Fermium
Encyclopedia
Fermium is a synthetic element
with the symbol Fm. It is the 100th element in the periodic table and a member of the actinide
series. It is 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 fermium metal has not yet been prepared. A total of 19 isotopes are known, with 257Fm being the longest-lived one with a half-life of 100.5 days.
It was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Nobel laureate
Enrico Fermi
, one of the pioneers of nuclear physics
. Its chemistry is typical of 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 its short half-life, there are currently no uses for it outside of basic scientific research. Like all synthetic elements, isotopes of fermium are extremely radioactive and are considered highly toxic.
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
, : this could only have formed by the absorption of six neutron
s 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 the cloud 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
20.5 days) that was made by the capture
of 15 neutron
s by uranium-238
nuclei – which then underwent seven successive beta decay
s:
Some 238U atoms, however, could capture another amount of neutrons (most likely, 16 or 17).
The discovery of fermium (Z = 100) 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 (7.1 MeV) 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 (t½ = 20.07(7) hours).
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 were 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 (t½ = 30 minutes) 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 the recently deceased Enrico Fermi
, the developer of the first artificial self-sustained nuclear reactor.
of 100.5 days. 253Fm has a half-life of 3 days, while 251Fm of 5.3 h, 252Fm of 25.4 h, 254Fm of 3.2 h, 255Fm of 20.1 h, and 256Fm 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, 258Fm, undergoes spontaneous fission
with a half-life of just 370(14) microseconds; 259Fm and 260Fm are also unstable with respect to spontaneous fission (t½ = 1.5(3) s and 4 ms respectively). This means that neutron capture cannot be used to create nuclide
s with a mass number
greater than 257, unless carried out in a nuclear explosion. As 257Fm is an α-emitter
, decaying to 253Cf, fermium is also the last element that can be prepared by a neutron-capture process.
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 nanogram quantities.All isotopes of elements Z > 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 = 101) per hour of irradiation (see reference 1 below)). 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. 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 and microgram 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; 40 picograms of 257Fm was recovered from 10 kilograms of debris from the 'Hutch
' test (16 July 1969).
After production, the fermium must be separated from other actinides and from lanthanoid 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 (t½ = 20.07(7) hours) as this isotope can be easily isolated as required as the decay product of 255Es (t½ = 39.8(12) days).
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 of the order 1023 neutrons/cm² within a microsecond, i.e. about 1029 neutrons/(cm²·s). In comparison, the flux of the HFIR reactor is 5 neutrons/(cm²·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 that 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 was however 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 explosion, shafts were drilled at the site not after but before the test, so that 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 kilograms of material, but with actinide concentration 3 times lower than in samples obtained after drilling; whereas such method could have been efficient in scientific studies of short-lived isotopes, it could not improve the overall collection efficiency of the produced actinides.
Although 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. So 6 atoms of 257Fm could be recovered after the Hutch detonation. They were then used in the studies of thermal-neutron induced fission of 257Fm and in discovery of a new fermium isotope 258Fm. Also the rare 250Cm isotope was synthesized in large quantities, which is very difficult to produce in nuclear reactors from its progenitor 249Cm – the half-life of 249Cm (64 minutes) is much too short for months-long reactor irradiations, but is very "long" on the explosion timescale.
of 1.6 (pKa = 3.8). Fermium(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 Fm3+ ion is expected to be smaller than the preceding An3+ 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 coprecipitates. 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 Fm2+/Fm0 couple has an electrode potential of −2.37(10) V based on polarographic
measurements.
has set annual exposure limits for the two most stable isotopes. For fermium-253, the ingestion limit was set at 107 Becquerel
s (1 Bq is equivalent to one decay per second), and the inhalation limit at 105 Bq; for fermium-257, at 105 Bq and 4000 Bq respectively.
Synthetic element
In chemistry, a synthetic element is a chemical element that is too unstable to occur naturally on Earth, and therefore has to be created artificially. So far 30 synthetic elements have been discovered—that is, synthesized...
with the symbol Fm. It is the 100th element in the periodic table and a member of the actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
series. It is the heaviest element that can be formed by neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities, although fermium metal has not yet been prepared. A total of 19 isotopes are known, with 257Fm being the longest-lived one with a half-life of 100.5 days.
It was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Nobel laureate
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...
, one of the pioneers of nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
. Its chemistry is typical of the late actinides, with a preponderance of the +3 oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
but also an accessible +2 oxidation state. Owing to the small amounts of produced fermium and its short half-life, there are currently no uses for it outside of basic scientific research. Like all synthetic elements, isotopes of fermium are extremely radioactive and are considered highly toxic.
Discovery
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...
