Our studies of drip-line nuclei focus on breakup reactions induced by the Coulomb and nuclear fields from a target nucleus. A critical issue is to develop a realistic description of breakup mechanisms as a necessary tool for extracting or testing the nuclear structure properties of drip-line nuclei. An example of particular interest to solar neutrino physics is the low-lying E1 strength of 8B which determines the radiative proton capture on 7Be in the sun. We have developed a single-particle model which reproduces the low-lying dipole strength of 8B extracted in recent Coulomb dissociation experiments. We have tested the model against other observables that are sensitive to the size of the valence proton state. Thus we find that the model is consistent with the measured cross sections of the predominantly nuclear-induced breakup on a carbon target.
We have extended our studies of drip-line nuclei to include the structure and decay of deformed proton emitters. This is described in a coupled-channels treatment of the particle-rotor model, supplemented with a Green's function technique. The calculation of decay rates is commonly performed by using complex energies. This is a difficult task because the decay widths of interest are extremely small. However, by employing the Green's function method it is sufficient to solve the coupled equations with an energy that is real. The results we obtain for the decay of low-spin states are quite encouraging in comparison to measurements. The decay from high-spin states, on the other hand, is much more difficult to predict. It is influenced by the Coriolis force, which is too strong without the effect of pairing.
Our studies of superdeformed nuclei, at both low and high spins, address the issues of possible new regions of superdeformation and hyperdeformation. Special emphasis is being put on the study of fission barriers at high spin, and the relation between fission barriers and the possibility of producing very extended nuclear shapes. Other areas of interest are the structure of heavy elements and superheavy elements, density dependence of two-body interactions, and the phenomenon of proton radioactivity. The techniques we use to study these problems are a deformed central potential approach for surveying nuclear structure over a large region, self-consistent mean-field calculations for more detailed studies of particular nuclides, and many-body wave functions when residual interaction effects are small and a mean-field approach is inadequate.
Much of our work is computer intensive, and we are adapting our codes to exploit the massively-parallel supercomputer systems at Argonne. Their use allows us to calculate energy surfaces as a function of angular momentum, using the Strutinsky method with cranking, in a four-dimensional space consisting of quadrupole, octupole, hexadecapole and necking degrees of freedom. We carried out studies of nuclear energy surfaces in nuclei with masses ranging from A~80 to A~200. We are analyzing these calculations to look for nuclei that are good candidates for experimental investigation of superdeformation. Our recent analysis of energy surfaces near A=100 suggests that very extended minima in several nuclides near 108Cd are experimentally accessible. Experimental studies motivated by these calculations have led to the observation of a superdeformed band in 108Cd. We have adapted our many-body code for parallel computer systems and have modified it so as to allow the use of general two-body matrix elements. We are using this code to study the nuclear structure of nuclides near the proton drip-line. A complementary effort, in conjunction with J. L. Egido and L. M. Robledo (Madrid), is also focused in this area. In the latter studies, we are using a Gogny interaction in the HFB approximation. We have completed an analysis of excited states in 256Fm, the heaviest nuclide for which there is extensive spectroscopic information on non-intrinsic states. The pairing force strength obtained from this analysis will improve the reliability of future theoretical studies of superheavy elements.
Nuclear forces and nuclear systems
Theoretical Physics Research
Atomic theory and fundamental quantum mechanics