Nuclear isomer
Some references recommend 5 × 10−9 seconds to distinguish the metastable half life from the normal "prompt" gamma-emission half-life.
During internal conversion, energy of nuclear de-excitation is not emitted as a gamma ray, but is instead used to accelerate one of the inner electrons of the atom.
These fragments are usually produced in a highly excited state, in terms of energy and angular momentum, and go through a prompt de-excitation.
If the half-life of the isomers is long enough, it is possible to measure their production rate and compare it to that of the ground state, calculating the so-called isomeric yield ratio.
A nucleus produced this way generally starts its existence in an excited state that relaxes through the emission of one or more gamma rays or conversion electrons.
Most actinide nuclei in their ground states are not spherical, but rather prolate spheroidal, with an axis of symmetry longer than the other axes, similar to an American football or rugby ball.
Most nuclear excited states are very unstable and "immediately" radiate away the extra energy after existing on the order of 10−12 seconds.
Quantum mechanics predicts that certain atomic species should possess isomers with unusually long lifetimes even by this stricter standard and have interesting properties.
The low excitation energy of the isomeric state causes 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 electron capture to hafnium or beta decay to tungsten to be suppressed due to spin mismatches.
It was first reported in 1988 by C. B. Collins[4] that theoretically 180mTa can be forced to release its energy by weaker X-rays, although at that time this de-excitation mechanism had never been observed.
However, the de-excitation of 180mTa by resonant photo-excitation of intermediate high levels of this nucleus (E ≈ 1 MeV) was observed in 1999 by Belic and co-workers in the Stuttgart nuclear physics group.
One gram of pure 178m2Hf contains approximately 1.33 gigajoules of energy, the equivalent of exploding about 315 kg (700 lb) of TNT.
These reports indicate that the energy is released very quickly, so that 178m2Hf can produce extremely high powers (on the order of exawatts).
[7][8][9] This low energy produces "gamma rays" at a wavelength of 148.3821828827(15) nm, in the far ultraviolet, which allows for direct nuclear laser spectroscopy.
Such ultra-precise spectroscopy, however, could not begin without a sufficiently precise initial estimate of the wavelength, something that was only achieved in 2024 after two decades of effort.
Neutral 229m90Th decays by internal conversion with a half-life of 7±1 μs, but because the isomeric energy is less than thorium's second ionization energy of 11.5 eV, this channel is forbidden in thorium cations and 229m90Th+ decays by gamma emission with a half-life of 1740±50 s.[7] This conveniently moderate lifetime allows the development of a nuclear clock of unprecedented accuracy.
[15][16][9] The most common mechanism for suppression of gamma decay of excited nuclei, and thus the existence of a metastable isomer, is lack of a decay route for the excited state that will change nuclear angular momentum along any given direction by the most common amount of 1 quantum unit ħ in the spin angular momentum.
Each additional unit of spin change larger than 1 that the emitted gamma ray must carry inhibits decay rate by about 5 orders of magnitude.
[citation needed] Hafnium[18][19] isomers (mainly 178m2Hf) have been considered as weapons that could be used to circumvent the Nuclear Non-Proliferation Treaty, since it is claimed that they can be induced to emit very strong gamma radiation.
Nonetheless a 12-member Hafnium Isomer Production Panel (HIPP) was created in 2003 to assess means of mass-producing the isotope.
