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= Fermium =
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Introduction
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Fermium is a synthetic chemical element; it has symbol Fm and atomic
number 100. It is an actinide and the heaviest element that can be
formed by neutron bombardment of lighter elements, and hence the last
element that can be prepared in macroscopic quantities, although pure
fermium metal has not been prepared yet. A total of 20 isotopes are
known, with 257Fm being the longest-lived with a half-life of 100.5
days.
Fermium was discovered in the debris of the first hydrogen bomb
explosion in 1952, and named after Enrico Fermi, one of the pioneers
of nuclear physics. Its chemistry is typical for the late actinides,
with a preponderance of the +3 oxidation state but also an accessible
+2 oxidation state. Owing to the small amounts of produced fermium and
all of its isotopes having relatively short half-lives, there are
currently no uses for it outside basic scientific research.
Discovery
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Fermium was first discovered in the fallout from the 'Ivy Mike'
nuclear test (1 November 1952), the first successful test of a
hydrogen bomb. Initial examination of the debris from the explosion
had shown the production of a new isotope of plutonium, plutonium-244:
this could only have formed by the absorption of six neutrons by a
uranium-238 nucleus followed by two β− decays. At the time, the
absorption of neutrons by a heavy nucleus was thought to be a rare
process, but the identification of raised the possibility that still
more neutrons could have been absorbed by the uranium nuclei, leading
to new elements.
Element 99 (einsteinium) was quickly discovered on filter papers which
had been flown through clouds from the explosion (the same sampling
technique that had been used to discover ). It was then identified in
December 1952 by Albert Ghiorso and co-workers at the University of
California at Berkeley. They discovered the isotope 253Es (half-life )
that was made by the capture of 15 neutrons by uranium-238 nuclei -
which then underwent seven successive beta decays:
{{NumBlk|:||}}
Some 238U atoms, however, could capture another amount of neutrons
(most likely, 16 or 17).
The discovery of fermium () required more material, as the yield was
expected to be at least an order of magnitude lower than that of
element 99, and so contaminated coral from the Enewetak atoll (where
the test had taken place) was shipped to the University of California
Radiation Laboratory in Berkeley, California, for processing and
analysis. About two months after the test, a new component was
isolated emitting high-energy α-particles () with a half-life of about
a day. With such a short half-life, it could only arise from the β−
decay of an isotope of einsteinium, and so had to be an isotope of the
new element 100: it was quickly identified as 255Fm ().
The discovery of the new elements, and the new data on neutron
capture, was initially kept secret on the orders of the U.S. military
until 1955 due to Cold War tensions. Nevertheless, the Berkeley team
was able to prepare elements 99 and 100 by civilian means, through the
neutron bombardment of plutonium-239, and published this work in 1954
with the disclaimer that it was not the first studies that had been
carried out on the elements. The "Ivy Mike" studies were declassified
and published in 1955.
The Berkeley team had been worried that another group might discover
lighter isotopes of element 100 through ion-bombardment techniques
before they could publish their classified research, and this proved
to be the case. A group at the Nobel Institute for Physics in
Stockholm independently discovered the element, producing an isotope
later confirmed to be 250Fm () by bombarding a target with oxygen-16
ions, and published their work in May 1954. Nevertheless, the priority
of the Berkeley team was generally recognized, and with it the
prerogative to name the new element in honour of Enrico Fermi, the
developer of the first artificial self-sustained nuclear reactor.
Fermi was still alive when the name was proposed, but had died by the
time it became official.
Isotopes
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There are 20 isotopes of fermium listed in NUBASE 2016, with atomic
weights of 241 to 260, of which (257)Fm is the longest-lived with a
half-life of 100.5 days. (253)Fm has a half-life of 3 days, while
(251)Fm of 5.3 h, (252)Fm of 25.4 h, (254)Fm of 3.2 h, (255)Fm of 20.1
h, and (256)Fm of 2.6 hours. All the remaining ones have half-lives
ranging from 30 minutes to less than a millisecond.
The neutron capture product of fermium-257, (258)Fm, undergoes
spontaneous fission with a half-life of just 370(14) microseconds;
(259)Fm and (260)Fm also undergo spontaneous fission ('t'1/2 = 1.5(3)
s and 4 ms respectively). This means that neutron capture cannot be
used to create nuclides with a mass number greater than 257, unless
carried out in a nuclear explosion. As (257)Fm alpha decays to
(253)Cf, and no known fermium isotopes undergo beta minus decay to the
next element, mendelevium, fermium is also the last element that can
be synthesized by neutron-capture. Because of this impediment in
forming heavier isotopes, these short-lived isotopes (258-260)Fm
constitute the "fermium gap."
