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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,[1] 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.[2]
The half-lives of a number of isomers are far longer than this and may be minutes, hours, or years. For example, the180m
73Ta nuclear isomer survives so long (at least2.9×1017 years[3]) that it has never been observed to decay spontaneously, and occurs naturally as aprimordial nuclide, though uncommon at only 1/8000 of all tantalum. The second most stable isomer is210m
83Bi, which does not occur naturally; its half-life is3.04×106 years toalpha decay. The half-life of a nuclear isomer can exceed that of the ground state of the same nuclide, as with the two above, as well as, for example,186m
75Re,192m2
77Ir,212m
84Po,242m
95Am and multipleholmium isomers.
Thegamma decay from a metastable state is referred to asisomeric transition (IT), orinternal transition, though it resembles shorter-lived "prompt" gamma decays in all external aspects with the exception of the longer life. This is generally associated with a highnuclear spin change, or"forbiddenness", which would be required in gamma emission to reach the ground state; this is even more true of beta decays. A low transition energy both slows the transition rate and makes it more likely that only highly forbidden decays are available, so most long-lived isomers have a relatively low excitation energy above the ground state (in the extreme case ofthorium-229m, low excitation alone causes the measurably long life). In210m
83Bi, the forbiddenness of available beta and gamma decays is so high that alpha decay is observed exclusively, though even that is more slow than for the ground state. For most lighter isomers including180m
73Ta, alpha decay is not practically available, but others are not quite so forbidden as those two.
The first nuclear isomer and decay-daughter system (uranium X2/uranium Z, now known as234m
91Pa/234
91Pa) was discovered byOtto Hahn in 1921.[4]
Metastable isomers can be produced through any nuclear reaction, includingradioactive decay,neutron capture,nuclear fission, and bombardment byaccelerated charged particles. A nucleus produced this way generally starts its existence in an excited state that loses its excess energy through the emission of one or moregamma rays orconversion electrons. This is normally a "prompt" process, but sometimes does not rapidly reach to the nuclearground state, in which case a metastable isomer has formed. 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, and if the excitation energy is low; such excited states will generally have long lives and be considered metastable.
After fission, several of thefission fragments may be produced in a metastable isomeric state, after their 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 an isomer is long enough, it is possible to measure its production rate, and comparing it to that of the ground state gives the so-calledisomeric yield ratio.[5]
A particular kind of metastable 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.
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). The index may be omitted if only one isomer is relevant.
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 140.5 keV energy; this is similar to the energy of medical diagnostic X-rays.
Nuclear isomers have long half-lives because their decay to the ground state is highly "forbidden" from the large change innuclear spin required. For example,180m
73Ta has a spin of 9 and the lower states have spins 1 and 2. Similarly,99m
43Tc has a spin of 1/2 and the lower states 7/2 and 9/2.[6] Clearly, the latter is less "forbidden" and, as expected, much faster.
Nuclear transitions, including the 'isomeric' variety, occur not only through gamma-ray emission, but alsointernal conversion where the transition energy instead ejects an electron from the atom. The two process always compete, with gamma emission normally the most common, but as the proportion converted increases with lower energy and also with forbiddenness, it often becomes important for metastable isomers. In fact, the usual decay of99m
43Tc involves conversion to the spin-7/2 state, then prompt gamma emission to the spin-9/2 ground state; similarly,180m
73Ta could decay through conversion to the spin-2 state, followed by a gamma decay to the ground state. This gamma was looked for in,[3] which assumed that to be the likely decay scheme, and not found.
In isotopes whose ground state is unstable, isomers can decay by the same modes rather than going to the ground state. Often both are seen, but rates can differ so much that only one is. Both isomers discussed just above have unstable ground states:99
43Tc undergoes beta decay, though slowly (half-life 211 ky) due to forbiddenness, and the isomer, which is less so, beta-decays over 10,000 times faster (though still a small minority of decays);180
73Ta can fall to either beta decay or electron capture, and quickly (half-life 8.15 h) as it is not forbidden, there the isomer is much more so to either as well as to isomeric transition, explaining its stability.
It was first reported in 1988 by C. B. Collins[7] 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.[8]
178m2
72Hf is another reasonably stable nuclear isomer, with a half-life of 31 years and a remarkably high excitation energy for that life. In the natural decay of178m2
Hf, the energy is released as gamma rays with a total energy of 2.45 MeV. As with180m
Ta, it is thought that178m2
Hf can bestimulated into releasing its energy. Due to this, the substance has been studied as a possible source forgamma-ray lasers, and reports have indicated that the energy could be 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.[2][9]
Holmium's nuclear isomer166m1
67Ho has a half-life of 1,133 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. Both the excitation energy of the former, and the decay energy of the latter, are less than 10 keV.
229m
90Th is a remarkably low-lying metastable isomer only8.355733554021(8) eV above the ground state.[10][11][12] 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.[13][14][15][16][17][11] 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.[10] This conveniently moderate lifetime allows the development of anuclear clock of unprecedented accuracy.[18][19][12]
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 in any given step by 0 or 1 quantum unit (ħ) ofspin angular momentum. This change is necessary to emit a gamma photon in an (electric dipole)allowed transition, as the photon has a spin of 1 unit. Changes of 2 or more units (any possible change is always integer) in angular momentum are possible, but the emitted photon must carry off the additional angular momentum. Changes of more than 1 unit are known asforbidden transitions. Each additional unit of spin larger than 1 that the emitted gamma ray must carry inhibits decay rate by about 5 orders of magnitude,[20] but this again increases at lower energies, and finally IC takes over, as can be seen in figures 14.61 and 14.62 of 'Quantum Mechanics for Engineers' by Leon van Dommelen.[20] From that it should be seen that the spin change of no less than 7 units that would occur in the hypothetical gamma decay of180mTa should result in essentially total suppression and replacement by IC, in agreement with the above.
Gamma emission is impossible when the nucleus begins and ends in a zero-spin state, as such an emission would not conserve angular momentum. Internal conversion remains possible for such transitions.[20]
Hafnium[21][22] 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.[23]DARPA had a program to investigate this use of both nuclear isomers.[24] 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.[25]
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.[26]
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.[26]
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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