Metastable isomers can be produced through nuclear fusion or other nuclear reactions. A nucleus thus produced generally starts its existence in an excited state that relaxes through the emission of one or more gamma rays or conversion electrons. However, sometimes it happens that the de-excitation does not proceed rapidly all the way to the nuclear ground state. This usually occurs because of the formation of an intermediate excited state with a spin far different from that of the ground state. Gamma-ray emission is far slower (is "hindered") if the spin of the post-emission state is very different from that of the emitting state, particularly if the excitation energy is low. The excited state in this situation is therefore 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" (or, in the case of isotopes with more than one isomer, m1, m2, m3, and so on). This designation is placed after the mass number of the atom; for example, Cobalt-58m (abbreviated 58m
27Co, where 27 is the atomic number of cobalt). Increasing indices, m1, m2, etc., correlate with increasing levels of excitation energy stored in each of the isomeric states (e.g., hafnium-177m2 or 177m2
72Hf).
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 spheroidal—specifically, prolate, with an axis of symmetry longer than the other axes (similar to an American football or rugby ball, although with a less pronounced departure from spherical symmetry). In some of these, quantum-mechanical states can exist in which the distribution of protons and neutrons is farther yet from spherical (in fact, about as non-spherical as an American football), so much so that de-excitation to the nuclear ground state is strongly hindered. In general, these states either de-excite to the ground state (albeit far more slowly than a "usual" excited state) or undergo spontaneous fission with half-lives of the order of nanoseconds or microseconds—a very short time, but many orders of magnitude longer than the half-life of a more usual nuclear excited state. Fission isomers are usually denoted with a postscript or superscript "f" rather than "m", so that a fission isomer in, e.g., plutonium 240 is denoted plutonium-240f or 240f
94Pu.
Read more about this topic: Nuclear Isomer