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.
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 or other
nuclear reactions. A nucleus thus produced generally starts its existence in an
excited state that de-excites through the emission of one or more
gamma rays (or, equivalently,
conversion electrons), usually in a time far shorter than a
picosecond. 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, than if the two states are of similar spin. 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, 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.,
58mCo). Increasing indices, m, 2m, etc. correlate with increasing levels of excitation energy stored in each of the isomeric states.
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 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 Pu-240f or
240fPu.
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 10
15 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 Collins
[C.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 group
[D. 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
.
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.
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)
*
Ballotechnics*
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.
