A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state (higher energy) levels. "Metastable" describes nuclei whose excited states have half-lives 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). The term "metastable" is usually restricted to isomers with half-lives of 10−9 seconds or longer. Some references recommend 5 × 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, the 180m 73Ta nuclear isomer survives so long (at least 1015 years) 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 by 180m 73Ta as well as 186m 75Re , 192m2 77Ir , 210m 83Bi , 212m 84Po , 242m 95Am and multiple holmium isomers.
Sometimes, the gamma 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 be forbidden 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 as 234m 91Pa /234 91Pa ) was discovered by Otto Hahn in 1921.[2]
Nuclei of nuclear isomers
The nucleus of a nuclear isomer occupies a higher energy state than the non-excited nucleus existing in the ground state. In an excited state, one or more of the protons or neutrons in a nucleus occupy a nuclear 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 by fluorescence. In electronic transitions, this process usually involves emission of light near the visible range. The amount of energy released is related to bond-dissociation energy or ionization energy and is usually in the range of a few to few tens of eV per bond. However, a much stronger type of binding energy, the nuclear binding energy, is involved in nuclear processes. Due to this, most nuclear excited states decay by gamma ray emission. For example, a well-known nuclear isomer used in various medical procedures is 99m 43Tc , which decays with a half-life of about 6 hours by emitting a gamma ray of 140keV 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 in nuclear spin needed to emit a gamma ray. For example, 180m 73Ta has a spin of 9 and must gamma-decay to 180 73Ta with a spin of 1. Similarly, 99m 43Tc has a spin of 1/2 and must gamma-decay to 99 43Tc with a spin of 9/2.
While most metastable isomers decay through gamma-ray emission, they can also decay through internal 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 re-arrange in a different way.
In nuclei that are far from stability in energy, even more decay modes are known.
After fission, several of the fission fragments that may be produced have a metastable isomeric state. These fragments are usually produced in a highly excited state, in terms of energy and angular 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-called isomeric yield ratio.[3]
Metastable isomers can be produced through nuclear fusion or other nuclear reactions. 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. Sometimes the de-excitation does not completely proceed rapidly to the nuclear ground state. This usually occurs as a spin isomer when the formation of an intermediate excited state has a spin 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 particular isotope are usually designated with an "m". This designation is placed after the mass number of the atom; for example, cobalt-58m1 is abbreviated 58m1 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, or 178m2 72Hf ).
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 prolate spheroidal, with an axis of symmetry longer than the other axes, similar to an American football or rugby 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 undergo spontaneous fission with half-lives of the order of nanoseconds or microseconds—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. of plutonium-240, can be denoted as plutonium-240f or 240f 94Pu .
Nearly stable isomers
Most nuclear excited states are very unstable and "immediately" radiate away the extra energy after existing on the order of 10−12seconds. As a result, the characterization "nuclear isomer" is usually applied only to configurations with half-lives of 10−9seconds 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 is 180m 73Ta , which is present in all tantalum samples at about 1 part in 8,300. Its half-life is at least 1015 years, markedly longer than the age of the universe. The low excitation energy of the isomeric state causes both gamma de-excitation to the 180 Ta 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. The origin of this isomer is mysterious, though it is believed to have been formed in supernovae (as are most other heavy elements). Were it to relax to its ground state, it would release a photon with a photon energy of 75keV.
It was first reported in 1988 by C. B. Collins[4] that theoretically 180m 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 of 180m Ta by resonant photo-excitation of intermediate high levels of this nucleus (E≈1MeV) was observed in 1999 by Belic and co-workers in the Stuttgart nuclear physics group.[5]
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. One gram of pure 178m2 Hf contains approximately 1.33 gigajoules of energy, the equivalent of exploding about 315kg (700lb) of TNT. In the natural decay of 178m2 Hf , the energy is released as gamma rays with a total energy of 2.45MeV. As with 180m Ta , there are disputed reports that 178m2 Hf can be stimulated into releasing its energy. Due to this, the substance is being studied as a possible source for gamma-ray lasers. These reports indicate that the energy is released very quickly, so that 178m2 Hf can produce extremely high powers (on the order of exawatts). Other isomers have also been investigated as possible media for gamma-ray stimulated emission.[1][6]
Holmium's nuclear isomer 166m1 67Ho has a half-life of 1,200 years, which is nearly the longest half-life of any holmium radionuclide. Only 163 Ho , with a half-life of 4,570 years, is more stable.
229 90Th has a remarkably low-lying metastable isomer only 8.355733554021(8)eV above the ground state.[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.[10][11][12][13][14][8] The energy is so low that the ionization state of the atom affects its half-life. Neutral 229m 90Th decays by internal conversion with a half-life of 7±1μs, but because the isomeric energy is less than thorium's second ionization energy of 11.5eV, this channel is forbidden in thorium cations and 229m 90Th+ decays by gamma emission with a half-life of 1740±50s.[7] This conveniently moderate lifetime allows the development of a nuclear clock of unprecedented accuracy.[15][16][9]
High-spin suppression of decay
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. 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 as forbidden 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.[17] The highest known spin change of 8 units occurs in the decay of 180mTa, 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 a half-life of more than 1023 seconds, or at least 3×1015 years, and thus has yet to be observed to decay.
