John Ennis

For more than thirty years, the structure of nuclei has been understood within the frameworks of two superficially quite different models. The independent particle shell model is characterized by a spherical equilibrium shape and independent motion of constituent nucleons in the mean field created by all others present. The nuclear collective model is described in terms of spheroidal equilibrium shapes and of vibrations and rotations of these shapes. Quadrupole deformations dominate the nuclear problem, reflecting the large energy required to separate the nuclear centroids of charge and mass to give rise to a dipole deformation.

It has also long been recognized that the special symmetry and stability of the alpha particle gives it enhanced freedom to participate in so-called cluster configurations, wherein the nuclear many body system spontaneously resolves itself into a small number of well defined clusters which then experience relatively simple relative motion. The very observation of natural alpha particle radioactivity suggests preformation of alpha particle clusters in the heavy radioactive nuclei.

While cluster models have had by far their greatest use, apart from natural radioactivity, in light nuclei in the region from (‘8)Be to (‘40)Ca, Iachello and Jackson recently suggested that even in heavy nuclei, just above closed shells, four valence nucleons could form a valence alpha particle cluster orbiting the remaining core. In heavy nuclei, as opposed to light, reflecting the neutron excess (N > Z), such cluster separation results in a separation of the centers of charge and mass and thus generate a static electric dipole moment.

We have undertaken a detailed experimental study of this question using a wide range of techniques and focussing upon the (‘218)Ra nucleus. Using a (‘208)Pb target and a (‘13)C beam at Yale and the reverse reaction at GSI, we have identified the quadrupole and dipole band members in the level spectrum of this nucleus. From measurements of the absolute lifetimes of many states ranging up to that having J = 15(H/2PI), we have shown that the El electromagnetic deexcitation matrix elements are indeed enhanced and that they exhaust as much as 15% of a molecular sum rule appropriate for these cluster configurations.

Having measured many of the pertinent parameters, we have shown that the Iachello-Jackson dipole model can reproduce what we have found in (‘218)Ra. Our new data also provides a stringent test of the spectrum generating algebraic and other approaches to this understanding.