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High-energy processes |
Anuclear isomer is ametastable state of anatomic nucleus, in which one or morenucleons (protons or neutrons) occupy excited state levels (higher energy levels). "Metastable" describes nuclei whose excited states havehalf-lives of 10−9 seconds or longer, 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10−12 seconds). Some references recommend5×10−9 seconds to distinguish the metastable half life from the normal "prompt"gamma-emission half-life.[1] Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example, the180m
73Ta
nuclear isomer survives so long (at least2.9×1017 years[2]) that it has never been observed to decay spontaneously. The half-life of a nuclear isomer can even exceed that of the ground state of the same nuclide, as shown by180m
73Ta
as well as186m
75Re
,192m2
77Ir
,210m
83Bi
,212m
84Po
,242m
95Am
and multipleholmium isomers.
Sometimes, thegamma decay from a metastable state is referred to as isomeric transition, but this process typically resembles shorter-lived gamma decays in all external aspects with the exception of the long-lived nature of the meta-stable parent nuclear isomer. The longer lives of nuclear isomers' metastable states are often due to the larger degree of nuclear spin change which must be involved in their gamma emission to reach the ground state. This high spin change causes these decays to beforbidden transitions and delayed. Delays in emission are caused by low or high available decay energy.
The first nuclear isomer and decay-daughter system (uranium X2/uranium Z, now known as234m
91Pa
/234
91Pa
) was discovered byOtto Hahn in 1921.[3]
The nucleus of a nuclear isomer occupies a higher energy state than the non-excited nucleus existing in theground state. In an excited state, one or more of the protons or neutrons in a nucleus occupy anuclear orbital of higher energy than an available nuclear orbital. These states are analogous to excited states of electrons in atoms.
When excited atomic states decay, energy is released byfluorescence. In electronic transitions, this process usually involves emission of light near thevisible range. The amount of energy released is related tobond-dissociation energy orionization energy and is usually in the range of a few to few tens of eV per bond. However, a much stronger type ofbinding energy, thenuclear binding energy, is involved in nuclear processes. Due to this, most nuclear excited states decay bygamma ray emission. For example, a well-known nuclear isomer used in various medical procedures is99m
43Tc
, which decays with a half-life of about 6 hours by emitting a gamma ray of 140 keV of energy; this is close to the energy of medical diagnostic X-rays.
Nuclear isomers have long half-lives because their gamma decay is "forbidden" from the large change innuclear spin needed to emit a gamma ray. For example,180m
73Ta
has a spin of 9 and must gamma-decay to180
73Ta
with a spin of 1. Similarly,99m
43Tc
has a spin of 1/2 and must gamma-decay to99
43Tc
with a spin of 9/2.
While most metastable isomers decay through gamma-ray emission, they can also decay throughinternal conversion. 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 excited electrons then leave at a high speed. This occurs because inner atomic electrons penetrate the nucleus where they are subject to the intense electric fields created when the protons of the nucleus rearrange in a different way.
In nuclei that are far from stability in energy, even more decay modes are known.
After fission, several of thefission fragments that may be produced have a metastable isomeric state. These fragments are usually produced in a highly excited state, in terms of energy andangular momentum, and go through a prompt de-excitation. At the end of this process, the nuclei can populate both the ground and the isomeric states. 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-calledisomeric yield ratio.[4]
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Metastable isomers can be produced throughnuclear fusion or othernuclear reactions. A nucleus produced this way generally starts its existence in an excited state that relaxes through the emission of one or moregamma rays orconversion electrons. Sometimes the de-excitation does not completely proceed rapidly to the nuclearground state. This usually occurs as aspin isomer when the formation of an intermediate excited state has aspin far different from that of the ground state. Gamma-ray emission is hindered if the spin of the post-emission state differs greatly from that of the emitting state, especially if the excitation energy is low. The excited state in this situation is 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 particularisotope are usually designated with an "m". This designation is placed after the mass number of the atom; for example,cobalt-58m1 is abbreviated58m1
27Co
, where 27 is the atomic number of cobalt. For isotopes with more than one metastable isomer, "indices" are placed after the designation, and the labeling becomes m1, m2, m3, and so on. Increasing indices, m1, m2, etc., correlate with increasing levels of excitation energy stored in each of the isomeric states (e.g., hafnium-178m2, or178m2
72Hf
).
