Radioactive nuclear beams: Present and future

Some results of investigations into a new nuclear-physics field associated with the production of radioactive nuclear beams and physical studies with these beams are presented. The most recent results obtained by studying the structure of nuclei and reaction mechanisms with radioactive nuclear beams are surveyed. Data obtained in Dubna at the DRIBs accelerator complex are presented along with allied results from other research centers. In this connection, existing experimental data on light loosely bound exotic nuclei are discussed. The parameters of DRIBs3, which is a new accelerator complex, are presented, and the physics research program that will be implemented with the aid of new setups, including a high-resolution magnetic analyzer (MAVR) and a 4π neutron detector (TETRA), is described. A collaboration in the realms of employing radioactive nuclear beams at the DRIBs complex together with other accelerator complexes [SPIRAL2 (France), RIKEN (Japan), FAIR (Germany), and RIBF (CIIIA)] on the basis of employing new highly efficient experimental facilities has already led to the discovery of new phenomena in nuclear physics and will make it possible to study in the future new regions of nuclear matter in extreme states.     

The Mass Programme at GANIL Using the CSS2 and CIME Cyclotrons

The mass-measurement programme at GANIL aims to measure the masses of heavy nuclei close to the N=Z line which is the ideal region to study neutron-proton pairing. An original direct time-of-flight mass-measurement method was developed at GANIL which uses the CSS2 cyclotron as a high-resolution spectrometer. The masses of ions of A=68,76,80 and 100 have been measured with a precision of a few 10⁻⁶. Mass measurements will be performed with the new CIME cyclotron of SPIRAL using a similar method based on the measurement of the phase of the accelerated ions for different radio-frequencies. A recently approved experiment will help develop this new technique and aims to measure the mass of ³¹Ar radioactive nuclei with a precision of 10⁻⁶. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of surface ionization and LASER ionization processes using the SOMEIL ion source: application to the Spiral 2 laser ion source development

SPIRAL2 is the new project under construction at GANIL to provide radioactive ion beams to the Nuclear Physics Community and in particular neutron rich ion beams. For the production of condensable radioactive elements, a resonant ionization laser ion source is under development at GANIL. In order to generate the ions of interest with a good selectivity and purity, our group is studying the way to minimize surface ionization process by using refractory materials with low work function as ionizer tube. To do those investigations a dedicated ion source, called SOMEIL (Source Optimisée pour les Mesures d‘Efficacité d‘Ionisation Laser) is used. Numerous types of ionizer tubes made in various materials and geometry are tested. Surface ionization and laser ionization efficiencies can be measured for each of them.     

Status of GISELE: a resonant ionization laser ion source for the production of radioactive ions at GANIL

Laser ion sources are in use extensively to ionize the nuclear reaction products at many on-line radioactive ion beam facilities. They have proven to be reliable and to combine good production efficiencies with chemical selectivity. A Laser ion source is currently under development for the SPIRAL2 project at GANIL. A status update of this project called GISELE is presented.     

Structural evolution along iso-chains of nuclei

The advent of radioactive nuclear beams (RNBs) has led to a rapid growth in the study of exotic nuclei. Already a number of major discoveries have been made. Examples are halo nuclei, mapping the bounds of nuclear existence, assessing the fragility of magicity, producing special nuclei such as ¹⁰⁰Sn and ⁴⁸Ni, and measuring key reaction rates of astrophysic interest. The growth in this field in the next decade will be enormous. With advanced RNB facilities being planned or under construction, more and more exotic nuclei will become accessible. One of the most interesting opportunities will be the study of the evolution of nuclear structure along extended iso-chains of nuclei. A prime example is Ni, where four magic numbers (20, 28, 40, and 50) and five major shells will be accessible. Structural evolution will be discussed from several standpoints, both theoretical and experimental, with emphasis on methods to obtain a maximum of information on new nuclei from the sparse data that will be available at the extremes of accessible N/Z ratios.     

