Rotation of Elongated Atomic Nuclei

Abstract

Many nuclei exhibit rotational spectra much as molecules do. A rotational spectrum is recognized by the ratios of the successive spacings between the levels. The magnitude of this spacing is inversely proportional to the moment of inertia of the rotating object. For a molecule, the moment of inertia is quite simply determined by the fairly definite positions of the atomic nuclei where practically all the mass is concentrated. In nuclei there are no fixed masses and the very possibility of rotation depends on the nuclear shape being out-of-round. The moment of inertia is determined by the way the internal motions of the nucleons are influenced by the rotation. The hydrodynamic calculation based on treating the nuclear “fluid” like water with no friction gives too small a moment of inertia, which means calculated levels are more widely spaced than observed, by about a factor five. Among all the complicated aspects of nuclear structure, one aspect of the motion of nucleons that can be described in simple graphic terms is the influence of rotation. It is shown that this leads to large contributions to the moment of inertia by the few nucleons not in closed shells. Thus it is possible to understand in fairly simple terms how the moment of inertia can be greater than that calculated for a distorted droplet of “nuclear fluid,” though one manner of estimating the effect somewhat overshoots the experimental result.