# 4: Nuclear Models


There are two important classes of nuclear models: single particle and microscopic models, that concentrate on the individual nucleons and their interactions, and collective models, where we just model the nucleus as a collective of nucleons, often a nuclear ﬂuid drop. Microscopic models need to take into account the Pauli principle, which states that no two nucleons can occupy the same quantum state. This is due to the Fermi-Dirac statistics of spin 1/2 particles, which states that the wavefunction is antisymmetric under interchange of any two particles

• 4.1: Nuclear Shell Model
The simplest of the single particle models is the nuclear shell model. It is based on the observation that the nuclear mass formula, which describes the nuclear masses quite well on average, fails for certain “magic numbers”, i.e., for neutron number N=20, 28, 50, 82, 126 and proton number Z=20, 28, 50, 82.
• 4.2: Collective Models
Another, and actually older, way to look at nuclei is as a drop of “quantum fluid”. This ignores the fact that a nucleus is made up of protons and neutrons, and explains the structure of nuclei in terms of a continuous system, just as we normally ignore the individual particles that make up a fluid.
• 4.3: Fission
Another, and actually older, way to look at nuclei is as a drop of “quantum fluid”. This ignores the fact that a nucleus is made up of protons and neutrons, and explains the structure of nuclei in terms of a continuous system, just as we normally ignore the individual particles that make up a fluid.
• 4.4: Barrier Penetration
To understand quantum mechanical tunnelling in fission it makes sense to look at the simplest fission process: the emission of a He nucleus, so called α radiation

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