Nearly-stable isomers

Most nuclear isomers are very unstable, and radiate away the extra energy immediately (on the order of 10-12 seconds). As a result, the term is usually restricted to refer to isomers with half-lives of 10-9 seconds or more. Quantum mechanics predicts that certain atomic species will possess isomers with unusually long lifetimes even by this stricter standard, and so have interesting properties. By definition, there is no such thing as a "stable" isomer; however, some are so long-lived as to be nearly stable, and can be produced and observed in quantity.The only nearly-stable nuclear isomer occurring in nature is Ta-180m, which is present in all tantalum samples at about 1 part in 8,300. Its half-life is at least 1015 years, markedly longer than the age of the universe. This remarkable persistence results from the fact that the excitation energy of the isomeric state is low and both gamma de-excitation to the Ta-180 ground state (which is radioactive and not particularly long lived) and beta decay to hafnium or tungsten are suppressed owing to spin mismatches. The origin of this isomer is mysterious, though it is believed to have been formed in supernovas (as are most other heavy elements). When it relaxes to its ground state, it releases a photon with an energy of 75 keV. It was first reported in 1988 by Collinsthat Ta-180m can be forced to release its energy by weaker x-rays. After 11 years of controversy those claims were confirmed in 1999 by Belic and co-workers in the Stuttgart nuclear physics group.Another reasonably stable nuclear isomer (with a half-life of 31 years) is hafnium-178m2, which has the highest excitation energy of any comparably long-lived isomer. One gram of pure Hf-178-m2 contains approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. Further, in the natural decay of Hf-178-m2, the energy is released as gamma rays with a total energy of 2.45 MeV. As with Ta-180m, there are disputed reports that Hf-178-m2 can be stimulated into releasing its energy, and as a result the substance is being studied as a possible source for gamma ray lasers. These reports also indicate that the energy is released very quickly, so that Hf-178-m2 can produce extremely high powers (on the order of exawatts. Other isomers have also been investigated as possible media for gamma-ray stimulated emission.