Gamma emission is impossible when the nucleus begins in a zero-spin state, as such an emission would not conserve angular momentum.[citation needed]
Applications
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. This claim is generally discounted.[20]DARPA had a program to investigate this use of both nuclear isomers.[21] 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.[22]
Technetium isomers 99m 43Tc (with a half-life of 6.01hours) and 95m 43Tc (with a half-life of 61days) are used in medical and industrial applications.
Nuclear batteries
Nuclear decay pathways for the conversion of lutetium-177 to hafnium-177
Nuclear batteries use small amounts (milligrams and microcuries) of radioisotopes with high energy densities. In one betavoltaic device design, radioactive material sits atop a device with adjacent layers of P-type and N-typesilicon. Ionizing radiation directly penetrates the junction and creates electron–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 include 108Ag, 166Ho, 177Lu, and 242Am. As of 2004, the only successfully triggered isomer was 180mTa, which required more photon energy to trigger than was released.[23]
An isotope such as 177Lu 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 than high explosive or other traditional chemical energy storage.[23]
Decay processes
An isomeric transition or internal transition (IT) is the decay of a nuclear isomer to a lower-energy nuclear state. The actual process has two types (modes):[24][25]
γ (gamma ray) emission (emission of a high-energy photon),
internal conversion (the energy is used to eject one of the atom's electrons).
Isomers may decay into other elements, though the rate of decay may differ between isomers. For example, 177mLu can beta-decay to 177Hf with a half-life of 160.4d, or it can undergo isomeric transition to 177Lu with a half-life of 160.4d, which then beta-decays to 177Hf with a half-life of 6.68d.[23]
The emission of a gamma ray from an excited nuclear state allows the nucleus to lose energy and reach a lower-energy state, sometimes its ground state. In certain cases, the excited nuclear state following a nuclear reaction or other type of radioactive decay can become a metastable 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 an alpha particle, beta particle, or some other type of particle.
The gamma ray may transfer its energy directly to one of the most tightly bound electrons, causing that electron to be ejected from the atom, a process termed the photoelectric effect. This should not be confused with the internal conversion process, in which no gamma-ray photon is produced as an intermediate particle.
In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is only slightly pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope. Bowling pins show similar metastability by either merely wobbling for a moment or tipping over completely. A common example of metastability in science is isomerisation. Higher energy isomers are long lived because they are prevented from rearranging to their preferred ground state by barriers in the potential energy.
Stable nuclides are isotopes of a chemical element whose nucleons are in a configuration that does not permit them the surplus energy required to produce a radioactive emission. The nuclei of such isotopes are not radioactive and unlike radionuclides do not spontaneously undergo radioactive decay. When these nuclides are referred to in relation to specific elements they are usually called that element's stable isotopes.
Neutron emission is a mode of radioactive decay in which one or more neutrons are ejected from a nucleus. It occurs in the most neutron-rich/proton-deficient nuclides, and also from excited states of other nuclides as in photoneutron emission and beta-delayed neutron emission. As only a neutron is lost by this process the number of protons remains unchanged, and an atom does not become an atom of a different element, but a different isotope of the same element.
Internal conversion is an atomic decay process where an excited nucleus interacts electromagnetically with one of the orbital electrons of an atom. This causes the electron to be emitted (ejected) from the atom. Thus, in internal conversion, a high-energy electron is emitted from the excited atom, but not from the nucleus. For this reason, the high-speed electrons resulting from internal conversion are not called beta particles, since the latter come from beta decay, where they are newly created in the nuclear decay process.
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, which are repelled electrostatically.
In physics, induced gamma emission (IGE) refers to the process of fluorescent emission of gamma rays from excited nuclei, usually involving a specific nuclear isomer. It is analogous to conventional fluorescence, which is defined as the emission of a photon by an excited electron in an atom or molecule. In the case of IGE, nuclear isomers can store significant amounts of excitation energy for times long enough for them to serve as nuclear fluorescent materials. There are over 800 known nuclear isomers but almost all are too intrinsically radioactive to be considered for applications. As of 2006 there were two proposed nuclear isomers that appeared to be physically capable of IGE fluorescence in safe arrangements: tantalum-180m and hafnium-178m2.
Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is relatively stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of 230Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.
Astatine (85At) has 41 known isotopes, all of which are radioactive; their mass numbers range from 188 to 229. There are also 24 known metastable excited states. The longest-lived isotope is 210At, which has a half-life of 8.1 hours; the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of 56 seconds.
Natural tantalum (73Ta) consists of two stable isotopes: 181Ta (99.988%) and 180m Ta (0.012%).
Technetium (43Tc) is one of the two elements with Z < 83 that have no stable isotopes; the other such element is promethium. It is primarily artificial, with only trace quantities existing in nature produced by spontaneous fission or neutron capture by molybdenum. The first isotopes to be synthesized were 97Tc and 99Tc in 1936, the first artificial element to be produced. The most stable radioisotopes are 97Tc, 98Tc, and 99Tc.
