The low energy nuclear excitation spectrum

Abstract

Various types of nuclear reactions yield information on different aspects of nuclear structure. Inelastic scattering reactions which proceed by direct interaction are especially suited for the study of collective modes of excitation of the nucleus. Most quantitative information is obtained from Coulomb excitation processes. The evidence available from low energy nuclear spectra is summarized. For even-even nuclei, the first few excited levels can in most cases be rather well described in terms of collective modes of motion. In odd-A (or odd-odd) nuclei, the relevant degrees of freedom include also those of the last nucleon(s). The collective modes correspond to oscillations of the nuclear shape and are influenced in an essential manner by the shell structure. For configurations with relatively few particles in unfilled shells, the collective excitations represent vibrations about a spherical equilibrium shape. If the nucleus contains sufficiently many particles in unfilled shells, an ellipsoidal equilibrium shape results. The collective excitations then separate into vibrations about the equilibrium shape and rotations with preservation of shape. While the dominating shape oscillations and deformations are of quadrupole type and give rise to excitations with no parity change, evidence has also recently become available on collective excitations of odd parity.