Nearly-stable Isomers
Most nuclear excited states 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 longer. 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 most stable nuclear isomer occurring in nature is 180m
73Ta, 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 180Ta ground state (which itself is radioactive by beta decay, with a half-life of only 8 hours), and direct beta decay to hafnium or tungsten are all suppressed, owing to spin mismatches. The origin of this isomer is mysterious, though it is believed to have been formed in supernovae (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 Collins that 180mTa 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 178m2
72Hf, which has the highest excitation energy of any comparably long-lived isomer. One gram of pure 178m2Hf contains approximately 1.33 gigajoules of energy, the equivalent of exploding about 315 kg (690 lb) of TNT. Further, in the natural decay of 178m2Hf, the energy is released as gamma rays with a total energy of 2.45 MeV. As with 180mTa, there are disputed reports that 178m2Hf 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 178m2Hf can produce extremely high powers (on the order of exawatts). Other isomers have also been investigated as possible media for gamma-ray stimulated emission.
Holmium has an interesting nuclear isomer, 166m1
67Ho with a half-life of 1,200 years, which is nearly the longest half-life of any holmium radionuclide (only 163Ho, with a half-life of 4,570 years is longer).
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90Th has a remarkably low-lying metastable isomer, only 7.6 ± 0.5 electron volts above the ground state, as calculated from spectroscopic measurements. This direct decay has not been observed, however. If this isomer were to decay it would produce a gamma ray (defined by its origin, not its wavelength) in the ultraviolet range. These "ultraviolet gamma rays" were thought to have been detected at one time, but this observation has since been found to be from nitrogen gas excited by higher energy emissions.
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