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THE SHEARS MECHANISM IN NUCLEI*
Key Words nuclear structure, gamma-ray spectroscopy, shears mechanism, effective forces
Abstract This chapter reviews the experimental properties ofshears bands. The most puzzling characteristic of these structures is the emergence of rotational-like behavior while the nucleus retains a small quadrupole deformation. Regardless of the details of particular theoretical models, it can be shown that the most important degree of freedom in describing the shears mechanism is the shears angle. It is then possible to develop a semiclassical description of the shears mechanism, in which the nature (multipole order) of the interaction between valence protons and neutrons constituting the shears "blades" may be derived and the dynamics of the system described. We discuss the competition between the shears mechanism and collective rotation and mention the connection to "magnetic rotation." Directions for future theoretical and experimental efforts are suggested.
1. ROTATIONS IN QUANTAL SYSTEMS
1.1 A Brief History and Introduction
The rotation of quantum objects has a long and distinguished history in physics. In 1912, the Danish scientist Bjerrum was the first to recognize that the rotation of molecules was quantized (1). In 1938, Teller & Wheeler observed similar features in the spectra of excited nuclei and suggested that they were caused by the rotation of the nucleus (2). A more complete explanation was developed in 1951 when Bohr pointed out that rotation was a consequence of nuclear deformation (3). We owe much of our understanding of nuclear rotation to the work of Bohr & Mottelson who, with Rainwater, shared the 1975 Nobel Prize for Physics for developing a model of nuclei that combined the individual and collective motions of the nucleons (4).
Because a perfect sphere has no preferred direction in space against which a change in orientation can be measured, quantum mechanically it cannot rotate. For a quantum system to rotate, the spherical symmetry in space must be broken. For example, a diatomic molecule can rotate about the axes perpendicular to its axis of symmetry. A three-dimensional quantum mechanical treatment of a diatomic molecule (5) leads to a very simple relationship between rotational energy, E, and...