Editing Fermium (section)

==Occurrence==
===Production===
[[File:Elutionskurven Fm Es Cf Bk Cm Am.png|thumb|[[Elution]]: chromatographic separation of Fm(100), Es(99), Cf, Bk, Cm and Am]]
Fermium is produced by the bombardment of lighter [[actinide]]s with [[neutron]]s in a nuclear reactor. Fermium-257 is the heaviest isotope that is obtained via neutron capture, and can only be produced in picogram quantities.{{efn|All isotopes of elements Z &gt; 100 can only be produced by accelerator-based nuclear reactions with charged particles and can be obtained only in tracer quantities (e.g., 1 million atoms for Md (''Z''&nbsp;{{=}}&nbsp;101) per hour of irradiation (see Silva 2006).}}<ref>{{cite book |last1=Luig |first1=Heribert |last2=Keller |first2=Cornelius |last3=Wolf |first3=Walter |last4=Shani |first4=Jashovam |last5=Miska |first5=Horst |last6=Zyball |first6=Alfred |last7=Gervé |first7=Andreas |last8=Balaban |first8=Alexandru T. |last9=Kellerer |first9=Albrecht M. |last10=Griebel |first10=Jürgen |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |doi=10.1002/14356007.a22_499 |chapter=Radionuclides |isbn=978-3527306732}}</ref> The major source is the 85&nbsp;MW [[High Flux Isotope Reactor]] (HFIR) at the [[Oak Ridge National Laboratory]] in [[Tennessee]], USA, which is dedicated to the production of transcurium (''Z''&nbsp;>&nbsp;96) elements.<ref>{{cite web|title = High Flux Isotope Reactor|url = http://neutrons.ornl.gov/facilities/HFIR/|publisher = Oak Ridge National Laboratory|access-date = 2010-09-23}}</ref> Lower mass fermium isotopes are available in greater quantities, though these isotopes (<sup>254</sup>Fm and <sup>255</sup>Fm) are comparatively short-lived. In a "typical processing campaign" at Oak Ridge, tens of grams of [[curium]] are irradiated to produce decigram quantities of [[californium]], milligram quantities of [[berkelium]] and [[einsteinium]], and picogram quantities of fermium.<ref>{{cite journal|first1 = C. E.|last1 = Porter| first2 = F. D. Jr. |last2 = Riley|first3 = R. D.|last3 = Vandergrift|first4 = L. K.|last4 = Felker|title = Fermium Purification Using Teva Resin Extraction Chromatography|journal = Sep. Sci. Technol.|volume = 32|issue = 1–4|date = 1997|pages = 83–92|doi = 10.1080/01496399708003188|url = https://zenodo.org/record/1234415}}</ref> However, nanogram<ref>{{cite journal|first1 = M.|last1 = Sewtz|first2 = H.|last2 = Backe|first3 = A.|last3 = Dretzke|first4 = G.|last4 = Kube|first5 = W.|last5 = Lauth|first6 = P.|last6 = Schwamb|first7 = K.|last7 = Eberhardt|first8 = C.|last8 = Grüning|first9 = P.|last10 = Trautmann|first10 = N.|last11 = Kunz|first11 = P.|last12 = Lassen|first12 = J.|last13 = Passler|first13 = G.|last14 = Dong|first14 = C.|last15 = Fritzsche|first15 = S.|last16 = Haire|first16 = R.|last9 = Thörle
|s2cid = 16234935|title = First Observation of Atomic Levels for the Element Fermium (''Z''=100)|journal = Phys. Rev. Lett.|volume = 90|issue = 16|page = 163002|date = 2003|doi = 10.1103/PhysRevLett.90.163002|bibcode=2003PhRvL..90p3002S|pmid=12731975}}</ref> quantities of fermium can be prepared for specific experiments. The quantities of fermium produced in 20–200&nbsp;kiloton thermonuclear explosions is believed to be of the order of milligrams, although it is mixed in with a huge quantity of debris; 4.0&nbsp;picograms of <sup>257</sup>Fm was recovered from 10&nbsp;kilograms of debris from the "[[Operation Mandrel|Hutch]]" test (16&nbsp;July 1969).<ref>{{cite journal|last1 = Hoff|first1 = R. W.|last2 = Hulet|first2 = E. K.|date = 1970|title = Engineering with Nuclear Explosives|volume = 2|pages = 1283–1294}}</ref> The Hutch experiment produced an estimated total of 250 micrograms of <sup>257</sup>Fm.

