Spectroscopy of exotic nuclei  Nuclei far from Stability  Exploring the nuclear landscape

Ground state properties

A substantial amount of information exists on the most basic properties of exotic nuclei in the ground state or at low excitation energies. Masses, moments, radii, spins, and decay modes have been studied for a large number of isotopes and are in general well known near stability. Further away our information becomes scarce and at the borderline of known nuclei often only a few decay properties are known. The fact that it is now realized that it is currently impossible to predict the exact limits of stability provides the forum for future exploration of the nuclear landscape (see figure ). The employment of new techniques and the efforts devoted to the development of radioactive ion beams provide the possibility of extending experimental studies to more and more exotic regions, and to obtain detailed information on nuclei presently known only to exist.
 

    Nuclear Masses

Masses contain basic information about nuclear structure: (i) Deviations of binding energies from the smooth trend given by a liquid-drop model directly reveal nuclear shell structure. (ii) Mass differences between odd and even nuclei show the strength of the pairing force. (iii) Departures from a smooth trend of the two-nucleon separation energies between shell closures reveal the onset of strong ground state deformations. New accurate experimental mass data are essential for testing and improving theoretical models and will yield more reliable predictions of experimentally inaccessible isotopes, e.g. r-process path nuclei. For a general mapping, experimental accuracies of 100 keV and less are required. In some specific cases such as halo nuclei or nuclei used to test the conserved vector current hypothesis, accuracies better than 10 keV are mandatory. Very far from stability Q-value measurements obtained from a reaction or a radioactive decay are often the first and only source of information on nuclear binding. An extension of the studies of halo nuclei towards heavier systems may be expected from reactions induced by the new radioactive beams. With regard to Q-values from decay studies, outstanding examples are isotopes along the proton drip-line and the new elements 110 to 112.

 

Figure: Divergence of different mass predictions far from stability, underlining the need for mass measurements on exotic nuclei.

 

Recently, direct measurements on radioactive isotopes have made important contributions. The renaissance of mass spectrometry is due to new time-of-flight and frequency measurement techniques, which are well adapted to radioactive ion beams. Time-of-flight spectrometers like TOFI and SPEG have paved the way to access large regions of nuclei near the drip-lines. Applying a new concept, masses were measured for $A\approx$80 isobars and ¹⁰⁰Sn and the isotopes in its vicinity by means of the SARA and GANIL cyclotrons, respectively. New perspectives for very short-lived species will also arise from time-of-flight mass measurements at storage rings like the ESR. With regard to frequency measurements, it has already been demonstrated (using a Schottky technique at the ESR) that cooler storage rings are a very powerful tool. Mass values of more than 100 neutron-deficient heavy isotopes were determined with an accuracy of $\delta m/m \approx 10^{-6}$ for half-lives as short as 10 s. Unprecedented accuracy, $\delta m/m \approx 10^{-7}$, in direct mass measurements is today achieved with Penning traps. ISOLTRAP has already studied long isotopic chains for alkali, alkali earth and lanthanide elements. This method has the potential to be applied to essentially all nuclei with half-lives as short as 0.1 s. The new radiofrequency transmission spectrometer MISTRAL will provide a complementary technique at ISOL separators, to measure with similar accuracy, isotopes with half-lives down to milliseconds.
 

    Charge Radii and Moments

The determination of nuclear charge radii and electromagnetic moments has been a domain of atomic spectroscopy at ISOL facilities via the measurement of the isotopic shifts and the hyperfine structure in optical transitions. A variety of newly developed laser spectroscopy techniques gave access to long isotopic chains for a large number of elements covering, in certain regions, even the isotonic dependence of nuclear radii and moments. At ISOLDE, a novel detection scheme has allowed a systematic mapping of the radii around N=20 and N=28 by isotopic shift measurements on Ar, K, Ca nuclei. These data serve as an important test of shell model predictions for the sd and fp shell. Optical polarisation, combined with $\beta$-NMR provided access to the electric quadrupole moments of neutron rich sodium isotopes. The N=20 isotope ³¹Na is now within immediate reach. For neutron deficient Sr and Kr nuclei, new isotope shift data are interpreted in terms of a core polarisation effect that drives the odd isotopes into a strong stable deformation. Recent investigations for refractory elements by laser-desorbed resonance ionisation spectroscopy have allowed the measurement of the isomeric and ground-state nuclear deformation of ^184m,gAu. These promising developments at ISOLDE will be complemented by the laser spectroscopy program at IGISOL where refractory fission products are of particular interest. Examples of laser spectroscopy applied to extreme nuclear states are the determination of spin, magnetic moment and deformation of the fission isomer ^242fAm and the study of the high spin isomers ^178mHf and ^177mLu. In the future, optical spectroscopy of radioactive isotopes in magneto-optical traps may provide the means for atomic parity violation experiments. In addition to the determination of radii and moments via atomic spectroscopy, nuclear orientation techniques provide complementary ways to obtain nuclear structure information. Besides optical pumping, experiments involving orientation at ISOL facilities have been achieved by developing implantation into low-temperature environments. In combination with moderate post-acceleration, the tilted foil method is thought to have a very important potential in the future. At higher energy, orientation is provided by the projectile fragmentation reaction itself. Pioneered in Japan for light neutron-rich nuclei, these promising first experiments have been started at other fragmentation facilities.
 

