The Nuclear Response to  Nuclei far from Stability  Ground state properties

Spectroscopy of exotic nuclei

Ground state proton decay clearly establishes the location of the proton drip-line. In addition, sensitivity to the orbital angular momentum of the unpaired proton can be used to assign shell structure even beyond the drip-line. So far proton radioactivity has been observed from odd-Z nuclei in the two regions from Z=51-55 and Z=69-83. Studies of new examples from the ``missing region'' of light rare earth nuclei will be particularly interesting since this is where large deformation effects are expected. Another form of proton radioactivity -- two proton radioactivity -- has yet to be discovered but may possibly be observed from even-Z nuclei bound to single proton emission. It is of interest for the possible observation of ²He clusters but will require increased yields of exotic nuclei since the drip-line for even-Z nuclei lies further from stability. Exotic ions detected in a recoil spectrometer subsequently decaying by proton or alpha-emission can be used to tag the prompt gamma-rays generated in the reaction which produces such nuclei. This powerful new experimental method, developed at Daresbury, will enable detailed structure investigations to be undertaken at the proton drip-line, as exemplified by a recent study on light Po isotopes at the gas-filled recoil separator RITU at Jyväskylä. Self-conjugate nuclei with N=Z are of great interest due to the high degree of symmetry displayed between the proton and neutron degrees of freedom which provides a unique way to study the effective n-p interaction. For A<40, such nuclei are $\beta$-stable, whereas for larger A, the Coulomb interaction drives the beta-stability line towards neutron-rich isotopes. Correspondingly, the N=Z nuclei become increasingly unstable as the proton drip-line is approached. These nuclei exhibit a rich variety of phenomena, such as spherical - prolate - oblate shape coexistence, superdeformation, alignment of proton-neutron pairs and discrete line proton decay from highly excited states, with rapid changes of structure from one nucleus to the next providing a stringent testing ground for theoretical models. The doubly magic N=Z nuclei ⁵⁶Ni and ¹⁰⁰Sn are fixpoints, where the prerequisites of any theoretical description, single particle energies and residual interaction, can be determined. In order to give one example, the nuclear binding energies are very sensitive to the residual interactions and correlations between weakly bound nucleons. In addition, the gamma-ray decay properties of self conjugate nuclei and Fermi and Gamov-Teller decay provide important information concerning the purity of isospin symmetry. As an example of work on mirror nuclei, a recent experiment concerning the mirror pair ⁴⁹Mn, ⁴⁹Cr shows a correlation between the alignment of the nucleons and the Coulomb energy differences between excited states. Also important are the precision beta-decay studies concerning the distribution of Gamov-Teller strength in the vicinity of ¹⁰⁰Sn. Studies at GSI and ISOLDE have concentrated in particular on the structure of Cd, In and Sn nuclei in the range N=50 to 56. Very promising is recent work at the GSI on-line mass separator, based on the combination of higher efficiency Ge detector arrays, such as the cluster cube, and total absorption gamma ray spectroscopy. These data show that the missing strength in Gamov-Teller beta-decay lies at higher energies than previously thought, and clearly indicate the Gamov-Teller resonance in beta-decay of heavy nuclei ($A\approx$ 100-150) for the first time. Improved sensitivity and detection efficiency will allow us to exploit the large decay energy windows in beta decay far from stability to the full. Such studies can provide insight into the structure of halos, or information on highly excited states which subsequently undergo exotic multinucleonic decay processes. Alpha and beta decay studies regain prime importance for studying nuclei near the drip-line. In addition, for proton-rich nuclei direct emission of protons may become possible. Alpha decay studies of nuclei near the Z=82 and N=Z=50 shell closures provide detailed information, e.g. concerning shape coexistence and the onset of the double-shell closure. Decay spectroscopy of neutron-rich nuclei will play an increasingly important role in providing a deeper understanding of highly asymmetric nuclear matter, e.g. testing the large scale shell model and other microscopic calculations for selected n-rich nuclei or by focusing on the evolution of nuclear structure between extreme deformations and double shell closures.