R. Raabe

Fusion and direct reactions of halo nuclei at energies around the Coulomb barrier

Abstract The present understanding of reaction processes involving light unstable nuclei at energies around the Coulomb barrier is reviewed. The effect of coupling to direct reaction channels on elastic scattering and fusion is investigated, with the focus on halo nuclei, for which such effects are expected to be most important. With the aim of resolving possible ambiguities in the terminology a short list of definitions for the relevant processes and quantities is proposed. This is followed by a review of the experimental and theoretical tools and information presently available. The effect of breakup couplings on elastic scattering and of transfer couplings on fusion is investigated with a series of model calculations within the coupled-channels framework. The experimental data on fusion are then compared to ‘bare’ no-coupling one-dimensional barrier penetration model calculations employing reasonably realistic double-folded potentials. On the basis of these model calculations and comparisons with experimental data, conclusions are drawn from the observation of recurring features. The total fusion cross-sections for halo nuclei show a suppression with respect to the ‘bare’ calculations at energies just above the barrier that is probably due to single neutron transfer reactions. The data for total fusion are also consistent with a possible sub-barrier enhancement; however, this observation is not conclusive and other couplings besides the single-neutron channels would be needed in order to explain any actual enhancement. We find that a characteristic feature of halo nuclei is the dominance of direct reactions over fusion at near and sub-barrier energies; the main part of the cross-section is related to neutron transfers, while calculations indicate only a modest contribution from the breakup process.

The SPIG, improvement of the efficiency and beam quality of an ion-guide based on-line isotope separator

Abstract A SPIG (Sextupole Ion Beam Guide) has been constructed and tested to couple an ion-guide based ion source to an on-line isotope separator. In comparison to a skimmer, the SPIG offers both an increased beam quality and the possibility to operate the ion-guide at higher buffer gas pressures.

Elastic 2n-transfer in the 4He(6He,6He)4He scattering

The elastic scattering He-4(He-6,He-6)He-4 has been investigated at center-of-mass energies of 11.6 and 15.9 MeV. Differential cross sections are determined using a post-accelerated He-6 (T-1/2 = 0.807 s) beam in the center-of-mass angular range between 50 and 140 degrees. The comparison of the measured data with calculations using a double folding potential shows evidence for the 2n-transfer process in the He-4(He-6,He-6)He-4 elastic scattering

The Leuven isotope separator on-line laser ion source

An element-selective laser ion source has been used to produce beams of exotic radioactive nuclei and to study their decay properties. The operational principle of the ion source is based on selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a noble gas at high pressure. The ion source has been installed at the Leuven Isotope Separator On-Line (LISOL), which is coupled on-line to the cyclotron accelerator at Louvain-la-Neuve. 54,55Ni and 54,55Co isotopes were produced in light-ion-induced fusion reactions. Exotic nickel, cobalt and copper nuclei were produced in proton-induced fission of 238U. The b decay of the 68–74Ni, 67–70Co, 70–75Cu and 110–114Rh isotopes has been studied by means of β–γ and γ–γ spectroscopy. Recently, the laser ion source has been used to produce neutron-deficient rhodium and ruthenium isotopes (91–95Rh, 98Rh, 90,91Ru) near the N=Z line in heavy ion-induced fusion reactions.

Shape isomerism at N=40: Discovery of a proton intruder state in Co67

The nuclear structure of 67Co has been investigated through 67Fe beta-decay. The 67Fe isotopes were produced at the LISOL facility in proton-induced fission of 238U and selected using resonant laser ionization combined with mass separation. The application of a new correlation technique unambiguously revealed a 496(33) ms isomeric state in 67Co at an unexpected low energy of 492 keV. A 67Co level scheme has been deduced. Proposed spin and parities suggest a spherical (7/2-) 67Co ground state and a deformed first excited (1/2-) state at 492 keV, interpreted as a proton 1p-2h prolate intruder state.

Shape Isomerism at N = 40: Discovery of a Proton Intruder in 67Co

The nuclear structure of 67Co has been investigated through 67Fe beta-decay. The 67Fe isotopes were produced at the LISOL facility in proton-induced fission of 238U and selected using resonant laser ionization combined with mass separation. The application of a new correlation technique unambiguously revealed a 496(33) ms isomeric state in 67Co at an unexpected low energy of 492 keV. A 67Co level scheme has been deduced. Proposed spin and parities suggest a spherical (7/2-) 67Co ground state and a deformed first excited (1/2-) state at 492 keV, interpreted as a proton 1p-2h prolate intruder state.

Experimental Study of the Two-Body Spin-Orbit Force in Nuclei

Energies and spectroscopic factors of the first 7/2-, 3/2-, 1/2-, and 5/2- states in the (35)Si21 nucleus were determined by means of the (d, p) transfer reaction in inverse kinematics at GANIL using the MUST2 and EXOGAM detectors. By comparing the spectroscopic information on the Si35 and S37 isotones, a reduction of the p3/2-p1/2 spin-orbit splitting by about 25% is proposed, while the f7/2-f5/2 spin-orbit splitting seems to remain constant. These features, derived after having unfolded nuclear correlations using shell model calculations, have been attributed to the properties of the two-body spin-orbit interaction, the amplitude of which is derived for the first time in an atomic nucleus. The present results, remarkably well reproduced by using several realistic nucleon-nucleon forces, provide a unique touchstone for the modeling of the spin-orbit interaction in atomic nuclei.

Radioactive ion beam physics at the cyclotron research centre Louvain-la-Neuve

The first beam of post-accelerated radioactive ions was realized in 1989 at the Louvain-la-Neuve research facility. The method employed two coupled cyclotrons to produce, separate and re-accelerate the species of interest. Several technological challenges were solved in the process to obtain pure and intense beams for the use in nuclear physics research. Similarly, new techniques were developed and refined for the measurement of the nuclear reactions induced by the radioactive beams. The available energy range made the facility particularly suited for nuclear astrophysics studies, and important results were obtained in the determination of stellar reaction rates using beams of 7Be, 13N, 18F and 18, 19Ne. A beam of 6He ions was extensively used in studies of the nuclear structure (halos, molecular states) and dynamics (the reaction process at energies around the potential barrier).

A detector setup to study the decay properties of exotic nuclei

Abstract General features of a detector setup for decay studies at on-line mass-separators are discussed. The detection system was used for measurements of β-decay properties of exotic nuclei with A ≈70. Results of GEANT Monte-Carlo simulations of the detector setup generally confirm the measurements. Both measurements and simulations show that even in case of low multiplicity events the Ge detector efficiency and peak to total ratio of γ-lines are reduced by large factor due to summing. Steps to improve the detection system are discussed.

Status of the laser ion source at the Leuven isotope separator online

An online laser ion source has been used at the Leuven isotope separator online for the production of pure beams of exotic nuclei. The operational principle of the ion source is based on the element-selective multistep laser resonance ionization of nuclear reaction products thermalized and neutralized in a high-pressure noble gas. A number of improvements has been carried out to obtain stable and reproducible operation. The ion source has been optimized for the production of beams of exotic nuclei, created in proton-induced fission reactions. The efficiency of the ion source has been improved by incorporating the sextupole ion guide to separate laser-produced ions from the gas jet and to transport them to the acceleration stage of the mass separator. A gas purification system has been installed to purify the noble gas down to ppb level. High selectivity and efficiency of the ion source allowed to collect nuclear spectroscopic information for the neutron deficient 54Ni and neutron-rich 68–74Ni isotopes.