Recent experiments with radioactive ion beams at GSI

A review is given over recent experiments performed by means of A·keV to A·GeV radioactive ion beams at GSI Darmstadt. Nuclear structure information on exotic nuclei is discussed as obtained by using an online isotope separator, a velocity filter or a magnetic beam-line spectrometer.     

Fusion using radioactive ion beams

The capture-fission cross-section is measured for the collision of the massive nucleus ¹³²Sn with ⁹⁶Zr at near-barrier energies and compared with the collision of ¹²⁴Sn with ⁹⁶Zr. This study gives insight into fusion enhancement and hindrance in systems involving neutron-rich nuclei. The dinuclear system model (DNS) calculations describe the excitation function reasonably well and if we use the barrier heights predicted by this model we can conclude that fusion hindrance (represented by extra push energy) is greater for the more neutron-rich systems.     

Surface physics with radioactive ion beams

Nuclear methods using radiation detection are very well suited for surface and interface investigations, since they generally require only a small number of radioactive probe nuclei. Virtually isolated probe atoms can be investigated. The use of an isotope separator to solve the central problem in the application of radioactive atoms for the study of surfaces, the clean deposition of the probe nuclei, is described. First physics experiments include studies of desorption isochrones and characterization of adatom sites on flat and vicinal surfaces by PAC. A particularly complete picture could be obtained for Cd and In on the Pd(111) surface, where five different sites successively populated in the surface diffusion process were observed. Other nuclear methods suitable for a future extension of the surface investigations, such as Mössbauer spectroscopy, decay recoil angular distribution and emission channeling, are briefly discussed.     

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.     

Testing fundamental symmetries using radioactive ion beams at TRIUMF-ISAC

The ISAC Facility at TRIUMF, Canada’s national laboratory for particle and nuclear physics, provides rare isotope beams for a diverse research program. In this paper we summarize some recent experimental developments at TRIUMF pertaining to fundamental symmetry tests. These tests use the atomic nucleus as a probe to search for physics beyond the Standard Model. Some recent results and future plans are discussed.     

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

As a tool for studying the structure of nuclei far off stability the technique of γ-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable ³⁶S beam on deuteron and ⁹Be targets. It is demonstrated that the Doppler broadening of γ lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 10⁶ particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the γ background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross-sections of neutron-rich light nuclei on ²H and ⁹Be targets at REX-ISOLDE energies of 2.2 MeV ^. A are confirmed. Received: 9 October 2000 / Accepted: 28 January 2001     

Relativistic many-body theory with radioactive ion beams: Surface pion condensation

We critically review the present relativistic mean-field theory from the viewpoint of missing pions. We introduce the interesting experimental data on pionic states taken at RCNP. These data seem to suggest the occurrence of pion condensation in the nuclear surface. Qualitative discussion is made on the consequence of surface pion condensation on Gamow-Teller transitions and spin response functions and others. The radioactive ion beams are the tools of studying the unstable nuclei, which have extended nuclear surfaces. We shall start with radioactive ion beams the nuclear surface science, which includes the surface pion condensation as the important ingredient in addition to spin-orbit splitting and surface pairing. Received: 1 May 2001 / Accepted: 4 December 2001     

Physics with beams of radioactive nuclei

A survey of the latest results from experiments devoted to studying the structure of nuclei and nuclear-reaction mechanisms by means of accelerated radioactive beams is given. Results obtained in Dubna with the aid of the DRIBs accelerator complex and at other radioactive-beam factories are presented.     

Nuclear astrophysics with fast radioactive beams

I summarize some of the open questions and recent developments in nuclear astrophysics concerning the role of unstable nuclei in cosmic phenomena. This includes a brief discussion of the rapid neutron capture and rapid proton capture processes, as well as some recent experimental results using radioactive beams from fragmentation at the National Superconducting Cyclotron Laboratory.     

A multi-reflection time-of-flight mass separator for isobaric purification of radioactive ion beams

A multi-reflection time-of-flight mass separator (MR-ToF-MS) for the enhancement of the performance of the Penning-trap mass spectrometer ISOLTRAP at the on-line isotope separator ISOLDE/CERN has been built and investigated at an off-line test facility. The MR-ToF-MS consists of two ion-optical mirrors between which oscillating ions are separated according to their different mass-over-charge ratios m/q. Flight paths of several hundreds of meters are folded to an apparatus length of less than one meter. Preliminary tests resulted in a mass-resolving power of up to m/??m????80,000, and the separation was demonstrated for the isobaric ions CO^?+? and $\mathrm{N}_2^+$ . The MR-ToF-MS will support the existing purification methods of ISOLTRAP and will extend the access to nuclides produced with high isobaric contamination yields at the ISOLDE facility.     

