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Exploring the nuclear landscape

Today, our knowledge about nuclei, is restricted to about 2500 of the potentially existing 6000 combinations of protons and neutrons (see Figure [*]).
 
Figure: The nuclear landscape. Black squares indicate the stable nuclei, the coloured nuclei are the 2500 nuclei synthesised so far. For many of them only their existence is verified today and basic nuclear properties need to be measured. The drip-lines are from the macroscopic-microscopic FRDM model by Möller & Nix.
\begin{figure}\epsfig{file=final/fig1.eps, width=\columnwidth}\end{figure}
 

Depending on the balance between the nuclear force and the Coulomb force, the limitations imposed on this number are the following: the western shore of the chart of nuclei, the proton drip-line, is reached when the binding energy of the last proton becomes zero. Correspondingly, the eastern shore is approached when the binding for the last neutrons vanishes. For the heaviest elements, the disappearance of the fission barrier defines the northern edge. Shell closures are the characteristic landmarks in this continent. In the following, we sketch the recent highlights in the mapping of this landscape including the shores and note that future advances will depend upon a significant increase in the available luminosity.
 

    Nuclei at the drip-lines and around closed shells.

Milestones were reached with the fragment separators at GANIL and GSI. The availability of intense primary beams, made from isotopically enriched material such as 36S, 48Ca, 112Sn... was an essential component of these experiments. In the N=20 region, one surprise was the instability of the semi- and doubly- magic 26O and 28O. Decay measurements in the region around 44S gave the first evidence for an unexpected deformation around N=28, shedding light on the astrophysical isotopic-abundance anomaly in this region. Coulomb excitation experiments with radioactive beams have started to map the deformation in the N=20-28 region. Fission has been the richest source of neutron-rich intermediate-mass nuclei. Historically, the bulk of these nuclei ( $\approx 500$) were made at reactors. Now fission from fast particles has become an important tool with great future potential. It is employed for fissioning targets at ISOL-type separators and for fissioning projectiles at fragment separators. The discovery of the doubly closed-shell nucleus 78Ni, Z=28, N=50, in the fission of 750 MeV/u 238U projectiles is an important landmark. Nuclear properties in this region are relevant to the understanding of the astrophysical r-process path. More than 100 new neutron-rich nuclides were identified from 62V to 160Nd, covering nearly 40 atomic and 100 mass units. Highlights from the ISOL technique are the first synthesis of Ag and Cd isotopes located on the r-process path and the study of single particle states around 132Sn, Z=50 and N=82. Very neutron-rich neutron emitters of refractory elements are now accessible by the IGISOL (=Ion Guide ISOL) technique. The fragment separators have also been very successful on the western shore of the nuclear chart. Precision spectroscopy of 37Ca and 40Ti has now become possible and will be of prime importance for the calibration of the Cl- and Ar- based solar neutrino detectors. Studies of nuclei near or at the proton drip-line ranging from 22Si to 100Sn have become possible. The new nuclei 45Fe and 49Ni are the most proton-rich nuclei ever produced. The long awaited synthesis of the heaviest self-conjugate Z=N=50 100Sn paves the way for future experiments. Studies of rp-process relevant nuclei close to the proton drip-line are successfully started. Fusion-evaporation reactions will continue to be an important tool in studies of proton-rich isotopes. This is e.g. shown by the 100Sn mass measurement and ISOL experiments where the heaviest known N=Z, odd-odd nucleus 94Ag was identified. For Z=82, a large number of new isotopes close to the proton drip-line were detected with recoil separator techniques which also allow the investigation of the fission properties of hitherto inaccessible species. Close to N=126 ISOL techniques allowed the study of 215Pb and 217Bi. To probe the contributions of closed shells in a fissioning nucleus, e.g. the N=162 neutron shell, and in nascent fragments, e.g. at Z=50 and N=82, even more neutron-rich isotopes beyond those presently known are needed for the heavy actinides and transactinides. Fusion-evaporation and multinucleon-transfer reactions with radioactive beams will provide the only access to this neutron-rich region. European laboratories active in the use of the ISOL technique are CERN, GANIL, GSI, Jyväskylä, Louvain-la-Neuve, and Studsvik.
 

