed below the Coulomb barrier for the production of ¹⁹⁸Au in the interaction of ⁶He with ¹⁹⁷Au. Possible mechanisms of formation and decay of transfer reaction products are discussed.     

Neutron excess nuclei of hydrogen and helium at ACCULINNA

Attempt to observe a ⁷H resonance produced in the reaction ²H(⁸He,³He)⁷H resulted only in setting a limit dσ/dΩ≤ 20 μb/sr for the reaction exit channel which could populate a resonance lying between 0 and 3 MeV above the ⁷H decay threshold. The quasi-free scattering of the α core bound in ⁶He was explored keeping in mind the possible study of the cluster structure of this halo nucleus. For the first time coincident particles emitted in the ⁴He(⁶He, 2α)nn reaction were detected in wide angular ranges giving a wide kinematical range of the measured angular and momentum distributions. The contribution of processes, competing with QFS in the α+ α+n+n output channel, was considerably suppressed by the selection of events with E_α1(2)-nn>10 MeV. A number of experimental distributions, relevant to the reaction mechanism and to the ⁶He structure, were compared with the results of MC simulations based on the PWIA formalism. The PWIA predictions showed consistency with the experimental data.     

Giant-resonance studies with radioactive beams: Perspectives

First-generation radioactive ion-beam facilities have already been in operation for some time. Advanced facilities that will deliver high-intensity radioactive nuclear beams ranging in energy from below the Coulomb barrier to up to several hundred MeV per nucleon (MeV/u) are either starting operation, or under construction or in the planning stage. In this paper the perspectives of using radioactive nuclear beams to study giant resonances in nuclei far from the valley of stability are explored. In particular, emphasis will be made on information on certain nuclear properties that can be gained from such studies. Received: 1 May 2001 / Accepted: 4 December 2001     

Measurements of fusion reactions induced by radioactive 132Sn on 64Ni

Evaporation residue and fission cross sections of radioactive ¹³²Sn on ⁶⁴Ni were measured. Statistical model calculations using parameters simultaneously fitting stable Sn+⁶⁴Ni data reproduce the ¹³²Sn induced reaction very well. A large sub-barrier fusion enhancement was observed. The enhancement can be accounted for by coupled-channels calculations including inelastic excitation of the projectile and target, and neutron transfer.     

Model calculations of a two-step reaction scheme for the production of neutron-rich secondary beams

A two-step reaction scheme for the production of extremely neutron-rich radioactive beams, fission followed by cold fragmentation, is considered. The cross-sections of the second step, the cold fragmentation of neutron-rich fission fragments, are estimated with different computer codes. Discrepancies between an empirical systematics and nuclear-reaction codes are found. Received: 27 January 2003 / Accepted: 6 March 2003 / Published online: 27 May 2003     

Implication of proton-nucleus scattering for density distributions of unstable nuclei

The use of elastic proton scattering at intermediate and high energies to obtain information about the density distributions of unstable nuclei is investigated. A comparison between the relativistic impulse approximation (RIA) and Glauber model for proton scattering from ¹⁶O, ⁴⁰Ca, ⁴⁴Ca and ⁴⁸Ca at medium energies is performed. We used density distributions derived from the relativistic mean-field theory, employing the recent relativistic force NL-RA1, as well as experimental and phenomenological densities. It is found that the eikonal approximation can describe the cross-section only at small scattering angles and is weakly sensitive to the density distributions, while the RIA nicely produced the experimental cross-sections, even at medium and larger angles, and was very sensitive to the nuclear densities. Furthermore, the RIA better describes the isospin dependence of the cross-section. We used the RIA to investigate the density distribution of ⁵⁸Ca for proton scattering at different energies. It is found that the cross-section strongly depends on the parameters of the density distribution even at a small scattering angle. These results are important in extracting information about the structure of unstable nuclei. We also investigated the RIA and its sensitivity in describing halo nuclei such as ⁶He. We used for ⁶He a no-halo Gaussian density and a realistic-halo density that derived in the cluster orbital shell model approximation and contains the extended distribution of the valence nucleons. Comparison with the recent experimental data at GSI at 717 MeV/nucleon shows that the RIA successfully described the data at all considered range of the momentum transfer and on the other hand favor the halo structure of ⁶He. Received: 1 December 2001 / Accepted: 31 July 2002 / Published online: 11 February 2003 RID="a" ID="a"e-mail: mrashdan@hotmail.com Communicated by P. Schuck     

Shape coexistence and the N = 20 shell closure far from stability by inelastic scattering

Following mass measurements in the region N = 20 and N = 28, we have studied inelastic nuclear scattering for the nuclei ³⁴Si, ³³Al and ³²Mg. No evidence for a low-lying shape isomeric 0⁺ state was found in ³⁴Si, and an upper limit for the population cross-section could be established, rendering its existence very unlikely. A new transition was found in ³³Al, that is a good candidate for a 2p-2h state and therefore a determination of the 2p-2h gap at N = 20. Inelastic nuclear scattering strongly excites 3^- states, as seen in ³⁴Si. A strong transition was found in ³²Mg that should correspond to the first 3^- in this nucleus, lying very low as compared to theory and systematics in this region. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: mittig@ganil.fr     