    Halos and Skins

The halo structure, with its long tails in the density distribution and the strong clustering, has spurred a great deal of activity during the last few years. A better theoretical understanding has been obtained through work on few-body correlations and the halo degrees of freedom. Halo nuclei behave differently from normal nuclei both in beta decays and in essentially all types of nuclear reactions that have been attempted with them so far. They break up easily, and such processes are understood for one-neutron halos like ¹¹Be, and progress is being made for two-neutron halos like ¹¹Li. Open questions at the moment include how large the neutron-neutron correlations are for well-developed halos and how the structure develops as the drip-lines are approached. In addition, there is a real need to extend the understanding already obtained for nuclei with A=11 to other halo systems. Almost nothing is known about heavier halo-systems although several candidates exist and there are encouraging results on ¹⁹C.

 

Figure: Presently studied halo nuclei.

 

To a large extent recent progress came from reaction experiments at 30-1000 MeV/u. Reaction cross sections and transverse momentum distributions, which were measured first are now complemented by new techniques, like complete kinematics experiments, longitudinal momentum distribution measurements via the use of the fragment separators as energy-loss spectrometers, Coulomb excitation and elastic scattering. There have been several searches for proton halos in nuclei such as ⁸B, ¹⁷Ne and ²⁰Mg, but the only reasonably well established case at present is the first excited state in ¹⁷F. To answer open questions such as the interpretation of the existing conflicting data on ⁸B or the problem of the role of NN pairing in the low density halo environment, new ways of probing are needed. One such probe is beta decay, another probe is reaction experiments at lower energy. As an example, transfer reactions could help in elucidating the exact structure of the neutron configuration in the ¹¹Li ground-state. The experiments in this energy range will be cleaner and thus easier to interpret, but will also put greater demands on beam quality and intensity since thinner targets are a must. The gradual occurrence of a neutron skin as a function of the Fermi-energy differences was seen at GSI for the Na isotopes by means of an interaction cross-section measurement at relativistic energies. There is a huge potential for this type of experiment conducted with other nuclei, and the results would complement the data on nuclear charge distributions obtained by laser measurements at ISOL facilities. Work in this direction has already begun and will constitute an important frontier in the coming years. Other progress for light nuclei in the last years has involved either nuclear astrophysics experiments or particle unbound systems, i.e. nuclei beyond the drip-line. Examples of the latter are the systems ¹⁰He, ¹⁰Li, ¹¹N and ¹²O. Their structure must be known if a coherent picture of the nuclear structure in this region is to emerge. Such a picture would be able to address questions like will the parity inversion seen in ¹¹Be show up in the related unbound systems ¹¹N and ¹⁰Li and would be essential for understanding two-neutron halo nuclei since in all known candidates the corresponding ``core plus one neutron'' subsystems are unbound. The unbound nuclei can be created e.g. as part of the final state in break-up reactions of radioactive beams; this method can yield rather clean spectroscopic information as seen from the cases of ¹⁰He and ¹⁰Li. In a similar way excited states in ¹¹Li have been probed. Although developments in detection technology certainly are important, the progress in research on halo and skin systems is driven by the increase in beam intensities for (mainly) very neutron rich light nuclei. The constant progress towards higher intensities makes the future look very promising, in particular for skin nuclei where the relevant degrees of freedom are unknown. The biggest challenge for the field will be to find ways of producing the nuclei close to the neutron drip line above Na. This is a very ambitious challenge for the future.