' 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
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, : this could only have formed by the absorption of six neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s by a uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
nucleus followed by two β− decays
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
. 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
Einsteinium
Einsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
) was quickly discovered on filter papers which had been flown through the cloud from the explosion (the same sampling technique that had been used to discover ). It was then identified in December 1952 by Albert Ghiorso
Albert Ghiorso
Albert Ghiorso was an American nuclear scientist and co-discoverer of a record 12 chemical elements on the periodic table. His research career spanned five decades, from the early 1940s to the late 1990s.-Early life:...
and co-workers at the University of California at Berkeley. They discovered the isotope 253Es (half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
20.5 days) that was made by the capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they 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...
of 15 neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s by uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
nuclei – which then underwent seven successive beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
s:
Some 238U atoms, however, could capture another amount of neutrons (most likely, 16 or 17).
The discovery of fermium (Z = 100) 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
Berkeley, California
Berkeley is a city on the east shore of the San Francisco Bay in Northern California, United States. Its neighbors to the south are the cities of Oakland and Emeryville. To the north is the city of Albany and the unincorporated community of Kensington...
, for processing and analysis. About two months after the test, a new component was isolated emitting high-energy α-particles (7.1 MeV) with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
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 (t½ = 20.07(7) hours).
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
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
tensions. Nevertheless, the Berkeley team were able to prepare elements 99 and 100 by civilian means, through the neutron bombardment of plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
, 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
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
later confirmed to be 250Fm (t½ = 30 minutes) 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 the recently deceased Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...
, the developer of the first artificial self-sustained nuclear reactor.
Isotopes
There are 19 isotopes of fermium listed in NUBASE 2003, with atomic weights of 242 to 260, of which 257Fm is the longest-lived with a half-lifeHalf-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 100.5 days. 253Fm has a half-life of 3 days, while 251Fm of 5.3 h, 252Fm of 25.4 h, 254Fm of 3.2 h, 255Fm of 20.1 h, and 256Fm 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, 258Fm, undergoes spontaneous fission
Spontaneous fission
Spontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units , spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses...
with a half-life of just 370(14) microseconds; 259Fm and 260Fm are also unstable with respect to spontaneous fission (t½ = 1.5(3) s and 4 ms respectively). This means that neutron capture cannot be used to create nuclide
Nuclide
A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
s with a mass number
Mass number
The mass number , also called atomic mass number or nucleon number, is the total number of protons and neutrons in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion...
greater than 257, unless carried out in a nuclear explosion. As 257Fm is an α-emitter
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
, decaying to 253Cf, fermium is also the last element that can be prepared by a neutron-capture process.
Production
Fermium is produced by the bombardment of lighter actinideActinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
s with neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s in a nuclear reactor. Fermium-257 is the heaviest isotope that is obtained via neutron capture, and can only be produced in nanogram quantities.All isotopes of elements Z > 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 = 101) per hour of irradiation (see reference 1 below)). The major source is the 85 MW High Flux Isotope Reactor
High Flux Isotope Reactor
The High Flux Isotope Reactor is a nuclear research reactor located at Oak Ridge National Laboratory in Oak Ridge, Tennessee, United States...
(HFIR) at the Oak Ridge National Laboratory
Oak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
in Tennessee
Tennessee
Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...
, USA, which is dedicated to the production of transcurium (Z > 96) elements. In a "typical processing campaign" at Oak Ridge, tens of grams of curium
Curium
Curium is a synthetic chemical element with the symbol Cm and atomic number 96. This radioactive transuranic element of the actinide series was named after Marie Skłodowska-Curie and her husband Pierre Curie. Curium was first intentionally produced and identified in summer 1944 by the group of...
are irradiated to produce decigram quantities of californium
Californium
Californium is a radioactive metallic chemical element with the symbol Cf and atomic number 98. The element was first made in the laboratory in 1950 by bombarding curium with alpha particles at the University of California, Berkeley. It is the ninth member of the actinide series and was the...
, milligram quantities of berkelium
Berkelium
Berkelium , is a synthetic element with the symbol Bk and atomic number 97, a member of the actinide and transuranium element series. It is named after the city of Berkeley, California, the location of the University of California Radiation Laboratory where it was discovered in December 1949...
and einsteinium
Einsteinium
Einsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
and picogram quantities of fermium. However, nanogram and microgram 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; 40 picograms of 257Fm was recovered from 10 kilograms of debris from the 'Hutch
Operation Mandrel
Operation Mandrel was a series of 53 nuclear test explosions conducted in 1969 and 1970. This test series included a 1.2 megaton "calibration shot" code-named Milrow, which was detonated underground at Amchitka Island, Alaska, and the 40 kiloton gas stimulation experiment code-named Rulison,...
