Most nuclei near closed shells have a spherical shape. However, many nuclei away from these magic numbers become deformed. Their proton and neutron distributions stretch into an elongated or flattened form.
The Nilsson model explains this deformation. Sven Gösta Nilsson developed it as an extension of the nuclear shell model. The model adds a deformed potential instead of a purely spherical one.
In this approach, single-particle energy levels split according to the degree of deformation. Nucleons occupy these new levels. As a result, the nucleus gains extra binding energy when it takes a non-spherical shape.
Shape evolution appears clearly across the nuclear chart. Nuclei with few valence nucleons stay nearly spherical. As more nucleons fill the mid-shell regions, quadrupole deformation usually increases.
Some regions show prolate shapes, where the nucleus stretches like a rugby ball. Other regions favour oblate shapes, which flatten like a disc. A few nuclei even display more complex shapes such as triaxial deformation.
The Nilsson model successfully predicts these trends. It accounts for the observed energy levels and electromagnetic properties of deformed nuclei. Moreover, it helps explain rotational bands seen in gamma-ray spectroscopy.
Researchers continue to test the model with new data from exotic nuclei. These studies reveal how shape changes as one moves far from stability. The Nilsson framework therefore remains a key tool for understanding nuclear structure.