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.
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 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
for half-lives as short as 10 s. Unprecedented accuracy, ,
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 -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.
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.