View source for Nuclear isomer

{{Short description|Metastable excited state of a nuclide}}
{{Use dmy dates|date=March 2020}}
{{Nuclear physics|cTopic=Nuclides' classification}}
A '''nuclear isomer''' is a [[metastable]] state of an [[atomic nucleus]], in which one or more [[nucleon]]s (protons or neutrons) occupy excited state levels (higher energy levels). "Metastable" describes nuclei whose excited states have [[Half-life|half-lives]] of 10<sup>−9</sup> seconds or longer,<ref>The standard reference {{NUBASE2020}} uses approximately 10<sup>-7</sup> seconds</ref> 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<sup>−12</sup> seconds). Some references recommend {{val|5|e=-9}} seconds to distinguish the metastable half-life from the normal "prompt" [[Induced gamma emission|gamma-emission]] half-life.<ref name=Walker>{{cite journal |url=http://epubs.surrey.ac.uk/137/1/fulltext.pdf |title=Nuclear Isomers: Recipes from the Past and Ingredients for the Future |last1=Walker |first1=Philip M. |last2=Carroll |first2=James J. | journal = [[Nuclear Physics News]] | volume = 17 | issue = 2 | pages = 11–15 | year = 2007 | doi = 10.1080/10506890701404206|s2cid=22342780 }}</ref>

The half-lives of a number of isomers are far longer than this and may be minutes, hours, or years. For example, the [[Isotopes of tantalum#Tantalum-180m|{{nuclide|Ta|180|m}}]] nuclear isomer survives so long (at least {{val|2.9|e=17}} years<ref name="180mTa_2023">{{Cite journal|last1=Arnquist|first1=I. J.|last2=Avignone III|first2=F. T.|last3=Barabash|first3=A. S.|last4=Barton|first4=C. J.|last5=Bhimani|first5=K. H.|last6=Blalock|first6=E.|last7=Bos|first7=B.|last8=Busch|first8=M.|last9=Buuck|first9=M.|last10=Caldwell|first10=T. S.|last11=Christofferson|first11=C. D.|last12=Chu|first12=P.-H.|last13=Clark|first13=M. L.|last14=Cuesta|first14=C.|last15=Detwiler|first15=J. A.|last16=Efremenko|first16=Yu.|last17=Ejiri|first17=H.|last18=Elliott|first18=S. R.|last19=Giovanetti|first19=G. K.|last20=Goett|first20=J.|last21=Green|first21=M. P.|last22=Gruszko|first22=J.|last23=Guinn|first23=I. S.|last24=Guiseppe|first24=V. E.|last25=Haufe|first25=C. R.|last26=Henning|first26=R.|last27=Aguilar|first27=D. Hervas|last28=Hoppe|first28=E. W.|last29=Hostiuc|first29=A.|last30=Kim|first30=I.|last31=Kouzes|first31=R. T.|last32=Lannen V.|first32=T. E.|last33=Li|first33=A.|last34=López-Castaño|first34=J. M.|last35=Massarczyk|first35=R.|last36=Meijer|first36=S. J.|last37=Meijer|first37=W.|last38=Oli|first38=T. K.|last39=Paudel|first39=L. S.|last40=Pettus|first40=W.|last41=Poon|first41=A. W. P.|last42=Radford|first42=D. C.|last43=Reine|first43=A. L.|last44=Rielage|first44=K.|last45=Rouyer|first45=A.|last46=Ruof|first46=N. W.|last47=Schaper|first47=D. C.|last48=Schleich|first48=S. J.|last49=Smith-Gandy|first49=T. A.|last50=Tedeschi|first50=D.|last51=Thompson|first51=J. D.|last52=Varner|first52=R. L.|last53=Vasilyev|first53=S.|last54=Watkins|first54=S. L.|last55=Wilkerson|first55=J. F.|last56=Wiseman|first56=C.|last57=Xu|first57=W.|last58=Yu|first58=C.-H.|date=13 October 2023|title=Constraints on the Decay of <sup>180m</sup>Ta|arxiv=2306.01965|doi=10.1103/PhysRevLett.131.152501|volume=131|issue=15|article-number=152501|journal=Phys. Rev. Lett.|pmid=37897780 }}</ref>) that it has never been observed to decay spontaneously, and occurs naturally as a [[primordial nuclide]], though uncommon at only 1/8000 of all tantalum. The second most stable isomer is [[Isotopes of bismuth#Bismuth-210m|{{nuclide|Bi|210|m}}]], which does not occur naturally; its half-life is {{val|3.04|e=6}} years to [[alpha 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, [[isotopes of rhenium|{{nuclide|Re|186|m}}]], [[isotopes of iridium|{{nuclide|Ir|192|m2}}]], [[isotopes of polonium|{{nuclide|Po|212|m}}]], [[isotopes of americium#Americium-242m|{{nuclide|Am|242|m}}]] and multiple [[isotopes of holmium|holmium isomers]].