After production, the fermium must be separated from other actinides and from [[lanthanide]] fission products. This is usually achieved by [[ion-exchange chromatography]], with the standard process using a cation exchanger such as Dowex&nbsp;50 or T<small>EVA</small> eluted with a solution of ammonium α-hydroxyisobutyrate.<ref name="Silva" /><ref>{{cite journal|last1 = Choppin|first1 = G. R.|last2 = Harvey|first2 = B. G.|last3 = Thompson|first3 = S. G.|date = 1956|title = A new eluant for the separation of the actinide elements|journal = J. Inorg. Nucl. Chem.|volume = 2|issue = 1|pages = 66–68|doi = 10.1016/0022-1902(56)80105-X|url = https://escholarship.org/content/qt73d377r3/qt73d377r3.pdf?t=p0fvlf}}</ref> Smaller cations form more stable complexes with the α-hydroxyisobutyrate anion, and so are preferentially eluted from the column.<ref name="Silva" /> A rapid [[fractional crystallization (chemistry)|fractional crystallization]] method has also been described.<ref name="Silva" /><ref>{{cite journal|last1 = Mikheev|first1 = N. B.|last2 = Kamenskaya|first2 = A. N.|last3 = Konovalova|first3 = N. A.|last4 = Rumer|first4 = I. A.|last5 = Kulyukhin|first5 = S. A.|title = High-speed method for the separation of fermium from actinides and lanthanides|date =1983|journal = Radiokhimiya|volume = 25|issue = 2|pages = 158–161}}</ref>

Although the most stable isotope of fermium is <sup>257</sup>Fm, with a [[half-life]] of 100.5&nbsp;days, most studies are conducted on <sup>255</sup>Fm (''t''<sub>1/2</sub>&nbsp;= 20.07(7)&nbsp;hours), since this isotope can be easily isolated as required as the decay product of <sup>255</sup>Es (''t''<sub>1/2</sub>&nbsp;= 39.8(12)&nbsp;days).<ref name="Silva" />

===Synthesis in nuclear explosions===
The analysis of the debris at the 10-[[TNT equivalent|megaton]] ''Ivy Mike'' nuclear test was a part of long-term project, one of the goals of which was studying the efficiency of production of transuranium elements in high-power nuclear explosions. The motivation for these experiments was as follows: synthesis of such elements from uranium requires multiple neutron capture. The probability of such events increases with the neutron flux, and nuclear explosions are the most powerful neutron sources, providing densities on the order 10{{sup|23}} neutrons/cm{{sup|2}} within a microsecond, i.e. about 10{{sup|29}} neutrons/(cm{{sup|2}}·s). For comparison, the flux of the HFIR reactor is 5{{e|15}} neutrons/(cm{{sup|2}}·s). A dedicated laboratory was set up right at [[Enewetak Atoll]] for preliminary analysis of debris, as some isotopes could have decayed by the time the debris samples reached the U.S. The laboratory was receiving samples for analysis, as soon as possible, from airplanes equipped with paper filters which flew over the atoll after the tests. Whereas it was hoped to discover new chemical elements heavier than fermium, those were not found after a series of megaton explosions conducted between 1954 and 1956 at the atoll.<ref name="s39">Seaborg, p. 39</ref>

[[File:ActinideExplosionSynthesis.png|thumb|upright=1.4|left|Estimated yield of transuranium elements in the U.S. nuclear tests Hutch and Cyclamen.<ref name="s40" />]]
The atmospheric results were supplemented by the underground test data accumulated in the 1960s at the [[Nevada National Security Site|Nevada Test Site]], as it was hoped that powerful explosions conducted in confined space might result in improved yields and heavier isotopes. Apart from traditional uranium charges, combinations of uranium with americium and thorium have been tried, as well as a mixed plutonium-neptunium charge. They were less successful in terms of yield, which was attributed to stronger losses of heavy isotopes due to enhanced fission rates in heavy-element charges. Isolation of the products was found to be rather problematic, as the explosions were spreading debris through melting and vaporizing rocks under the great depth of 300–600 meters, and drilling to such depth in order to extract the products was both slow and inefficient in terms of collected volumes.<ref name="s39" /><ref name="s40">Seaborg, p. 40</ref>