' test (16 July 1969).
After production, the fermium must be separated from other actinides and from lanthanoid fission products. This is usually achieved by ion exchange chromatography
Ion exchange chromatography
Ion-exchange chromatography is a process that allows the separation of ions and polar molecules based on their charge. It can be used for almost any kind of charged molecule including large proteins, small nucleotides and amino acids. The solution to be injected is usually called a sample, and the...
, 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
Fractional crystallization (chemistry)
In chemistry, fractional crystallization is a method of refining substances based on differences in solubility. If a mixture of two or more substances in solution is allowed to crystallize, for example by allowing the temperature of the solution to decrease, the precipitate will contain more of...
method has also been described.
Although the most stable isotope of fermium is 257Fm, with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 100.5 days, most studies are conducted on 255Fm (t½ = 20.07(7) hours) as this isotope can be easily isolated as required as the decay product of 255Es (t½ = 39.8(12) days).
Synthesis in nuclear explosions
The analysis of the debris at the 10-megatonTNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...
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 of the order 1023 neutrons/cm² within a microsecond, i.e. about 1029 neutrons/(cm²·s). In comparison, the flux of the HFIR reactor is 5 neutrons/(cm²·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 that 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
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...
, 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 was however 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 explosion, shafts were drilled at the site not after but before the test, so that 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 kilograms of material, but with actinide concentration 3 times lower than in samples obtained after drilling; whereas such method could have been efficient in scientific studies of short-lived isotopes, it could not improve the overall collection efficiency of the produced actinides.
Although 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. So 6 atoms of 257Fm could be recovered after the Hutch detonation. They were then used in the studies of thermal-neutron induced fission of 257Fm and in discovery of a new fermium isotope 258Fm. Also the rare 250Cm isotope was synthesized in large quantities, which is very difficult to produce in nuclear reactors from its progenitor 249Cm – the half-life of 249Cm (64 minutes) is much too short for months-long reactor irradiations, but is very "long" on the explosion timescale.
Chemistry
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 constantAcid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
of 1.6 (pKa = 3.8). Fermium(3+) forms complexes with a wide variety of organic ligands with hard
HSAB theory
The HSAB concept is an acronym for 'hard and soft acids and bases. Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways....
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
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
or nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
and, again, these complexes appear to be more stable than those formed by einsteinium
Einsteinium
Einsteinium is a synthetic element with the symbol Es and atomic number 99. It is the seventh transuranic element, and an actinide.Einsteinium was discovered in the debris of the first hydrogen bomb explosion in 1952, and named after Albert Einstein...
or californium
Californium
Californium is a radioactive metallic chemical element with the symbol Cf and atomic number 98. The element was first made in the laboratory in 1950 by bombarding curium with alpha particles at the University of California, Berkeley. It is the ninth member of the actinide series and was the...
. It is believed that the bonding in the complexes of the later actinides is mostly ionic
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
in character: the Fm3+ ion is expected to be smaller than the preceding An3+ ions because of the higher effective nuclear charge
Effective nuclear charge
The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. The term "effective" is used because the shielding effect of negatively charged electrons prevents higher orbital electrons from experiencing the full nuclear charge by the repelling effect...
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
Samarium(II) chloride
Samarium chloride is a chemical compound, used as a radical generating agent in the ketone-mediated intraannulation reaction....
, with which fermium coprecipitates. The electrode potential
Electrode potential
Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...
has been estimated to be similar to that of the ytterbium
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...
(III)/(II) couple, or about −1.15 V with respect to the standard hydrogen electrode
Standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
, a value which agrees with theoretical calculations. The Fm2+/Fm0 couple has an electrode potential of −2.37(10) V based on polarographic
Polarography
Polarography is a subclass of voltammetry where the working electrode is a dropping mercury electrode or a static mercury drop electrode ., useful for its wide cathodic range and renewable surface...
measurements.
Toxicity
Although few people come in contact with fermium, the International Commission on Radiological ProtectionInternational Commission on Radiological Protection
The International Commission on Radiological Protection is an advisory body providing recommendations and guidance on radiation protection; It was founded in 1928 by the International Society of Radiology and was then called the ‘International X-ray and Radium Protection Committee’...
has set annual exposure limits for the two most stable isotopes. For fermium-253, the ingestion limit was set at 107 Becquerel
Becquerel
The becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
s (1 Bq is equivalent to one decay per second), and the inhalation limit at 105 Bq; for fermium-257, at 105 Bq and 4000 Bq respectively.
Literature
- Robert J. Silva: 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; ISBN 1-4020-3555-1, p. 1621–1651; .
- Seaborg, G.T. (ed.) (1978) 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.