{{Anchor|Isomeric transition}}
{{Anchor|Internal transition}}
The [[gamma decay]] from a metastable state is referred to as '''isomeric transition''' (IT), or ''internal 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 high [[nuclear spin]] change, or [[forbidden transition|"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 of [[thorium-229m]], low excitation alone causes the measurably long life). In {{nuclide|Bi|210|m}}, 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 including {{nuclide|Ta|180|m}}, 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 X<sub>2</sub>/uranium Z, now known as {{nuclide|Pa|234|m|links=no}}/[[protactinium-234|{{nuclide|Pa|234}}]]) was discovered by [[Otto Hahn]] in 1921.<ref>{{cite journal | title = Über ein neues radioaktives Zerfallsprodukt im Uran | author = Hahn, Otto | journal = [[Die Naturwissenschaften]] | volume = 9 | issue = 5 | page = 84 | year = 1921 | doi = 10.1007/BF01491321|bibcode = 1921NW......9...84H | s2cid = 28599831 | url = https://zenodo.org/record/2482506 }}</ref>

Metastable isomers can be produced through any nuclear reaction, including [[radioactive decay]], [[neutron capture]], [[nuclear fission]], and bombardment by [[particle accelerator|accelerated]] 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 more [[gamma ray]]s or [[conversion electron]]s. This is normally a "prompt" process, but sometimes does not rapidly reach to the nuclear [[ground state]], in which case a metastable isomer has formed. This usually occurs as a '''spin isomer''' when the formation of an intermediate excited state has a [[Spin (physics)|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, and if the excitation energy is low; such excited states will generally have long lives and be considered metastable.

After fission, several of the [[Nuclear fission product|fission 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-called ''isomeric yield ratio''.<ref>{{Cite journal|last1=Rakopoulos|first1=V.|last2=Lantz|first2=M.|last3=Solders|first3=A.|last4=Al-Adili|first4=A.|last5=Mattera|first5=A.|last6=Canete|first6=L.|last7=Eronen|first7=T.|last8=Gorelov|first8=D.|last9=Jokinen|first9=A.|last10=Kankainen|first10=A.|last11=Kolhinen|first11=V. S.|date=2018-08-13|title=First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments|url=https://link.aps.org/doi/10.1103/PhysRevC.98.024612|journal=Physical Review C|language=en|volume=98|issue=2|article-number=024612|doi=10.1103/PhysRevC.98.024612|bibcode=2018PhRvC..98b4612R |s2cid=125464341 |issn=2469-9985}}</ref>

{{anchor|Fission isomer}}
A particular kind of metastable isomer is the '''fission isomer''' or '''shape isomer'''. Most [[actinide]] nuclei in their ground states are not spherical, but rather [[prolate spheroid]]al, 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-life|half-lives]] of the order of [[nanosecond]]s or [[microsecond]]s—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 {{nuclide|plutonium|240f}}.

==Nomenclature==
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 {{nuclide|cobalt|58m1}}, 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 {{nuclide|hafnium|178m2}}). The index may be omitted if only one isomer is relevant.

==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 light|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 [[technetium-99m|{{nuclide|Tc|99|m}}]], which decays with a half-life of about 6 hours by emitting a gamma ray of 140.5&nbsp;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 in [[nuclear spin]] required. For example, {{nuclide|Ta|180|m}} has a spin of 9 and the lower states have spins 1 and 2. Similarly, {{nuclide|Tc|99|m}} has a spin of 1/2 and the lower states 7/2 and 9/2.<ref>ENSDF data as compiled at {{NNDC}}</ref> 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 also [[internal 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 of {{nuclide|Tc|99|m}} involves conversion to the spin-7/2 state, then prompt gamma emission to the spin-9/2 ground state; similarly, {{nuclide|Ta|180|m}} could decay through conversion to the spin-2 state, followed by a gamma decay to the ground state. This gamma was looked for in,<ref name="180mTa_2023" /> 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: {{nuclide|Tc|99}} 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); {{nuclide|Ta|180}} 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.

