Experimental studies of strongly deformed nuclei

Present experimental nuclear research has three main frontiers. The search for anticipated new phenomena requires the study of exotic nuclear states, where either the excitation energy, the deformation or the so-called isospin takes extreme values. The recently developed, highly-efficient Germanium detector arrays have provided great opportunities for the high-resolution studies of nuclear states associated with extremely elongated nuclear shapes. In the mass region of A=130, more than a hundred superdeformed (with a nuclear axis ratio of 2:1) rotational bands have been identified so far. In this mass region, the centrifugal force related to the fast rotating motion is responsible for the large deformation which can be stabilized by strong nuclear shell effects (like in the case of the atomic shell closures in the noble gases). Contrary to the observations of discrete superdeformed (SD) transitions, the identification of discrete γ rays from hyperdeformed (HD) nuclear states, having an axis ratio of 3:1, represents one of the last frontiers of nuclear physics. Although a large community with such arrays have been searching for HD states in very long experiments, no discrete HD states have been identified so far.

In the actinide (Uranium-Plutonium-Thorium) region, however, the appearance of fission resonances gives a unique possibility for the identification and examination of extremely deformed nuclear states. The appearance of a deep and local 2^nd minimum in the potential energy surface (PES) of the nucleus at large quadrupole deformations (β₂=0.6) is a typical feature of the actinide nuclei. It can be described within the macroscopic-microscopic theoretical framework, in which the liquid drop deformation energy of the fissioning nucleus is superposed by periodically varying shell corrections. The ground state of the 2^nd potential minimum is called the fission isomeric state. So far 33 fission isomers have been identified in the region between U (Z=92) and Bk (Z=97) (''island of fission isomers'') having a decay lifetime in the ps-ms range [2].

Soon after the double-humped fission barrier concept was well-established having two minima in the PES, unexpected features of the fission cross-sections of the light actinides (called ''Thorium-anomaly'') pointed to extend the existing picture and introduce the triple-humped fission barrier concept with a shallow 3^rd (hyperdeformed) minimum in the PES at very large quadrupole and large octupole deformations. Almost two decades later we found experimentally in several measurements on ²³⁴U, on ²³⁶U and most recently on ²³²U, that the 3^rd minimum is in fact as deep as the 2^nd minimum for the uranium isotopes.

Our experimental approach to investigate extremely deformed nuclear states of the light actinides is based on the observation of resonances in the fission cross section. Resonances appear when states in the 1^st minimum energetically coincide with (β vibrational) states in the 2^nd (SD) or in the 3^rd (HD) potential minimum. Observing resonances as a function of the excitation energy caused by resonant tunnelling through excited states in the 3^rd minimum of the potential barrier, allows us to identify the excitation energies of the HD states. Moreover, the observed states can be ordered into rotational bands, with moments of inertia proving that the underlying nuclear shape of these states is indeed a HD configuration. For the identification of the rotational bands, the spins and their projections onto the nuclear symmetry axis (K values) can be obtained by measuring the angular distribution of the fission fragments. Furthermore, by analysing the overall structure of the fission cross section and by fitting it with nuclear reaction code calculations, the fission barrier parameters can be extracted very precisely.