Shell structure

Shell structure

According to the nuclear shell model, a nucleon that is, a proton or a neutron moves in a common attractive force field generated by all the other nucleons. Similar to an electron's motion in an atom, nucleonic orbits bunch together forming shells, and nuclei having filled nucleonic shells (nuclear noble gases) are exceptionally stable. This happens for specific "magic" numbers of protons (Z = 2, 8, 20, 28, 50 and 82) and neutrons (N = 2, 8, 20, 50, 82 and 126). The orbitals which make up the various shells can be described by various quantum numbers and follow specific rules. For example, any one orbital can have at most two protons and two neutrons.

The nuclear shell structure is illustrated below for an excited state in 16O (oxygen-16). The blue orbital is the first shell which contains an s-orbital. This shell can hold 2 protons and 2 neutrons. The three red orbits are the second shell and it contains the first p-orbital which can support 6 protons and 6 neutrons. These two shells are full when 16O is in its ground state. The green the orbital is in the third shell and contains the s and d orbitals. This shell can support 12 protons and 12 neutrons. For simplicity, only 1 of the 6 possible orbits is shown.

In an excited state of 16O a nucleon is located in this third shell resulting in a hole in the lower shells. As this nucleon changes from the third shell to the second, it must lose energy in the form of gamma-rays. Such changes

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This file last modified Tuesday March 13, 2007