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A deformed nucleus can increase its spin or angular momentum by collective rotation. However, because the nucleus is a many-body quantal system, such regular rotational motion must have an underlying microscopic basis, with the angular momentum being generated by small contributions from a sizeable number of valence nucleons. Since the number of valence particles (and holes) and their individual spin contributions are both finite there is a limiting angular momentum that can be generated. At this point the rotational band loses its collectivity and is said to terminate. Band termination spectroscopy enables a study of the balance and interplay between the two extremes of nuclear dynamics, namely collective and single-particle degrees of freedom.
GAMMASPHERE has played a most significant role in establishing a novel form of band termination in nuclei near mass A = 110. The structures of interest involve particle-hole excitations across the Z = 50 shell gap into specific high-j orbitals to create prolate collective rotational sequences or bands. As the rotational frequency and spin increases the Coriolis interaction forces the valence particles to gradually align their individual angular momenta along a common spin axis resulting in characteristic decreasing dynamic moments of inertia (J(2)). The observed E-ERLD (excitation energy - rotating liquid drop energy) versus spin curves also show characteristic hook shape behavior which are different for each configuration. These properties are illustrated in the middle row of figures for the yrast band in 110Sb. It is the ability to observe a specific configuration over such an extended range of spin values without interruption that is so special in these heavy nuclei.
Theoretical calculations predict that as the available (10-15) valence nucleons outside of the Z=N=50 double shell closure align, the nuclear shape gradually traces a path from a collective prolate shape (gamma = 0°) through triaxial shapes to a noncollective oblate shape (g = +60°) over many transitions. This feature called "smooth band termination" is illustrated in the upper row of figures where the potential energy surface is plotted for the yrast configuration in 110Sb as a function of shape and spin.
GAMMASPHERE experiments have established similar behavior in neighboring Sb isotopes and other surrounding nuclei. The lower figures show results for 109Sb where several terminating bands have also been found. Most importantly however, is the fact that detailed lifetime measurements using GAMMASPHERE have shown for the first time convincing evidence for decreasing quadrupole transition moments (Qt) or collectivity with increasing spin values in 109Sb (and 108Sn), (bottom right). These results form crucial confirmation of the "smooth band termination" model in A = 110 nuclei.