Americium (95Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was 241Am in 1944. The artificial element decays by ejecting alpha particles. Americium has an atomic number of 95. Despite 243 Am being an order of magnitude longer lived than 241 Am, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.
Seaborgium (106Sg) is a synthetic element and so has no stable isotopes. A standard atomic weight cannot be given. The first isotope to be synthesized was 263Sg in 1974. There are thirteen known radioisotopes from 258Sg to 271Sg and five known isomers. The longest-lived isotopes are 267Sg with a half-life of 9.8 minutes and 269Sg with a half-life of 5 minutes. Due to a low number of measurements, and the consequent overlapping measurement uncertainties at the confidence level corresponding to one standard deviation, a definite assignment of the most stable isotope cannot be made.
Darmstadtium (110Ds) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was 269Ds in 1994. There are 11 known radioisotopes from 267Ds to 281Ds and 2 or 3 known isomers. The longest-lived isotope is 281Ds with a half-life of 14 seconds. However, the unconfirmed 282Ds might have an even longer half-life of 67 seconds.
In spectroscopy, a forbidden mechanism is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particular selection rule but is allowed if the approximation associated with that rule is not made. For example, in a situation where, according to usual approximations, the process cannot happen, but at a higher level of approximation the process is allowed but at a low rate.
Yrast is a technical term in nuclear physics that refers to a state of a nucleus with a minimum of energy for a given angular momentum. Yr is a Swedish adjective sharing the same root as the English whirl. Yrast is the superlative of yr and can be translated whirlingest, although it literally means "dizziest" or "most bewildered". The yrast levels are vital to understanding reactions, such as off-center heavy ion collisions, that result in high-spin states.
Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. This effect, discovered by Rudolf Mössbauer in 1958, consists of the nearly recoil-free emission and absorption of nuclear gamma rays in solids. The consequent nuclear spectroscopy method is exquisitely sensitive to small changes in the chemical environment of certain nuclei.
The hafnium controversy was a debate over the possibility of "triggering" rapid energy releases, via gamma-ray emission, from 178m2Hf, a nuclear isomer of hafnium. The energy release per event is 5 orders of magnitude higher than in a typical chemical reaction, but 2 orders of magnitude less than a nuclear fission reaction. In 1998, a group led by Carl Collins in the University of Texas at Dallas reported having successfully initiated such a trigger. Signal-to-noise ratios were small in those first experiments, and to date no other group has been able to reproduce these results. Peter Zimmerman described claims of weaponization potential as having been based on "very bad science".
A nuclear clock or nuclear optical clock is a atomic clock being developed that will use the energy of a nuclear isomeric transition as its reference frequency, instead of the atomic electron transition energy used by conventional atomic clocks. Such a clock is expected to be more accurate than the best current atomic clocks by a factor of about 10, with an achievable accuracy approaching the 10−19 level.
A gamma ray, also known as gamma radiation (symbol γ ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays. With frequencies above 30 exahertz (3×1019 Hz) and wavelengths less than 10 picometers (1×10−11 m), gamma ray photons have the highest photon energy of any form of electromagnetic radiation. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900, he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power.
The isomeric shift is the shift on atomic spectral lines and gamma spectral lines, which occurs as a consequence of replacement of one nuclear isomer by another. It is usually called isomeric shift on atomic spectral lines and Mössbauer isomeric shift respectively. If the spectra also have hyperfine structure the shift refers to the center of gravity of the spectra. The isomeric shift provides important information about the nuclear structure and the physical, chemical or biological environment of atoms. More recently the effect has also been proposed as a tool in the search for the time variation of fundamental constants of nature.
↑ D. Belic; etal. (1999). "Photoactivation of 180Tam and Its Implications for the Nucleosynthesis of Nature's Rarest Naturally Occurring Isotope". Physical Review Letters. 83 (25): 5242–5245. Bibcode:1999PhRvL..83.5242B. doi:10.1103/PhysRevLett.83.5242.
1 2 Tiedau, J.; Okhapkin, M. V.; Zhang, K.; Thielking, J.; Zitzer, G.; Peik, E.; Schaden, F.; Pronebner, T.; Morawetz, I.; De Col, L. Toscani; Schneider, F.; Leitner, A.; Pressler, M.; Kazakov, G. A.; Beeks, K. (29 April 2024). "Laser Excitation of the Th-229 Nucleus". Physical Review Letters. 132 (18) 182501. doi:10.1103/PhysRevLett.132.182501.
1 2 Zhang, Chuankun; Ooi, Tian; Higgins, Jacob S.; Doyle, Jack F.; von der Wense, Lars; Beeks, Kjeld; Leitner, Adrian; Kazakov, Georgy; Li, Peng; Thirolf, Peter G.; Schumm, Thorsten; Ye, Jun (4 September 2024). "Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock". Nature. 633 (8028): 63–70. arXiv:2406.18719. doi:10.1038/s41586-024-07839-6. PMID39232152. The transition frequency between the I = 5/2 ground state and the I = 3/2 excited state is determined as: 𝜈Th = 1/6 (𝜈a + 2𝜈b + 2𝜈c + 𝜈d) = 2020407384335(2)kHz.
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