==Artificial de-excitation==
It was first reported in 1988 by C. B. Collins<!--he's not the C. B. Collins with an article, so don't link--><ref>{{cite journal | author=C. B. Collins | title=Depopulation of the isomeric state <sup>180</sup>Ta<sup>m</sup> by the reaction <sup>180</sup>Ta<sup>m</sup>(γ,γ′)<sup>180</sup>Ta | url=http://www.hafniumisomer.org/isomer/180ta.pdf | journal= Physical Review C | volume=37 | pages=2267–2269 | year=1988 | doi=10.1103/PhysRevC.37.2267 | bibcode = 1988PhRvC..37.2267C | issue=5 | pmid=9954706 |display-authors=etal |archive-url=https://web.archive.org/web/20190121175916/http://www.hafniumisomer.org/isomer/180ta.pdf |archive-date=21 January 2019}}</ref> that theoretically {{SimpleNuclide|Ta|180|m}} 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 {{SimpleNuclide|Ta|180|m}} by resonant photo-excitation of intermediate high levels of this nucleus (''E''&nbsp;≈&nbsp;1&nbsp;MeV) was  observed in 1999 by Belic and co-workers in the Stuttgart nuclear physics group.<ref>{{cite journal | author=D. Belic | title=Photoactivation of <sup>180</sup>Ta<sup>m</sup> and Its Implications for the Nucleosynthesis of Nature's Rarest Naturally Occurring Isotope | journal= Physical Review Letters | volume=83 | issue=25 | pages=5242–5245 | year=1999 | doi =10.1103/PhysRevLett.83.5242 | bibcode=1999PhRvL..83.5242B | display-authors=etal}}</ref>

<!-- Citations for this pargraph can be found in the linked article and farther down in this article, under Applications and External links. Please remove those later sections first, and move references if desired. -->
[[Hafnium-178m2|{{nuclide|Hf|178|m2}}]] 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 of {{SimpleNuclide|Hf|178|m2}}, the energy is released as gamma rays with a total energy of 2.45&nbsp;MeV. As with {{SimpleNuclide|Ta|180|m}}, it is thought that {{SimpleNuclide|Hf|178|m2}} can be [[stimulated emission|stimulated]] into releasing its energy. Due to this, the substance has been studied as a possible source for [[gamma-ray laser]]s, and reports have indicated that the energy could be released very quickly, so that {{SimpleNuclide|Hf|178|m2}} can produce extremely high powers (on the order of [[Orders of magnitude (power)|exawatts]]).

Other isomers have also been investigated as possible media for [[Induced gamma emission|gamma-ray stimulated emission]].<ref name=Walker/><ref>{{cite web | title=UNH researchers search for stimulated gamma ray emission | url=http://einstein.unh.edu/nuclear/NucNews/graser_news.html | website=UNH Nuclear Physics Group | year=1997 | access-date=1 June 2006 |archive-url = https://web.archive.org/web/20060905160103/http://einstein.unh.edu/nuclear/NucNews/graser_news.html |archive-date = 5 September 2006}}</ref>

==Other notable isomers==
[[Holmium]]'s nuclear isomer [[holmium-166|{{nuclide|Holmium|166|m1}}]] has a half-life of 1,133 years, which is nearly the longest half-life of any holmium radionuclide. Only {{SimpleNuclide|Holmium|163}}, 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.

