Editing Nuclear isomer (section)

==Metastable isomers==
{{Unreferenced section|date=October 2024}}
'''Metastable isomers''' can be produced through [[nuclear fusion]] or other [[nuclear reaction]]s. A nucleus produced this way generally starts its existence in an excited state that relaxes through the emission of one or more [[gamma ray]]s or [[conversion electron]]s. Sometimes the de-excitation does not completely proceed rapidly to the nuclear [[ground state]]. This usually occurs as a '''spin isomer''' when the formation of an intermediate excited state has a [[Spin (physics)|spin]] far different from that of the ground state. Gamma-ray emission is hindered if the spin of the post-emission state differs greatly from that of the emitting state, especially if the excitation energy is low. The excited state in this situation is a good candidate to be metastable if there are no other states of intermediate spin with excitation energies less than that of the metastable state.

Metastable isomers of a particular [[isotope]] are usually designated with an "m". This designation is placed after the mass number of the atom; for example, [[cobalt-58m1]] is abbreviated {{nuclide|cobalt|58m1}}, where 27 is the atomic number of cobalt. For isotopes with more than one metastable isomer, "indices" are placed after the designation, and the labeling becomes m1, m2, m3, and so on. Increasing indices, m1, m2, etc., correlate with increasing levels of excitation energy stored in each of the isomeric states (e.g., hafnium-178m2, or {{nuclide|hafnium|178m2}}).

{{anchor|Fission isomer}}
A different kind of metastable nuclear state (isomer) is the '''fission isomer''' or '''shape isomer'''. Most [[actinide]] nuclei in their ground states are not spherical, but rather [[prolate spheroid]]al, with an [[axis of symmetry]] longer than the other axes, similar to an American football or [[rugby ball]]. This geometry can result in quantum-mechanical states where the distribution of protons and neutrons is so much further from spherical geometry that de-excitation to the nuclear ground state is strongly hindered. In general, these states either de-excite to the ground state far more slowly than a "usual" excited state, or they undergo [[spontaneous fission]] with [[Half-life|half-lives]] of the order of [[nanosecond]]s or [[microsecond]]s—a very short time, but many orders of magnitude longer than the half-life of a more usual nuclear excited state. Fission isomers may be denoted with a postscript or superscript "f" rather than "m", so that a fission isomer, e.g. of [[plutonium]]-240, can be denoted as plutonium-240f or {{nuclide|plutonium|240f}}.