6. 3  Shapes of Atomic Nuclei by High-Energy Ion Beams
- Complete Spectroscopy Experiments with Coulomb Excitations
 


Fig. 6-5 Nuclear internal motion and deformation

When the number of nucleons (protons and neutrons) of an atomic nucleus is close to certain numbers ("magic numbers"), independent particle motion becomes dominant. Nuclei not close to those numbers can have collective rotation or vibration. Transitional nuclei can have various shapes, from spherical to highly deformed ones.


Fig. 6-6 "Complete" spectroscopy experiments at a tandem accelerator

A heavy-ion beam is directed at a target, which is placed at the center of the gamma-ray detector array "GEMINI. " The resulting scattered particles and gamma-rays are then measured in coincidence. This comprehensive body of information is used to determine the gamma-ray transition probabilities for all the low-lying excited states.


Fig. 6-7 Deformation of excited states in 70,72,74,76Ge

The solid lines show excited state energy levels plotted for
a series of Ge isotopes. Angular momentum quantum numbers are shown at both sides; parities are positive. The states with similar internal structure are connected by dashed lines. All the states are classified as either spherical (red) or deformed (blue). Different deformations may exist in the same nucleus, a phenomenon which is called shape coexistence.


The size of the atomic nucleus is on the order of 10-12 cm, so that its shape cannot be seen, even with sophisticated optical devices like microscopes. Instead, the shape is probed by means of the level structures of excited states of an assumed nuclear model. But since there is no nuclear model which can describe nuclear structure completely, the interpretation of such data for the shapes of nuclei remains ambiguous.
Recently, we have succeeded in developing a new technique of "complete" (i.e., comprehensive) spectroscopy utilizing Coulomb excitations, which enables us to determine nuclear shapes without relying on nuclear models. A nucleus of interest will be accelerated and impacted onto a lead target, at which point it is excited by a strong Coulomb field produced between the beam and target nuclei. The nuclear excitation process can be analyzed to determine all the electromagnetic properties (transition probabilities and quadrupole moments) for all the low-lying states. These quantities can then be used directly to derive nuclear shape parameters. With this approach we have made a complete spectroscopy experiment on Ge isotopes utilizing the gamma-ray detector array "GEMINI" at the tandem accelerator in Tokai. As a result we have been able to derive the shape parameters of all excited states up to an energy of 2 MeV.
The results show that there are different shapes coexisting in the same nucleus, and that the isotope dependence of the level structures is found to be related to shape-dependent behavior of the various states.


Reference
Y. Toh* et al., Coulomb Excitation of 74Ge Beam, Eur. Phys. J. A9, 353 (2000). *post doctoral fellow

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Persistent Quest - Research Activities 2001
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