[[Thorium-229m|{{nuclide|Thorium|229m}}]] is a remarkably low-lying metastable isomer only {{val|{{#expr:(2020407384335*6.62607015/1.602176634e12) round 12}}|(8)|u=eV}} above the ground state.<ref name=Tiedau2024>{{Cite journal
| last1=Tiedau | first1=J.
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| last3=Zhang | first3=K.
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| date=2024-04-29
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| journal=Physical Review Letters
| volume=132 | issue=18 | article-number=182501
| doi=10.1103/PhysRevLett.132.182501
| doi-access=free
| pmid=38759160
| bibcode=2024PhRvL.132r2501T
}}</ref><ref name=Zhang2024>{{cite journal
 |title=Frequency ratio of the <sup>229m</sup>Th nuclear isomeric transition and the <sup>87</sup>Sr atomic clock
 |first1=Chuankun |last1=Zhang  |first2=Tian |last2=Ooi  |first3=Jacob S. |last3=Higgins
 |first4=Jack F. |last4=Doyle  |first5=Lars |last5=von der Wense  |first6=Kjeld |last6=Beeks
 |first7=Adrian |last7=Leitner  |first8=Georgy |last8=Kazakov  |first9=Peng |last9=Li
 |first10=Peter G. |last10=Thirolf  |first11=Thorsten |last11=Schumm  |first12=Jun |last12=Ye |author-link12=Jun Ye
 |journal=[[Nature (journal)|Nature]] |volume=633 |issue=8028 |pages=63–70
 |date=4 September 2024
 |doi=10.1038/s41586-024-07839-6
 |pmid=39232152 |arxiv=2406.18719
 |bibcode=2024Natur.633...63Z |quote=The transition frequency between the {{math|1=''I'' = 5/2}} ground state and the {{math|1=''I'' = 3/2}} excited state is determined as: {{math|1=
''𝜈''<sub>Th</sub> = {{sfrac|1|6}} (''𝜈''<sub>a</sub> + 2''𝜈''<sub>b</sub> + 2''𝜈''<sub>c</sub> + ''𝜈''<sub>d</sub>) = {{val|2020407384335|(2)|u=kHz}}}}.
}}</ref><ref name=Conover2024>{{cite news
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 |first=Emily |last=Conover |author-link=Emily Conover 
 |date=4 September 2024
 |journal=[[ScienceNews]]
 |url=https://www.sciencenews.org/article/nuclear-clock-ultraprecise-timekeeping
}}</ref>  This low energy produces "gamma rays" at a wavelength of {{val|{{#expr:(299792458e6/2020407384335) round 10}}|(15)|u=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<!--2003 to 2024--> of effort.<ref>{{cite journal 
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| first2=Benedict | last2=Seiferle
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}}</ref><ref name=Zhang2024/> The energy is so low that the ionization state of the atom affects its half-life.  Neutral {{nuclide|Thorium|229|m}} decays by [[internal conversion]] with a half-life of {{val|7|1|u=us}}, but because the isomeric energy is less than thorium's second ionization energy of {{val|11.5|u=eV}}, this channel is forbidden in thorium [[cations]] and {{nuclide|Thorium|229|m|charge=+}} decays by gamma emission with a half-life of {{val|1740|50|u=s}}.{{r|Tiedau2024}}  This conveniently moderate lifetime allows the development of a [[nuclear clock]] of unprecedented accuracy.<ref name="Peik2003">{{cite journal
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==Mechanism of suppression of decay==
:''See also [[Selection rule]]s for technical discussion.
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'') of [[Spin (physics)|spin]] 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 as [[forbidden transition]]s. Each additional unit of spin larger than 1 that the emitted gamma ray must carry inhibits decay rate by about 5 orders of magnitude,<ref name="gamma">Leon van Dommelen, ''[https://web1.eng.famu.fsu.edu/~dommelen/quantum/style_a/ntgd.html Quantum Mechanics for Engineers], Section 14.20 {{Webarchive|url=https://web.archive.org/web/20140405004809/http://www.eng.fsu.edu/~dommelen/quantum/style_a/ntgd.html |date=5 April 2014 }}''</ref> 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.<ref name="gamma"/> From that it should be seen that the spin change of no less than 7 units that would occur in the hypothetical gamma decay of <sup>180m</sup>Ta 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.<ref name="gamma" />

==Applications==
[[Hafnium]]<ref>{{cite web
 |author       = David Hambling
 |title        = Gamma-ray weapons
 |url          = http://www.eurekalert.org/pub_releases/2003-08/ns-gw081303.php
 |publisher    = New Scientist
 |website      = Reuters EurekAlert
 |date         = 16 August 2003
 |access-date  = 12 December 2010
 |archive-date = 27 October 2012
 |archive-url  = https://web.archive.org/web/20121027222639/http://www.eurekalert.org/pub_releases/2003-08/ns-gw081303.php
 }}</ref><ref>{{cite magazine
| author     = Jeff Hecht
| title      = A perverse military strategy
| url        = https://www.newscientist.com/article/mg19025562.200-a-perverse-military-strategy.html
| magazine  = New Scientist
| date       = 19 June 2006
| access-date = 12 December 2010
}}</ref> isomers (mainly <sup>178m2</sup>Hf) 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 gamma emission|induced to emit very strong gamma radiation]]. This claim is generally discounted.<ref name="superbombdispute">{{cite web |last1=Davidson |first1=Seay |title=Superbomb Ignites Science Dispute |url=http://www.veteransforpeace.org/Superbomb_ignites_092903.htm |url-status=bot: unknown |archive-url=https://web.archive.org/web/20050510062612/http://www.veteransforpeace.org/Superbomb_ignites_092903.htm |archive-date=10 May 2005}}</ref> [[DARPA]] had a program to investigate this use of both nuclear isomers.<ref>
{{cite news
 | author=S. Weinberger
 | title=Scary things come in small packages
 | url=https://www.washingtonpost.com/ac2/wp-dyn?pagename=article&contentId=A22099-2004Mar24&notFound=true
 | archive-url=https://web.archive.org/web/20110823054905/http://www.washingtonpost.com/ac2/wp-dyn?pagename=article&contentId=A22099-2004Mar24&notFound=true
 | archive-date=23 August 2011
 | newspaper=[[Washington Post]]
 | date=28 March 2004
}}</ref> 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.<ref>{{cite news |url=http://www.commondreams.org/headlines03/0928-07.htm |title=Superbomb ignites science dispute |date=2003-09-28 |newspaper=San Francisco Chronicle |archive-url=https://web.archive.org/web/20120615015157/http://www.commondreams.org/headlines03/0928-07.htm |archive-date=15 June 2012}}</ref>

