Edward Paul, Paul Nolan and Andy Boston

Band Termination

The generation of angular momentum in atomic nuclei has long been a topical question in nuclear-structure physics. Macroscopically, or classically, deformed nuclei can rotate ever more faster to produce a collective spin. Microscopically, however, quantal effects of the large, yet finite, ensemble of strongly interacting fermions need to be considered. Each nucleon occupies a quantum state, or orbital, with a well-defined angular momentum. Starting with a fully paired (spin 0) doubly magic core, e.g. ¹⁰⁰Sn or ¹⁴⁶Gd, the addition of valence nucleons can potentially only add a finite amount of angular momentum. The lowest-energy (low-spin) state involves pairwise, time-reversed (I = 0) occupation of specific orbitals. A combination of Coriolis and centrifugal forces, induced by rapid rotation, can however break the valence pairs and align the individual nucleonic angular momentum along the collective rotation axis, in accordance with the Pauli exclusion principle. As more pairs are broken, the nucleons contribute more single-particle spin to the total angular momentum. These nucleons move in equatorial orbits giving the nucleus an oblate appearance. Eventually the available spin is exhausted. Hence, a given nucleus, or rather a specific nucleonic configuration, can only accommodate a limiting value of angular momentum. Experimentally, this effect is seen in gamma-ray spectra as "band termination" in which the regular behaviour breaks down at high spin.

Band termination occurs in many regions of the Segre chart with ¹⁵⁸Er as a classic textbook example. Here band termination is seen at spin 46 hbar. In addition, a new type of "smooth" termination has recently been observed in the A ~ 110 region, which involves a gradual shape change from prolate to oblate over many units of spin.

Beyond Band Termination

Superdeformation

Many nuclei take up extremely deformed shapes at high angular momentum. Superdeformed shapes have been studied for many years and a lot of their properties can be explained using a simple collective rotational model. Many recent experiments have shown evidence that the detailed behaviour of these deformed structures can only be understood if the properties of the valence particles are understood and included in any model calculations.

In ¹³²Ce a superdeformed band is known with a major to minor axis ratio of 3:2. Using the EUROBALL gamma-ray spectrometer, this band is known experimentally up to a spin close to 70h and, at the highest spins, has properties consistent with those expected for a smoothly terminating band approaching maximal spin. Indeed, this terminating spin is calculated to be 78h. An experiment will be carried out at GAMMASPHERE to investigate whether this state can be reached and to determine whether the rotational properties of the nuclei are replaced by single particle features.

Superdeformation in heavier, neutron deficient nuclei between polonium and thorium can be investigated using SACRED, which has a high sensitivity to weak decay sequences, such as those within superdeformed bands. Nuclear models that have been successful in predicting the properties of superdeformed nuclei also predict "hyperdeformed" shapes, ie sausage-like nuclei. These shapes are expected to occur when the nucleus is rotating at high speeds.

The unrivalled sensitivity of new arrays of gamma-ray spectrometers such as AGATA will be important in searching for them. The study of such states will allow the interplay between collective and single particle motion to be studied at the extremes of the stress caused by rapid rotation. It is expected that the hyperdeformed states be populated at spins where the nucleus would normally be expected to fission. The study of such states is one of the few possible probes of individual states at extreme angular momentum.

Novel Physics in Odd-Odd Nuclei

The level structures of nuclei with an odd number of both protons and neutrons are very complicated. However, interesting features of bands based on certain nuclear configurations have been found in such nuclei. These include low-spin signature inversion whereby the expected favoured signature component of a band is actually energetically "unfavoured" below some critical spin value, and evidence for a geometrical "handedness" (chirality) in the nucleus.