The nucleus is bound by short range forces, much as a droplet of water is bound by short range forces. Nevertheless it is a quantum system. The question arises as to whether it has a ``shell structure'' like that of the atom, which follows from the grouping of electron levels into orbitals. The understanding that it does, and how this arises, is a central issue in nuclear structure physics.
Quantum structure, in the form of internal stationary states (energy levels) of the nucleus, arises essentially from two aspects. From the `liquid drop' point of view, the nucleus can be deformed from an equilibrium configuration, or, if deformed, can rotate about an axis of low symmetry. This gives rise to quantum oscillators and rotators, which possess an energy spectrum after quantisation. These effects can be referred to as collective, as many of the nucleons participate in them.
At the other extreme, single particle behaviour is observed, that is, certain states can be understood as the behaviour of single nucleons moving in a potential, (or mean field) determined by the rest of the nucleons. In this picture, the nucleons can be regarded as independent particles, and the question is how to reconcile this with the strong short range force between nucleons. Interactions between the nucleons (by means of a residual interaction that omits the overall mean field effects) are taken into account in the shell model of the nucleus.
At this point, we need to note that the nucleus is a complicated composite object with an internal structure, and can exist in various states of excitation. Transitions between these states give rise to electromagnetic radiation, just as transitions in atoms give rise to light. In nuclei, this radiation is known as gamma radiation. The states of a nucleus can be represented as a function of increasing excitation energy in an energy level diagram. These are generally labelled by a set of quantum numbers such as `spin' (rather, total angular momentum) and parity, and often isospin. (cf. 2p or 3d states in the H atom).
Figure: Energy level diagram
An excited nucleus is less tightly bound (by an amount equal to the excitation energy) and might well decay into some other configuration (or channel).
The binding energy in the excited state becomes
where the subscript X indicates the excited state quantities, and is the ground state binding energy.