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Chapter I Overview of nuclear structure
Introduction
A nucleus is a system of A interacting particles. Besides the general nature of the problem as a many-body system, the nucleon-nucleon interaction is a challenge by itself. Compared to atomic structure, the problem is more complex. In an atom the nucleus provides a common center of attraction for all the electrons (central field) and the electron-electron forces generally play a smaller role. Furthermore, the predominant force (Coulomb) is well known. Nuclei, on the other hand, have no center of attraction; the nucleons are held together by their mutual nuclear interactions which are much more complicated than Coulomb interactions.
The difference between the components of the two systems is another source of difficulties.
While all atomic electrons are alike, there are two kinds of nucleons: protons and neutrons. This allows a richer variety of structures. Consequently, there are about 100 types of atoms (elements), but more than 1000 different nuclides.
Experimental evidence for the existence for a nuclear shell structure had already accumulated by the mid 1940’s. The early evidence included that nuclei with certain proton and neutron numbers (N or Z = 2, 8, 20, 28, 50, 82, and 126) are particularly stable. These numbers were called “magic numbers” because while the evidence of their special nature was considerable, an understanding of why these numbers were special was lacking.
The nuclear shell mode
Basic principles of the nuclear shell model
The basic assumption of the nuclear shell model is that to a first approximation each nucleon moves independently in a potential that represents the average interaction with the other nucleons in a nucleus. This independent motion can be understood qualitatively from a combination of the weakness of the long-range nuclear attraction and the Pauli exclusion principle.
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