The DESIR facility at SPIRAL2

The DESIR Collaboration proposes the construction of an experimental facility to exploit the low-energy beams from SPIRAL1, SPIRAL2 and S3. The high degree of purity required to push experiments towards the limits of stability will be achieved by the implementation in the SPIRAL2 production building of a high-efficiency RFQ cooler coupled to a high-resolution mass separator. Beams from the low-energy branch of the separator spectrometer S3 and from SPIRAL1 will allow complementary studies of refractory elements produced by means of fusion reactions as well as of light and intense exotic beams, respectively.     

Nuclear structure studies on halo nuclei by direct reactions with radioactive beams

The investigation of direct reactions with exotic beams in inverse kinematics gives access to a wide field of nuclear structure studies in the region far off stability. The basic concept and the methods involved are briefly discussed. The present contribution will focus on the investigation of light neutron-rich halo nuclei. Such nuclei reveal a new type of nuclear structure, namely an extended neutron distribution surrounding a nuclear core. An overview on this phenomenon, and on the various methods which gave first evidence and qualitative confirmation of our present picture of halo nuclei, is given. To obtain more quantitative information on the radial shape of halo nuclei, elastic proton scattering on neutron-rich light nuclei at intermediate energies was recently investigated for the first time. This method is demonstrated to be an effective means for studying the nuclear matter distributions of such nuclei. The results on the nuclear matter radii of ⁶He and ⁸He, the deduced nuclear matter density distributions, and the significance of the data on the halo structure is discussed. The present data allow also a sensitive test of theoretical model calculations on the structure of neutron-rich helium isotopes. A few examples are presented. The investigation of few-nucleon transfer reactions in inverse kinematics may provide new and complementary information on nuclear structure, as well as astrophysical questions. The physics motivation and the experimental concept for such experiments, to be performed due to momentum matching reasons at low incident energies around 5–20 MeV/u at the new generation low energy radioactive beam facilities SPIRAL, PIAFE, etc., is briefly discussed.     

Study of 19Na at SPIRAL

The excitation function for the elastic-scattering reaction p( ¹⁸Ne, p) ¹⁸Ne was measured with the first radioactive beam from the SPIRAL facility at the GANIL laboratory and with a solid cryogenic hydrogen target. Several broad resonances have been observed, corresponding to new excited states in the unbound nucleus ¹⁹Na. In addition, two-proton emission events have been identified and are discussed.     

Evolution of structure in exotic nuclei

Nuclear structure in exotic nuclei is likely to be quite different than in the nuclei we have been able to access near the valley of stability. With the development of advanced radioactive-beam facilities, we will, for the first time, have access to long isotopic chains of nuclei. A key facet of exotic nuclei therefore will be the study of the evolution of structure with N, Z, and A to an extent never before imagined. Yet, the beam intensities will be very low (relative to stable beams); hence, we will need to extract more physics from less data. We will discuss several aspects of structural evolution and its elucidation in exotic nuclei.     

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     

在放射性束装置上进行新核素鉴别及其衰变性质的研究

综述了近年来在放射束装置上进行新核素鉴别及其衰变性质研究取得的一些成果, 并简要介绍了即将在兰州放射性束装置上开展的奇异核衰变性质的研究工作． The progresses of new isotope identification and decay property studies on radioactive nuclear beam facilities are reviewed. Studies of the decay properties of exotic nuclei that will be performed at RIBLL are briefly introduced.     

Weak Interaction Studies Using a Paul Trap

New developments in ion cooling and ion bunching allow the trapping of radioactive ions from a low energy beam, in a very short time scale and with very good efficiency. The performances of Radio Frequency Quadrupole (RFQ) coolers, recently developed in several laboratories in Europe and USA, are now better known. In the present study, a RFQ device will be used to inject ⁶He⁺ ions in a transparent Paul trap. The ion beam will be delivered by the SPIRAL facility at GANIL. The careful measurement of the β-recoil ion coincidence spectrum is sensitive to the angular correlation parameter a, which depends on the coupling constants of the weak hamiltonian. It should be equal to −1/3 if the interaction is only of axial vector type (V-A theory). Deviation from this value would imply a new tensor-like interaction involving a new exchange boson thus introducing new physics beyond the Standard Model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Proton scattering from unstable nuclei

Recent improvements in the intensities and optical qualities of radioactive beams have made possible the study of elastic and inelastic proton scattering on unstable nuclei. The design and performances of an innovative silicon strip detector array devoted to such experiments are described. The quality of the data obtained are illustrated with recent results obtained at the GANIL facility for unstable oxygen, sulfur and argon isotopes. Methods to analyse the data using phenomenological and microscopic optical model potentials are discussed.     