A different kind of metastable nuclear state (isomer) is thefission isomer orshape isomer. Mostactinide nuclei in their ground states are not spherical, but ratherprolate spheroidal, with anaxis of symmetry longer than the other axes, similar to an American football orrugby ball. This geometry can result in quantum-mechanical states where the distribution of protons and neutrons is so much further from spherical geometry that de-excitation to the nuclear ground state is strongly hindered. In general, these states either de-excite to the ground state far more slowly than a "usual" excited state, or they undergospontaneous fission withhalf-lives of the order ofnanoseconds ormicroseconds—a very short time, but many orders of magnitude longer than the half-life of a more usual nuclear excited state. Fission isomers may be denoted with a postscript or superscript "f" rather than "m", so that a fission isomer, e.g. ofplutonium-240, can be denoted as plutonium-240f or240f
94Pu
.
Most nuclear excited states are very unstable and "immediately" radiate away the extra energy after existing on the order of 10−12 seconds. As a result, the characterization "nuclear isomer" is usually applied only to configurations with half-lives of 10−9 seconds or longer.Quantum mechanics predicts that certain atomic species should possess isomers with unusually long lifetimes even by this stricter standard and have interesting properties. Some nuclear isomers are so long-lived that they are relatively stable and can be produced and observed in quantity.
The most stable nuclear isomer occurring in nature is180m
73Ta
, which is present in alltantalum samples at about 1 part in 8,300. Its half-life is theorized to be at least2.9×1017 years, markedly longer than theage of the universe. The low excitation energy of the isomeric state causes both gamma de-excitation to the180
Ta
ground state (which itself is radioactive by beta decay, with a half-life of only 8 hours) and directelectron capture tohafnium orbeta decay totungsten to be suppressed due to spin mismatches. The origin of this isomer is mysterious, though it is believed to have been formed insupernovae (as are most other heavy elements). Were it to relax to its ground state, it would release aphoton with aphoton energy of 75 keV.
It was first reported in 1988 by C. B. Collins[5] that theoretically180m
Ta
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 of180m
Ta
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.[6]
178m2
72Hf
is another reasonably stable nuclear isomer. It possesses a half-life of 31 years and the highest excitation energy of any comparably long-lived isomer. Onegram of pure178m2
Hf
contains approximately 1.33 gigajoules of energy, the equivalent of exploding about 315 kg (700 lb) ofTNT. In the natural decay of178m2
Hf
, the energy is released as gamma rays with a total energy of 2.45 MeV. As with180m
Ta
, there are disputed reports that178m2
Hf
can bestimulated into releasing its energy. Due to this, the substance is being studied as a possible source forgamma-ray lasers. These reports indicate that the energy is released very quickly, so that178m2
Hf
can produce extremely high powers (on the order ofexawatts). Other isomers have also been investigated as possible media forgamma-ray stimulated emission.[1][7]
Holmium's nuclear isomer166m1
67Ho
has a half-life of 1,200 years, which is nearly the longest half-life of any holmium radionuclide. Only163
Ho
, with a half-life of 4,570 years, is more stable.