[[Technetium]] isomers [[Tc-99m|{{nuclide|Tc|99|m}}]] (with a half-life of 6.01&nbsp;hours) and {{nuclide|Tc|95|m}} (with a half-life of 61&nbsp;days) are used in [[Technetium#Nuclear medicine and biology|medical]] and [[Technetium#Industrial and chemical|industrial]] applications.

===Nuclear batteries===
[[File:Lutetium nuclear isomer energy levels.gif|thumb|upright=2|Nuclear decay pathways for the conversion of lutetium-177<sup>m</sup> to hafnium-177]]
[[Nuclear battery|Nuclear batteries]] use small amounts (milligrams and [[Curie (unit)|microcuries]]) of radioisotopes with high energy densities. In one betavoltaic device design, radioactive material sits atop a device with adjacent layers of [[Extrinsic semiconductor#The two types of semiconductor|P-type and N-type]] [[silicon]]. Ionizing radiation directly penetrates the junction and creates [[electron–hole pair]]s. 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 [[silver-108|<sup>108</sup>Ag]], [[holmium-166|<sup>166</sup>Ho]], [[Lutetium-177|<sup>177</sup>Lu]], and [[americium-242|<sup>242</sup>Am]].  As of 2004, the only successfully triggered isomer was [[Tantalum-180m|<sup>180m</sup>Ta]], which required more photon energy to trigger than was released.<ref name=Litz04 />

An isotope such as <sup>177</sup>Lu 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 10<sup>6</sup> times more concentrated than [[high explosive]] or other traditional chemical energy storage.<ref name=Litz04>{{cite web |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a433348.pdf |archive-url=https://web.archive.org/web/20160304042920/http://www.dtic.mil/dtic/tr/fulltext/u2/a433348.pdf |url-status=live |archive-date=4 March 2016 |title=Controlled extraction of energy from nuclear isomers |author1=M. S. Litz |author2=G. Merkel |name-list-style=amp |date=December 2004}}</ref>

==See also==
*[[Induced gamma emission]]
*[[Isomeric shift]]
*[[Mössbauer effect]]

==References==
{{Reflist}}

==External links==
*[http://www.utdallas.edu/research/quantum/ Research group which presented initial claims of hafnium nuclear isomer de-excitation control.] {{Webarchive|url=https://web.archive.org/web/20090225181815/http://www.utdallas.edu/research/quantum/ |date=25 February 2009 }} – The Center for Quantum Electronics, The University of Texas at Dallas.
*[[JASON Defense Advisory Group]] [http://www.fas.org/irp/agency/dod/jason/he.pdf report on high energy nuclear materials] mentioned in the ''Washington Post'' story above
*{{Cite magazine| url=http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_57/iss_5/21_1.shtml | date=May 2004| magazine= Physics Today| pages=21–24 | author=Bertram Schwarzschild| title=Conflicting Results on a Long-Lived Nuclear Isomer of Hafnium Have Wider Implications | doi = 10.1063/1.1768663 | volume=57| issue=5|bibcode = 2004PhT....57e..21S }}
*[https://web.archive.org/web/20070316182840/http://www.hafniumisomer.org/Hafnium_isomer_triggering.htm Confidence for Hafnium Isomer Triggering in 2006.] – The Center for Quantum Electronics, The University of Texas at Dallas.
*[https://web.archive.org/web/20070928081412/http://www.hafniumisomer.org/isomer/isomerPubl.htm Reprints of articles about nuclear isomers in peer reviewed journals.] – The Center for Quantum Electronics, The University of Texas at Dallas.

{{Nuclear processes}}
{{Authority control}}

{{DEFAULTSORT:Nuclear Isomer}}
[[Category:Nuclear physics|Isomer, nuclear]]