Among the nine underground tests, which were carried between 1962 and 1969 and codenamed Anacostia (5.2 [[TNT equivalent|kilotons]], 1962), Kennebec (<5 kilotons, 1963), Par (38 kilotons, 1964), Barbel (<20 kilotons, 1964), Tweed (<20 kilotons, 1965), Cyclamen (13 kilotons, 1966), Kankakee (20-200 kilotons, 1966), Vulcan (25 kilotons, 1966) and Hutch (20-200 kilotons, 1969),<ref>[http://www.nv.doe.gov/library/publications/historical/DOENV_209_REV15.pdf United States Nuclear Tests July 1945 through September 1992] {{webarchive |url=https://web.archive.org/web/20100615231826/http://www.nv.doe.gov/library/publications/historical/DOENV_209_REV15.pdf |date=June 15, 2010 }}, DOE/NV--209-REV 15, December 2000</ref> the last one was most powerful and had the highest yield of transuranium elements. In the dependence on the atomic mass number, the yield showed a saw-tooth behavior with the lower values for odd isotopes, due to their higher fission rates.<ref name="s40" /> The major practical problem of the entire proposal, however, was collecting the radioactive debris dispersed by the powerful blast. Aircraft filters adsorbed only about 4{{e|-14}} of the total amount and collection of tons of corals at Enewetak Atoll increased this fraction by only two orders of magnitude. Extraction of about 500 kilograms of underground rocks 60 days after the Hutch explosion recovered only about 10{{sup|−7}} of the total charge. The amount of transuranium elements in this 500-kg batch was only 30 times higher than in a 0.4&nbsp;kg rock picked up 7 days after the test. This observation demonstrated the highly nonlinear dependence of the transuranium elements yield on the amount of retrieved radioactive rock.<ref name="s43">Seaborg, p. 43</ref> In order to accelerate sample collection after the explosion, shafts were drilled at the site not after but before the test, so that the explosion would expel radioactive material from the epicenter, through the shafts, to collecting volumes near the surface. This method was tried in the Anacostia and Kennebec tests and instantly provided hundreds of kilograms of material, but with actinide concentrations 3 times lower than in samples obtained after drilling; whereas such a method could have been efficient in scientific studies of short-lived isotopes, it could not improve the overall collection efficiency of the produced actinides.<ref name="s44">Seaborg, p. 44</ref>

Though no new elements (apart from einsteinium and fermium) could be detected in the nuclear test debris, and the total yields of transuranium elements were disappointingly low, these tests did provide significantly higher amounts of rare heavy isotopes than previously available in laboratories. For example, 6{{e|9}} atoms of {{sup|257}}Fm could be recovered after the Hutch detonation. They were then used in the studies of thermal-neutron induced fission of {{sup|257}}Fm and in discovery of a new fermium isotope {{sup|258}}Fm. Also, the rare isotope {{sup|250}}Cm was synthesized in large quantities, which is very difficult to produce in nuclear reactors from its progenitor {{sup|249}}Cm; the half-life of {{sup|249}}Cm (64 minutes) is much too short for months-long reactor irradiations, but is very "long" on the explosion timescale.<ref name="s47">Seaborg, p. 47</ref>

===Natural occurrence===
Because of the short half-life of all known isotopes of fermium, any [[Primordial nuclide|primordial]] fermium, that is fermium present on Earth during its formation, has decayed by now. Synthesis of fermium from naturally occurring uranium and thorium in the Earth's crust requires multiple neutron captures, which is extremely unlikely. Therefore, most fermium is produced on Earth in laboratories, high-power nuclear reactors, or in [[nuclear test]]s, and is present for only a few months afterward. The [[transuranic element]]s [[americium]] to fermium did occur naturally in the [[natural nuclear fission reactor]] at [[Oklo]], but no longer do so.<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|date=2011|publisher=Oxford University Press|location=New York, NY|isbn=978-0-19-960563-7}}</ref>