    The heaviest nuclei, synthesis and chemical properties

The discovery of elements 110, 111, and 112 with SHIP at GSI was certainly the outstanding highlights at the top end of the Periodic Table. The results of these fusion experiments, have clearly demonstrated the existence of shell-stabilised nuclei. Very good agreement between these experimental results and theoretical calculations indicate that this region of, presumably deformed, superheavy elements will extend into the island of spherical superheavy elements around element 114 and neutron-number N=178-184. Recent calculations emphasise the importance of the presumably stronger Z=126 proton-shell. A number of different approaches to obtain access to this region of rapidly diminishing cross sections can be envisaged. To find the optimum path towards these superheavy elements, a much better understanding is needed of all nuclear reactions leading to the synthesis of the heaviest elements. ``Hot fusion'' reactions with light beams on actinide targets are studied at Dubna and GSI and have produced the most neutron-rich isotopes of element 106, 108 and 110. Fast chemical separation allowed half-life measurements in the range of seconds for Z=106, 265Sg and 266Sg. At what point nuclear stability may finally limit the existence of chemical elements remains an open question, and hence presents a challenge to experiment and theory. The most important immediate goal is to reach the island of spherical superheavy elements at $Z \cong 114$ and N=178-184. However, no target-projectile combination of stable isotopes will directly lead to the centre of the island in a fusion-evaporation reaction. The long-term perspective is the use of intense neutron-rich radioactive beams, mainly in combination with neutron-rich radioactive targets, to produce the most neutron-rich nuclei of heavy actinides, transactinides, and, ultimately, superheavy elements. With cross sections presumably ranging from fractions of a nanobarn to less than one picobarn, it is not obvious that intensity-limited radioactive beams may allow direct access to this region in the near future. However, nuclear reaction studies with radioactive beams can provide essential clues about how to proceed. Studies of the influence of just one important parameter at a time, like the fissility or the neutron-excess, are expected to shed much more light on the open questions about these mechanisms. Recent studies have rejuvenated the entire field of chemical research of the transactinide elements which have now reached Seaborgium, Z=106. It is the goal of these fast chemistry experiments to map out the architecture of the Periodic Table at its upper end. Increasing deviations from the periodicity of chemical properties have been predicted as a consequence of the increasingly strong Coulomb field of the highly-charged atomic nucleus (``relativistic effect''). For example, the non-Ta-like behaviour of Dubnium (Z=105), and the contradicting behaviour of Seaborgium (Z=106) which behaves like its lighter homologues, have demonstrated that the chemical properties of the heaviest elements cannot be reliably extrapolated from the trends observed in the lighter homologues. Detailed nuclear and atomic spectroscopy on radioactive beams can significantly increase our knowledge of the heaviest elements. Storage of these beams in traps for further investigations, e.g. optical spectroscopy with modern laser techniques, would provide another big leap forward, but they also are an ultimate experimental challenge. Perhaps, chemical research may even approach the elusive island of spherical superheavy nuclei around Z=114 and N=178 where theory predicts long half-lives. The key issue is the intensity of next generation radioactive beam facilities.
 

    Interplay between Nuclear Structure and Nuclear Reactions

Many of the novel features of exotic nuclei are explored in reactions induced by radioactive beams. The analysis of the experiments poses important questions with regard to the reaction mechanisms with which the new degrees of freedom are excited. Of special importance for these studies are reactions involving elastic scattering, inelastic excitation and transfer reactions involving one or more nucleons. In addition, fusion and deep-inelastic reactions may provide new insights to pave the paths to the production of superheavy elements and to get access to more neutron-rich isotopes. Furthermore, fission provides ways to produce a wide variety of neutron rich nuclei. Elastic and inelastic scattering induced by radioactive beams give information about the interaction potentials and the spatial extension of the projectile; they may also reveal directly the predicted existence of a neutron skin. For this purpose, it will also be interesting to study the angular distributions from inelastic reactions, since, via the nuclear-Coulomb interference, information about the mean square radius of neutron rN2 and proton rP2 densities may be obtained. Of particular interest in this field is the study of the energy dependence of the optical potential at energies close to the Coulomb barrier. It contains information about the open channels of the system under investigation. The presence of collective strength at low excitation energy, strongly excited by the Coulomb field, may give rise to large polarisations. This will considerably alter the shape and strength of both the real and imaginary parts of the optical potential over large distances. Because of the many open channels at low excitation energy and of the importance of the continuum, the standard analysis in terms of optical potentials and distorted wave Born approximation (DWBA) is not very suitable. To address these problems the use of complex coupled-channels codes, where elastic, inelastic and possibly transfer channels are treated on the same footing, may be needed. Coulomb excitation provides a direct and clean way to obtain information on the properties of nuclei in their ground state and low-lying excited states. Coulomb excitation of the ions of the radioactive beams allows the study of the development of deformation in the vicinity of closed shells as has been recently demonstrated at GANIL in measurements of magnesium isotopes up to A=32. Future exotic beams, obtained through post-acceleration with energies around the Coulomb barrier will allow for precision spectroscopy. The experimental information on giant resonances in exotic nuclei is at present marginal. Nevertheless they can be studied in inverse kinematics with secondary beams. Other standard tools for giant resonance studies, i.e. hadronic probes like (p,p'), ($\alpha$,$\alpha$') or (p,n) reactions can be adopted in principle, but they require new developments. Last but not least, inelastic electron scattering is a challenge, as it would require an ion-electron collider, but it would provide a powerful probe. One-nucleon transfer reactions are very good tools for the investigation of the nature of the single-particle level structure of interacting nuclei, while two-particle transfer can give information about pairing-correlations and thus provides insight into the nature of the pairing-interaction in the surface region.

