Nucleus Energy Levels
Analogies with the atomic energy levels …
At first glance, nuclei seem to be very different from atoms. More than a hundred thousand times smaller, they are also vastly more complex; atoms are primarily made up of empty space whereas the inside of the nucleus is extremely dense. Despite these differences, however, atoms and nuclei have a lot in common.
The behaviour of a nucleus is governed by the laws of quantum mechanics; laws which supersede those of classical mechanics at the microscopic scale. These quantum laws force the nucleus to exist in any one of a finite number of ‘states’, primarily defined in terms of the energy it possesses. This energy is at a minimum when the nucleus is in isolation – a state usually referred to as the ‘ground state’.
When the nucleus is at a different level, it finds itself with a surplus of energy. This extra energy is released in the form of a gamma photon, thereby allowing the nucleus to reach its ground state once more. These gamma photons are of the same kind as the X rays emitted by atoms, but have far greater energies – which can be of the order of a million electronvolts (MeV).
The energy states of the community of nucleons coexisting in any given nucleus can vary, primarily due to a kind of ‘layer’ structure not dissimilar to that which exists in the atom. The binding energy holding the nucleons together can take any of a selection of values that correspond to the number of ‘layers’ that exist.
Nuclei with 2, 8, 20, 28, 50, 82 or 126 nucleons are observed to be particularly stable; an interesting analogy with the stability of atoms of the noble gases whose outermost electron layers are said to be complete.
In addition to this layered structure, the nucleons can undergo collective movements which correspond to new nuclear energy states. If the entire cluster starts vibrating, for instance, the vibrational energies can only take certain specific values, precisely determined by the laws of quantum mechanics.
Finally, it is important to remember that these nuclei are not necessarily spherical in shape, and can undergo deformation or start rotating. The energies associated with these rotations can also only take well-defined values – they are said to be ‘quantified’.
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