229
90Th
has a remarkably low-lying metastable isomer only8.355733554021(8) eV above the ground state.[8][9][10] This low energy produces "gamma rays" at a wavelength of148.3821828827(15) nm, in thefar ultraviolet, which allows for direct nuclear laserspectroscopy. 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.[11][12][13][14][15][9] The energy is so low that the ionization state of the atom affects its half-life. Neutral229m
90Th
decays byinternal conversion with a half-life of7±1 μs, but because the isomeric energy is less than thorium's second ionization energy of11.5 eV, this channel is forbidden in thoriumcations and229m
90Th+
decays by gamma emission with a half-life of1740±50 s.[8] This conveniently moderate lifetime allows the development of anuclear clock of unprecedented accuracy.[16][17][10]
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 unitħ in thespin angular momentum. This change is necessary to emit a gamma photon, which has a spin of 1 unit in this system. Integral changes of 2 and more units in angular momentum are possible, but the emitted photons carry off the additional angular momentum. Changes of more than 1 unit are known asforbidden transitions. 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.[18] The highest known spin change of 8 units occurs in the decay of180mTa, which suppresses its decay by a factor of 1035 from that associated with 1 unit. Instead of a natural gamma-decay half-life of 10−12 seconds, it has yet to be observed to decay, and is believed to have a half-life on the order of at least 1025 seconds, or at least2.9×1017 years.
Gamma emission is impossible when the nucleus begins in a zero-spin state, as such an emission would not conserve angular momentum.[citation needed]
Hafnium[19][20] isomers (mainly178m2Hf) have been considered as weapons that could be used to circumvent theNuclear Non-Proliferation Treaty, since it is claimed that they can beinduced to emit very strong gamma radiation. This claim is generally discounted.[21]DARPA had a program to investigate this use of both nuclear isomers.[22] The potential to trigger an abrupt release of energy from nuclear isotopes, a prerequisite to their use in such weapons, is disputed. Nonetheless a 12-member Hafnium Isomer Production Panel (HIPP) was created in 2003 to assess means of mass-producing the isotope.[23]
Technetium isomers99m
43Tc
(with a half-life of 6.01 hours) and95m
43Tc
(with a half-life of 61 days) are used inmedical andindustrial applications.
Nuclear batteries use small amounts (milligrams andmicrocuries) of radioisotopes with high energy densities. In one betavoltaic device design, radioactive material sits atop a device with adjacent layers ofP-type and N-typesilicon. Ionizing radiation directly penetrates the junction and createselectron–hole pairs. Nuclear isomers could replace other isotopes, and with further development, it may be possible to turn them on and off by triggering decay as needed. Current candidates for such use include108Ag,166Ho,177Lu, and242Am. As of 2004, the only successfully triggered isomer was180mTa, which required more photon energy to trigger than was released.[24]
An isotope such as177Lu releases gamma rays by decay through a series of internal energy levels within the nucleus, and it is thought that by learning the triggering cross sections with sufficient accuracy, it may be possible to create energy stores that are 106 times more concentrated thanhigh explosive or other traditional chemical energy storage.[24]
Anisomeric transition orinternal transition (IT) is the decay of a nuclear isomer to a lower-energy nuclear state. The actual process has two types (modes):[25][26]
Isomers may decay into other elements, though the rate of decay may differ between isomers. For example,177mLu can beta-decay to177Hf with a half-life of 160.4 d, or it can undergo isomeric transition to177Lu with a half-life of 160.4 d, which then beta-decays to177Hf with a half-life of 6.68 d.[24]
The emission of a gamma ray from an excited nuclear state allows the nucleus to lose energy and reach a lower-energy state, sometimes itsground state. In certain cases, the excited nuclear state following anuclear reaction or other type ofradioactive decay can become ametastable nuclear excited state. Some nuclei are able to stay in this metastable excited state for minutes, hours, days, or occasionally far longer.
The process of isomeric transition is similar to gamma emission from any excited nuclear state, but differs by involving excited metastable states of nuclei with longer half-lives. As with other excited states, the nucleus can be left in an isomeric state following the emission of analpha particle,beta particle, or some other type of particle.
The gamma ray may transfer its energy directly to one of the most tightly boundelectrons, causing that electron to be ejected from the atom, a process termed thephotoelectric effect. This should not be confused with theinternal conversion process, in which no gamma-ray photon is produced as an intermediate particle.
The transition frequency between theI = 5/2 ground state and theI = 3/2 excited state is determined as:𝜈Th =1/6 (𝜈a + 2𝜈b + 2𝜈c +𝜈d) =2020407384335(2) kHz.
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