Nuclear Isomer

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Viewpoint: Free-Electron Lasers Trigger Nuclear Transitions
Félicie Albert, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

Published February 24, 2014 | Physics 7, 20 (2014) | DOI: 10.1103/Physics.7.20

New theoretical calculations show that an x-ray free-electron-laser pulse can generate transitions in excited nuclei via an indirect process involving electron capture

URL: http://physics.aps.org/articles/v7/20
Excerpt
The consequences of this work could spark new life into the field of nuclear energy storage, where scientists have been studying for years the prospects of using metastable nuclear isomers in high-energy batteries. The advantage of isomers, which are typically produced in nuclear reactions or through irradiation, is that they provide nuclear power with no long-lasting nuclear waste. However, the challenge is the control of the release of the stored nuclear energy. Gunst et al. have shown that the light from an XFEL could trigger the rapid decay of a nuclear isomer. Now, given that this process requires using a large and costly facility, it seems unlikely that there will be a practical energy-storage device in the near future, but this new work could stimulate research on other triggering schemes. Another application could be in the medical community. Long-lived nuclear isomers are of interest for a number of applications in nuclear medicine [8], where they can be used for therapeutic (endoradiotherapy) or diagnostic (nuclear imaging) purposes. In the example of 93mMo triggering described by Gunst et al., a 1-mega-electron-volt photon is emitted in the decay cascade from the triggering level to the ground state, and it can potentially be detected by modern imaging systems or target a specific tumor.

Well, well! We see Wikipedia, from the attempts to understand nuclear isomers below, shows that while research in long-lived nuclear isomers (up to seven minutes) have been 'studied for years', the wikipedia authors yet see reality through old-school ideoms.

A point to be learned: Wikipedia.org 'experts' are often professing from dusty textbooks of their earlier careers, now with time to edit Wikipedia to the yesteryear of their academic heyday, when life was simpler.

DonEMitchell (talk) 02:13, 28 February 2014 (MST)

Based on information from David Hudson and ORME

The following Wikipedia excerpts suggest the military's interest in Hudson's research [1] --gamma ray weapons by a population phase transition of ORME atomic nuclei from a high-spin state.

Concepts

Chemical Isomer

Wikipedia

In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. That is to say, they have different structural formulae. Many isomers share similar if not identical properties in most chemical contexts. This should not be confused with a nuclear isomer, which involves a nucleus at different states of excitement.

Conversion electron

Wikipedia

A conversion electron is an electron which results from interactions with metastable atomic nuclei, which results from radioactive decay processes. A metastable nucleus can transfer its energy to an electron that has a certain probability of being in the nucleus. If this happens, the electron becomes a free electron with a kinetic energy equal to the energy of the metastable state minus the binding energy of the electron. This electron is called a conversion electron. Because of its proximity to the nucleus, the conversion electron usually comes from the K shell. The hole in the electron shell is filled by electrons from other shells thus producing a characteristic X-ray peak. The x-ray may then reproduce the effect and cause the emission of an Auger electron.

Conversion occurs for the same nuclear decays as gamma decay, and hence competes with that process.

Auger electron

Wikipedia

Auger emission is a phenomenon in physics in which the emission of an electron from an atom causes the emission of a second electron. This second ejected electron is called an Auger electron.

The name Auger electron comes from one of its discoverers, Pierre Victor Auger. The name does not come from the similarly-named device, the auger.

When an electron is removed from a core level of an atom, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in a release of energy. Although sometimes this energy is released in the form of an emitted photon, the energy can also be transferred to another electron, which is then ejected from the atom.

Upon ejection the kinetic energy of the Auger electron corresponds to the difference between the energy of the initial electronic transition and the ionization energy for the shell from which the Auger electron was ejected. These energy levels depend on the type of atom and the chemical environment in which the atom was located. Auger electron spectroscopy involves the emission of Auger electrons by bombarding a sample with either X-rays or energetic electrons and measures the intensity of Auger electrons as a function of the Auger electron energy. The resulting spectra can be used to determine the identity of the emitting atoms and some information about their environment.

Auger recombination is a similar Auger effect which occurs in semiconductors. An electron and electron hole (electron-hole pair) can recombine giving up their energy to an electron in the conduction band, increasing its energy.

The reverse effect is known as impact ionization.

Decay processes

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)

Nuclear isomer

Wikipedia

A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons). A nuclear isomer occupies a higher energy state than the corresponding non-excited nucleus, called the ground state. Most nuclear excited states decay by gamma ray emission or internal conversion, though, far from stability, other decay modes are known. Only one long-lived nuclear isomer is found in nature, 180m73Ta. 180mTa has the unusual property that the excited state decays with a half life longer than 1015 years while the lower-energy ground state undergoes beta decay with a half-life of only 8 hours.

It is the observation of DonEMitchell that ORME satisfies the definition for nuclear isomer.

Metastable isomers

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, m2, m3, 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, m2, etc. correlate with increasing levels of excitation energy stored in each of the isomeric states e.g., Hf-177m2 or 177m2Hf).

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 -- specifically, prolate, with an axis of symmetry longer than the other axes (similar to an American football or rugby ball, although with a less pronounced departure from spherical symmetry). In some of these, quantum-mechanical states can exist in which the distribution of protons and neutrons is farther yet from spherical (in fact, about as non-spherical as a football), so much so that de-excitation to the nuclear ground state is strongly hindered. In general these states either de-excite to the ground state (albeit far more slowly than a "usual" excited state) or 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 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.

Nearly-stable Nuclear Isomers

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. By definition, there is no such thing as a "stable" isomer; however, some are so long-lived as to be nearly stable, and can be produced and observed in quantity.

The only nearly-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 1015 years, markedly longer than the age of the universe. This remarkable persistence results from the fact that the excitation energy of the isomeric state is low and both gamma de-excitation to the Ta-180 ground state (which is radioactive and not particularly long lived) and beta decay to hafnium or tungsten are suppressed owing to spin mismatches. 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 an energetic photon with an energy of 75 keV. It was first reported in 1988 by Collins[1] 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[[2].

Another reasonably stable nuclear isomer (with a half-life of 31 years) is hafnium-178m2, which has the highest excitation energy of any comparably long-lived isomer. One gram of pure Hf-178-m2 contains approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. Further, all of the energy is released as 2.45 MeV gamma rays. As with Ta-180m, there are disputed reports that Hf-178-m2 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-m2 can produce extremely high powers (on the order of exawatts). Other isomers have also been investigated as possible media for gamma-ray stimulated emission[[3].

Applications

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[Citation needed]. DARPA has (or had) a program to investigate this use of both nuclear isomers.

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.

See also

References

  1. David Hudson public lectures on formerly proprietary ORME research, MP3 series (http://groupkos.com/science2/library/ormus/hudson/hudson_ORME_lecture.htm)