Nuclear isomer

A nuclear isomer is a metastable state of an atom caused by the excitation of one or more protons and/or neutrons in its nucleus.

Explanation

An element is defined by the number of protons in its nucleus and isotopes of an element are distinguished by differing numbers of neutrons in the nucleus.

Metastable isomers can be produced through nuclear fusion. The collision of two nuclei forms a new nucleus in an excited state that then releases energy in the form of particles and photons. In some cases this de-excitation is inhibited and the resulting nucleus remains in an excited state for some period of time. So-called shape, spin and k-isomers describe different causes of this inhibition. This excitation is reflected in a different value of spin in the nucleus.

Metastable isomers of a particular isotope are usually designated with an "m" (or, in the case of isotopes with more than one isomer, 2m, 3m, and so on). This designation is usually placed after the atomic symbol and number of the atom (e.g., Co-58m), but is sometimes placed as a superscript before (e.g., ^mCo-58 or ^58mCo). Increasing indices, m, 2m, etc. correlate with increasing levels of excitation energy stored in each of the isomeric states.

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.

The only stable nuclear isomer occurring in nature is Ta-180m, which is present in all tantalum samples at about 1 part in 8300. Its half-life is at least 10¹⁵ years, and it may in fact be entirely stable. 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 energetic photons with wavelength of 16 nanometers—x-ray wavelengths. It was first reported in 1988 by CollinsC.B. Collins et al., Phys. Rev. C, 37, p 2267-2269 (1988). that 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 groupD. Belic et al., Phys. Rev. Lett., 83, p 5242 (1999)..

Another reasonably stable nuclear isomer (with a half-life of 31 years) is hafnium-178m, which has the highest excitation energy of any stable isomer. One gram of pure Hf-178-2m would contain approximately 1300 megajoules of energy, the equivalent of exploding about 226 kilograms (500 pounds) of TNT. Further, all of the energy released is in gamma rays at 0.05 nanometers. As with Ta-180m, there are disputed reports that Hf-178-2m 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-178-2m can produce extremely high powers (on the order of exawatts). Other isomers have also been investigated as possible media for gamma-ray stimulated emission.

Applications

These hafnium and tantalum isomers have been considered in some quarters as weapons that could be used to circumvent the Nuclear Non-Proliferation Treaty, since they can be induced to emit very strong gamma radiation. DARPA has or has had a program to investigate this usage of both isomers. However, given the difference in speed between a photon and a neutron, they can't be induced to chain react like a nuclear weapon, so there will probably never be such a weapon. Ta-180m is also one of the most expensive substances to procure in the world, at approximately $17 million per gram. In 1999, the entire world's supply of Ta-180m was only 6.7 milligrams.

Technetium isomers Tc-99m (with a half-life of 6.01 hours) and Tc-95m (with a half-life of 61 days) are used in medical and industrial applications.

Decay processes

Isomers decay to lower energy states of the nuclide through two isomeric transitions:

γ (gamma) emission (emission of a high-energy photon)
internal conversion (the energy is used to ionize the atom)

References

External links

Research group which presented initial claims of hafnium nuclear isomer de-excitation control. - The Center for Quantum Electronics, The University of Texas at Dallas.
Lengthy Washington Post article, March 2004
JASON Defense Advisory Group report on high energy nuclear materials mentioned in the Washington Post story above
May 2004 article in Physics Today which reviews the Hf controversy in a balanced manner.
Confidence for Hafnium Isomer Triggering in 2006. - The Center for Quantum Electronics, The University of Texas at Dallas.

Nuclear physics

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