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There are many different types of excites states in a nucleus. Since the nucleus is actually
comprised of many protons and neutrons, some excited states can be described as
single particle states. In this mode one or more protons and/or neutrons are not in their
lowest orbits. Another mode of excitation occurs when the nucleons appear to act
collectively. Much as a school of individual fish can turn and move as one entity, so can
the nucleus. As we learn more about the nucleus, we simplify our understanding of it
by considering it to be described by two very different models. In one model, the
nucleus is a bag of individual protons and neutrons interacting with each other. In this
model, we need to understand and be able to describe all of those interactions. For large
nuclei, one nucleon "interacts" with the other 200 and vice-versa. This quickly becomes
a "many-body problem" which is very difficult to solve. Therefore we usually describe
nuclei as consisting of an "inert core" and a few "valence nucleons" which are involved in
the excitation. This simplfied approach is surprisingly good at describing experiments
and we call this model, the shell model. Another view of the nucleus is
to treat it as a single entity or "liquid-drop". Think of a drop of cold honey.
In this case, the individual nucleons
have little influence on the whole and the entire system can be described more simply.
These models are called "mean-field" models and are very good at describing the nucleus
in its collective modes of excitation.
Several modes of excitation are described below.
Single particle excitation
In single-particle states, individual nucleons are promoted to high lying orbitals
within the various shells of the nucleus. Many particles may be raised to these
orbitals. Usually, these types of excitations are described as a spherical inert core
(nucleons which are in their normal orbitals) surrounded by valence nucleons which
take part in the excitation.