 
Figure: Predicted cross sections for thallium and mercury isotopes by bombarding a 208Pb target with a stable 136Xe beam (yellow) or a neutron-rich 144Xe beam (red).
\begin{figure}\epsfig{file=final/fig2.eps, width=\columnwidth}\end{figure}
 

The multinucleon transfer process plays a very important role, both as a doorway mechanism towards deep-inelastic and fusion reactions, and may also prove to be one of the mechanisms for the production of very neutron rich nuclei which cannot be produced in the standard fusion-evaporation reactions, see [*]. Fusion reactions are the ideal tool for the production of superheavy elements, and the use of radioactive beams could substantially increase these possibilities. It is still an open question whether the presence of a neutron skin and of collective strength at low excitation energy could enhance considerably the yields for these reactions at energies below the Coulomb barrier or could be counterproductive. Fusion reactions, of course, must not be pursued only for the quest of superheavy elements but are very important in their own right. The outcome of fusion reactions is strongly influenced by the coupling to inelastic scattering and transfer channels. Studies of the energy dependence of these reactions, especially at energies close to the Coulomb barrier, can thus provide complementary information on the properties of these degrees of freedom in the new exotic nuclei. Bi- or even multi-modal fission modes have been observed in the spontaneous fission of the most neutron-rich heavy actinides and light transactinides. Similar features have been seen in the fission of actinides at low excitation energy ( $E^\ast \cong$ 12 MeV) resulting from the excitation and decay of the giant dipole resonance (GDR); this opens a new field of studies with very interesting prospects. At GSI, cross sections were measured for the electromagnetic fission of a 238U target bombarded with 100 to 1000 MeV/u 208Pb beams and similarly for 600 to 1000 MeV/u 238U beams interacting with a variety of targets between Be and Pb. Cross sections of about 2 barn are reported for the heavy system at 1000 MeV/u. Secondary beams of radioactive, fissile heavy nuclei were produced by the fragmentation of a relativistic 238U-beam at the FRS, and their fission was studied in-flight in a secondary Pb target. These data give access to the fission barriers of highly fissile nuclei in the region of the N=126 neutron-shell and they are important for determining the end point of the astrophysical r-process and for the $\beta$-delayed fission process. For neutron-rich isotopes far off stability, our knowledge of isomeric states is scarce. By means of delayed $\gamma$-ray correlation techniques, it has recently been possible at GANIL to identify more than 20 new, neutron-rich isomers in the fragmentation of 86Kr. Such fragmentation reactions will permit, for the SPIRAL project, post-accelerated isomeric beams. Another method for producing such beams could be through the excitation of the GDR with relativistic actinide beams on high Z-targets followed by neutron emission to fission isomers. The availability of isomeric beams will also be of interest for nuclear astrophysics. The radioactive beams which are presently produced by the fragmentation facilities, or will be produced by the first generation ISOL+post-acceleration facilities, will already allow us to carry out experiments for a wide range of exotic nuclei, in particular for elastic and inelastic scattering and Coulomb excitation. Extension of these experiments to even more exotic nuclei will require radioactive beams of substantially increased intensities with emphasis on neutron-rich species for fusion-evaporation reactions at energies around the Coulomb barrier. 


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