Structure effects on reaction mechanisms in collisions induced by radioactive ion beams

The present paper concerns the study of reactions induced by radioactive beams of halo and weakly bound nuclei at energies around and above the Coulomb barrier. The results obtained for the reaction induced by the halo nucleus ⁶He on ⁶⁴Zn have been compared with the results for the reaction induced by ⁴He on the same target. The results of the reaction induced by the weakly bound unstable ¹³N on the weakly bound ⁹Be have been compared with those for the reaction ¹⁰B + ¹²C. The text was submitted by the authors in English.     

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.     

Multi-element focused ion beams using compact microwave plasma ion source

Focused ion beams (FIB) are widely used for research and applications in nanoscience and technology. We have developed a compact microwave plasma based multi element FIB (MEFIB) system in order to widen the applications and overcome the limitations faced by conventional Liquid Metal Ion Source (LMIS) based FIB systems, that provide primarily Ga ions. The MEFIB source provides high density plasmas (∼1.5 × 10¹¹ cm⁻³) in a compact cross section. Recently the ion energy spread in the plasma meniscus from where the beams are extracted is found to be small (∼5 eV) [1–3]. The beam extraction and focusing are carried out using electrostatic multi electrode assembly. AXCEL INP and SIMION simulation codes are employed for the design of electrostatic Einzel lenses for beam focusing. The beam focal point is measured using a specially designed three slit Faraday cup and the spot size is measured by the micrography of craters formed by the focused ion beams impinging on copper and aluminium substrates. The initial experimental results show a focused beam spot size of ∼ 25 micron which is in good agreement with the simulations. By further reduction of electrode apertures and operating the second Einzel lens at higher potentials, submicron focused ion beams can be expected.     

Projectile Coulomb excitation with fast radioactive beams

We report a search for γ rays emanating from Coulomb excitation of fast (30–46 MeV/u) radioactive projectiles⁸He,^11,12,14Be interacting with a lead target. These are clearly identified by their Doppler shift. The 320 keV 1/2^? → 1/2⁺ γ transition from¹¹Be was observed with a cross-section of 191±26 mb which is noticeably less than expected from the known lifetime and in the perturbation limit of pure Coulomb excitation. In the other nuclei rather stringent upper limits of 0.01 to 0.2 Weisskopf units, are placed on the hypothetical transition to 1^? states.     

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     

Nuclear reactions of astrophysical interest with radioactive beams

The ARENAS³ facility, coupling the two cyclotrons of Louvain-la-Neuve, is intensively been used over the past years for cross-section measurements of nuclear reactions of astrophysical interest. We will describe the characteristics of the available beams and the experiments going on using these beams. The ¹⁹Ne(p,γ)²⁰Na reaction, of considerable importance for the reaction flow between the CNO and the NeNa mass region in high-temperature hydrogen burning conditions, will be discussed in particular. The proposed low-energy resonance at 0.447 MeV has been measured in inverse kinematics using novel activation techniques. An upper limit (90% C.L.) of 18 meV is obtained for the strength of this first particle-unbound state of ²⁰Na together with preliminary values for the strength of the higher excited states.     

Reactions with fast radioactive beams of neutron-rich nuclei

The neutron dripline has presently been reached only for the lightest nuclei up to the element oxygen. In this region of light neutron-rich nuclei, scattering experiments are feasible even for dripline nuclei by utilizing high-energy secondary beams produced by fragmentation. In the present article, reactions of high-energy radioactive beams will be exemplified using recent experimental results mainly derived from measurements of breakup reactions performed at the LAND and FRS facilities at GSI and at the S800 spectrometer at the NSCL. Nuclear and electromagnetically induced reactions allow probing different aspects of nuclear structure at the limits of stability related to the neutron-proton asymmetry and the weak binding close to the dripline. Properties of the valence-neutron wave functions are studied in the one-neutron knockout reaction, revealing the changes of shell structure when going from the beta-stability line to more asymmetric loosely bound neutron-rich systems. The vanishing of the N = 8 shell gap for neutron-rich systems like ¹¹Li and ¹²Be, or the new closed N = 14, 16 shells for the oxygen isotopes are examples. The continuum of weakly bound nuclei and halo states can be studied by inelastic scattering. The dipole response, for instance, is found to change dramatically when going away from the valley of stability. A redistribution of the dipole strength towards lower excitation energies is observed for neutron-rich nuclei, which partly might be due to a new collective excitation mode related to the neutron-proton asymmetry. Halo nuclei, in particular, show strong dipole transitions to the continuum at the threshold, being directly related to the ground-state properties of the projectile. Finally, an outlook on future experimental prospects is given.