The Interacting Boson Model
The spectroscopy of medium mass and heavy nuclei is characterized by the occurrence of low-lying collective states. However, the structure of these states, their arrangement into bands and their electromagnetic properties change from nucleus to nucleus. The changes are related to the shell structure of the nucleus with major closed shells at nucleon number 50, 82, 126. The traditional way to analyze collective spectra has been the geometrical description of Bohr and Mottelson1. In this description two limiting cases can be discussed easily, that of a harmonic vibrator with energy levels proportional to the angular momentum I and that of a rigid rotor with energy levels proportional to the square of the angular momentum I(I+1). The description of the intermediate situations is much more difficult, although several calculations have been done by Kumar and Baranger2 to which the interested reader is referred.
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