Methods for organizing the interaction of circulating particle beams with internal targets in nuclear physics experiments at synchrotrons and storage rings

The first methods for using internal targets and their further development for organization of nuclear physics experiments at synchrotrons and storage rings are reviewed. With these methods, new results are obtained and discoveries are made in physics of interactions of elementary particles and nuclei. Current uses of various internal targets and circulating particle beams in ongoing and projected research in particle physics, relativistic nuclear and spin physics, and physics of interactions of exotic and radioactive nuclei and for producing fluxes of secondary particles (mesons, neutrons, exotic and radioactive nuclei, etc.) for physics experiments are described.     

Challenge towards muonic atom formation with unstable nuclei

X-ray spectroscopy of muonic atoms is an important tool to obtain information on the nuclear charge distribution of nuclei. It has been successfully used for many years to study stable isotopes in condensed or gaseous states. A new method has been proposed to extend muonic atom X-ray spectroscopy to the use of radioactive isotope beams to form muonic atoms with unstable nuclei. This new method allows studies of the nuclear properties and nuclear sizes of unstable atoms by means of the muonic X-ray method at facilities where both negative muon and radioactive nuclear beams would be available. Progress of a feasibility study at RIKEN-RAL muon facility is also reported. Received: 1 May 2001 / Accepted: 4 December 2001     

In-beam gamma-ray spectroscopy with exotic beams at the NSCL

Projectile fragmentation provides radioactive beams at intermediate velocities (v/c = 0.3-0.5) by physical means of fragment separation. With the development of position-sensitive photon detectors it has become possible to measure the energies and directions of photons emitted in-flight from such fast-moving exotic beams. This allows the reconstruction of the photons' energies emitted from an exotic projectile with high accuracy. It can be advantageous to employ photon detection in experiments with exotic beams since photons can traverse matter easily and their attenuation can be calculated. Experiments with standard luminosities can be carried out at intermediate beam energies with thick secondary targets (order of g/cm²) and very low incident beam rates (order of particle/s or less). Experimental success in this field is strongly correlated with the development of photon detectors such as position-sensitive scintillation detectors or segmented germanium detectors. In-beam gamma-ray spectroscopy of fast exotic beams has been successfully used at all projectile fragmentation facilities in intermediate-energy heavy-ion inelastic scattering experiments, knockout reactions and fragmentation reactions. Here, we focus on experimental results for neutron-rich exotic nuclei in the π(sd )-shell. Measurements and detector developments carried out at the NSCL at Michigan State University during the last four years are discussed. Received: 1 May 2001 / Accepted: 4 December 2001     

A new cross-section measurement of reactions induced by <Superscript>3</Superscript>He particles on a carbon target

The production of intense beams of light radioactive nuclei can be achieved at the SPIRAL2 facility using intense stable beams accelerated by the driver accelerator and impinging on light targets. The isotope ¹⁴O is identified to be of high interest for future experiments. The excitation function of the production reaction ¹²C(³He, n)¹⁴O was measured between 7 and 35MeV. Results are compared with literature data. As an additional result, we report the first cross-section measurement for the ¹²C(³He,a \alpha + n)¹⁰C reaction. Based on this new result, the potential in-target ¹⁴O yield at SPIRAL2 was estimated: 2.4×10¹¹ pps, for 1mA of ³He at 35MeV. This is a factor 140 higher than the in-target yield at SPIRAL1.     

Hydrogen targets for exotic-nuclei studies developed over the past 10 years

Hydrogen-induced reactions provide essential information on nuclear structure, complementary to other experimental probes. For studies at both low and relativistic incident energy, developments in hydrogen targets have been performed over the past 10 years in parallel with the development of new radioactive beams. We present a review of all major hydrogen target developments related to the study of exotic nuclei with direct reactions in inverse kinematics. Both polarized and non-polarized systems are presented.