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Nuclear Isomer Definition
Nuclear isomers are atoms with the same mass number and atomic number, but with different states of excitation in the atomic nucleus. The higher or more excited state is called a metastable state, while the stable, unexcited state is called the ground state.
How They Work
Most people are aware electrons can change energy levels and be found in excited states. An analogous process occurs in the atomic nucleus when protons or neutrons (the nucleons) become excited. The excited nucleon occupies a higher energy nuclear orbital. Most of the time, the excited nucleons return immediately to the ground state, but if the excited state has a half-life longer than 100 to 1000 times that of normal excited states, it is considered a metastable state. In other words, the half-life of an excited state is usually on the order of 10-12 seconds, while a metastable state has a half-life of 10-9 seconds or longer. Some sources define a metastable state as having a half-life greater than 5 x 10-9 seconds to avoid confusion with the half-life of gamma emission. While most metastable states decay quickly, some last for minutes, hours, years, or much longer.
The reason metastable states form is because a larger nuclear spin change is needed in order for them to return to the ground state. High spin change makes the decays "forbidden transitions" and delays them. Decay half-life is also affected by how much decay energy is available.
Most nuclear isomers return to the ground state via gamma decay. Sometimes gamma decay from a metastable state is named isomeric transition, but it's essentially the same as normal short-lived gamma decay. In contrast, most excited atomic states (electrons) return to the ground state via fluorescence.
Another way metastable isomers can decay is by internal conversion. In internal conversion, the energy that is released by the decay accelerates an inner electron, causing it to exit the atom with considerable energy and speed. Other decay modes exist for highly unstable nuclear isomers.
Metastable and Ground State Notation
The ground state is indicated using the symbol g (when any notation is used). The excited states are denoted using the symbols m, n, o, etc. The first metastable state is indicated by the letter m. If a specific isotope has multiple metastable states, the isomers are designated m1, m2, m3, etc. The designation is listed after the mass number (e.g., cobalt 58m or 58m27Co, hafnium-178m2 or 178m272Hf).
The symbol sf may be added to indicate isomers capable of spontaneous fission. This symbol is used in the Karlsruhe Nuclide Chart.
Metastable State Examples
Otto Hahn discovered the first nuclear isomer in 1921. This was Pa-234m, which decays in Pa-234.
The longest-lived metastable state is that of 180m73 Ta. This metastable state of tantalum has not been seen to decay and appears to last at least 1015 years (longer than the age of the universe). Because the metastable state endures so long, the nuclear isomer is essentially stable. Tantalum-180m is found in nature at an abundance of about 1 per 8300 atoms. It's thought perhaps the nuclear isomer was made in supernovae.
How They Are Made
Metastable nuclear isomers occur via nuclear reactions and can be produced using nuclear fusion. They occur both naturally and artificially.
Fission Isomers and Shape Isomers
A specific type of nuclear isomer is the fission isomer or shape isomer. Fission isomers are indicated using either a postscript or superscript "f" instead of "m" (e.g., plutonium-240f or 240f94Pu). The term "shape isomer" refers to the shape of the atomic nucleus. While the atomic nucleus tends to be depicted as a sphere, some nuclei, such as those of most actinides, are prolate spheres (football-shaped). Because of quantum mechanical effects, de-excitation of excited states to the ground state is hindered, so the excited states tend to undergo spontaneous fission or else return to the ground state with a half-life of nanoseconds or microseconds. The protons and neutrons of a shape isomer may be even further from a spherical distribution than the nucleons on the ground state.
Uses of Nuclear Isomers
Nuclear isomers may be used as gamma sources for medical procedures, nuclear batteries, for research into gamma ray stimulated emission, and for gamma ray lasers.