Nuclear Energy Levels. The nuclei of certain atoms have a property known as spin. A (charged) spinning nucleus generates a small magnetic field and can be thought of as being a small magnet. Normally this magnet can be orientated in any direction, but in the presence of an external magnetic field it can only align itself either with or against the field, i.e., it is quantized. Transitions between the differences energy within the nuclei can be made to occur by applying radiation of a specific energy to the sample.