Production
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Fermium is produced by the bombardment of lighter actinides with
neutrons in a nuclear reactor. Fermium-257 is the heaviest isotope
that is obtained via neutron capture, and can only be produced in
picogram quantities. The major source is the 85 MW High Flux Isotope
Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, USA,
which is dedicated to the production of transcurium ('Z' > 96)
elements. Lower mass fermium isotopes are available in greater
quantities, though these isotopes (254Fm and 255Fm) 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. However, nanogram quantities of
fermium can be prepared for specific experiments. The quantities of
fermium produced in 20-200 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 picograms of 257Fm was recovered
from 10 kilograms of debris from the "Hutch" test (16 July 1969). The
Hutch experiment produced an estimated total of 250 micrograms of
257Fm.
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 50 or TEVA eluted with a solution of ammonium
α-hydroxyisobutyrate. Smaller cations form more stable complexes with
the α-hydroxyisobutyrate anion, and so are preferentially eluted from
the column. A rapid fractional crystallization method has also been
described.
Although the most stable isotope of fermium is 257Fm, with a half-life
of 100.5 days, most studies are conducted on 255Fm ('t'1/2 = 20.07(7)
hours), since this isotope can be easily isolated as required as the
decay product of 255Es ('t'1/2 = 39.8(12) days).
Synthesis in nuclear explosions
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The analysis of the debris at the 10-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(23)
neutrons/cm(2) within a microsecond, i.e. about 10(29)
neutrons/(cm(2)·s). For comparison, the flux of the HFIR reactor is 5
neutrons/(cm(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.
The atmospheric results were supplemented by the underground test data
accumulated in the 1960s at the 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.
Among the nine underground tests, which were carried between 1962 and
1969 and codenamed Anacostia (5.2 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), 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. 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 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(−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 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. 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.
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 atoms of (257)Fm
could be recovered after the Hutch detonation. They were then used in
the studies of thermal-neutron induced fission of (257)Fm and in
discovery of a new fermium isotope (258)Fm. Also, the rare isotope
(250)Cm was synthesized in large quantities, which is very difficult
to produce in nuclear reactors from its progenitor (249)Cm; the
half-life of (249)Cm (64 minutes) is much too short for months-long
reactor irradiations, but is very "long" on the explosion timescale.
Natural occurrence
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Because of the short half-life of all known isotopes of fermium, any
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 tests, and is present for only a few months afterward. The
transuranic elements americium to fermium did occur naturally in the
natural nuclear fission reactor at Oklo, but no longer do so.
Chemistry
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The chemistry of fermium has only been studied in solution using
tracer techniques, and no solid compounds have been prepared. Under
normal conditions, fermium exists in solution as the Fm3+ ion, which
has a hydration number of 16.9 and an acid dissociation constant of
1.6 (p'K' = 3.8). Fm(3+) forms complexes with a wide variety of
organic ligands with hard donor atoms such as oxygen, and these
complexes are usually more stable than those of the preceding
actinides. It also forms anionic complexes with ligands such as
chloride or nitrate and, again, these complexes appear to be more
stable than those formed by einsteinium or californium. It is believed
that the bonding in the complexes of the later actinides is mostly
ionic in character: the Fm(3+) ion is expected to be smaller than the
preceding An(3+) ions because of the higher effective nuclear charge
of fermium, and hence fermium would be expected to form shorter and
stronger metal-ligand bonds.
Fermium(III) can be fairly easily reduced to fermium(II), for example
with samarium(II) chloride, with which fermium(II) coprecipitates. In
the precipitate, the compound fermium(II) chloride (FmCl) was
produced, though it was not purified or studied in isolation. The
electrode potential has been estimated to be similar to that of the
ytterbium(III)/(II) couple, or about −1.15 V with respect to the
standard hydrogen electrode, a value which agrees with theoretical
calculations. The Fm(2+)/Fm(0) couple has an electrode potential of
−2.37(10) V based on polarographic measurements.
Toxicity
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Though few people come in contact with fermium, the International
Commission on Radiological Protection has set annual exposure limits
for the two most stable isotopes. For fermium-253, the ingestion limit
was set at 10(7) becquerels (1 Bq equals one decay per second), and
the inhalation limit at 10(5) Bq; for fermium-257, at 10(5) Bq and
4,000 Bq respectively.
Further reading
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* Robert J. Silva:
[
https://web.archive.org/web/20100717155410/http://radchem.nevada.edu/classes/rdch710/files/Fm%20to%20Lr.pdf
Fermium, Mendelevium, Nobelium, and Lawrencium], in: Lester R. Morss,
Norman M. Edelstein, Jean Fuger (Hrsg.): 'The Chemistry of the
Actinide and Transactinide Elements', Springer, Dordrecht 2006; , p.
1621-1651; .
* Seaborg, Glenn T. (ed.) (1978)
'[
http://www.escholarship.org/uc/item/92g2p7cd.pdf Proceedings of the
Symposium Commemorating the 25th Anniversary of Elements 99 and 100]',
23 January 1978, Report LBL-7701
* 'Gmelins Handbuch der anorganischen Chemie', System Nr. 71,
Transurane: Teil A 1 II, p. 19-20; Teil A 2, p. 47; Teil B 1, p. 84.
External links
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* [
http://www.periodicvideos.com/videos/100.htm Fermium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
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http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Fermium