Shell effects and pairing correlations in fission investigated with radioactive beams

The secondary-beam facility at GSI allows to produce a large variety of exotic nuclei at relativistic energies. This technique offers a unique oportunity to investigate systematically fission in inverse kinematics. In the present experiment, the fission properties of more than 70 different actinides and preactinides were investigated at low excitation energy. The elemental yields and kinetic energies of the fission residues present new signatures of shell structure and pairing correlations. Received: 1 May 2001 / Accepted: 4 December 2001     

Spectroscopy on very neutron-rich nuclei beyond N = 20

Recent studies on nuclear structure by using radioactive isotope beams available at the RIKEN projectile-fragment separator (RIPS) are introduced. Special emphasis is given to two selected experiments from recent programs that highlight studies on the magicity loss observed for very neutron-rich nuclei beyond N = 20 in the “island-of-inversion” region; the particle stability of ³¹F, and the low-lying excited states of ³⁴Mg. Received: 1 May 2001 / Accepted: 4 December 2001     

Polarized proton target for RI Beam experiments

We have constructed a polarized proton solid target system for radioactive nuclear beam experiments at the Center for Nuclear Study, the University of Tokyo. The proton polarization is based on an electron population difference in a photo-excited triplet state of pentacene molecule. The target system was completed in 2003 and applied to a RI beam experiment in 2003 and 2005 by using the projectile fragment separator, RIPS at RIKEN. The maximum polarization reached 20% under the condition of T=100 K and B=0.09 T. Overview of the polarized target and its application in physics experiments at RIPS and RIBF of RIKEN are presented.     

Unexpected transparency in the scattering of fragile <Superscript>6</Superscript>Li and <Superscript>6</Superscript>He Nuclei

It is found that the scattering of the fragile nucleus ⁶Li from ¹²C and ¹⁶O is unexpectedly transparent. It is shown that the internal-wave contribution is significantly large in the scattering, which suggests that some transparency could persist in the scattering involving the fragile nucleus ⁶He.     

Investigation of α-nucleus interaction in the 27Al (α,α)27Al scattering and 27Al (α, d )29Si reaction

Full finite-range macroscopic calculations in the distorted-wave Born approximation have been performed using the molecular and Michel α-nucleus potentials to analyze the angular distributions of cross-sections of the ²⁷Al(α, d)²⁹Si reaction, at 26.5 and 27.2 MeV incident energies, leading to seven transitions up to the excitation energy E _X = 4.08 MeV of the final nucleus. The parameters of the two types of the α-nucleus potentials are determined from the elastic-scattering data. Both the molecular and Michel potentials, without any adjustment to the parameters needed to fit the elastic-scattering data, are able in most cases to reproduce, simultaneously, the absolute cross-sections particularly at large angles, where the previous calculations failed to reproduce by orders of magnitude, and the gross pattern of angular distributions of the reaction. The deuteron-cluster spectroscopic factors for most of the seven transitions, deduced using the two α-²⁷Al potentials, differ from those obtained in earlier works. The spectroscopic factor for the ground-state transition, deduced in the present work for the 25.8 MeV data, agrees well with the shell model prediction. Received: 15 July 2002 / Accepted: 8 August 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: akbasak2001@Yahoo.com Communicated by G. Orlandini     

Reaction mechanisms in collisions induced by halo and/or weakly bound radioactive nuclei around the barrier

In this paper a brief review of the status of theory and experiments concerning the study of reaction mechanisms, in particular fusion, with halo and loosely bound nuclei at energies around the Coulomb barrier is made. The data of the reactions ^4,6He+⁶⁴Zn and ¹³N+⁹Be are discussed in more detail.     

Reactions induced by 35 MeV 6He beam on 12C and 14C

The ⁶He+^12,14C scattering and reactions have been studied using a 35 MeV ⁶He radioactive beam and preliminary results are presented. The three-body reaction ⁶He+¹²C ↦ ¹⁰Be+2 α has been clearly identified and sequential decay of intermediate states in ¹⁴C has been studied. Search for an analog reaction on ¹⁴C, ⁶He+¹⁴C ↦ ¹²Be+2 α, has not given conclusive results yet. Other interesting reactions with this entrance channel are discussed.     

Inverse kinematics resonance scattering on thick target with EXCYT beams

The resonance scattering method in inverse kinematics on infinite target has been applied to two systems, ¹⁸O + α and ⁹Be + α, in order to test and to optimize an experimental apparatus in view of a set of resonance scattering experiments to be performed with ⁸Li and ⁹Li radioactive ion beams delivered by the facility EXCYT. The scattering excitation functions, deduced for the two systems, were compared with previous results reported in literature. Results show that reliable stopping power data are essential for the application of this technique.