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16 décembre 2011 5 16 /12 /décembre /2011 02:15

Human dietary intake and deficiency

The human body contains about 0.07 mg of molybdenum per kilogram of weight.Sony VAIO VGN-AW91JS Battery

It occurs in higher concentrations in the liver and kidneys and in lower concentrations in the vertebrae. Molybdenum is also present within human tooth enamel and may help prevent its decay.

The average daily intake of molybdenum varies between 0.12 and 0.24 mg, but it depends on the molybdenum content of the food.Sony VAIO VGN-AW91YS Battery

Pork, lamb and beef liver each have approximately 1.5 parts per million of molybdenum. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, cucumbers and cereal grain. Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state.Sony VAIO VGN-AW92CDS Battery

Studies on rats show a median lethal dose(LD50) as low as 180 mg/kg for some Mo compounds. Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/day of molybdenum can cause diarrhea, growth retardation, infertility, low birth weight and gout; it can also affect the lungs, kidneys and liver.Sony VAIO VGN-AW92CJS Battery

Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues.

Dietary molybdenum deficiency from low soil concentration of molybdenum has been associated with increased rates of esophageal cancer in a geographical band fromnorthern China to Iran.Sony VAIO VGN-AW92CYS Battery

Compared to the United States, which has a greater supply of molybdenum in the soil, people living in these areas have about 16 times greater risk for esophageal squamous cell carcinoma.

Molybdenum deficiency has also been reported as a consequence of non-molybdenum supplemented total parenteral nutrition (complete intravenous feeding) for long periods of time.Sony VAIO VGN-AW92DS Battery

It results in high blood levels of sulfite and urate, in much the same way as molybdenum cofactor deficiency. However, presumably since pure molybdenum deficiency from this mechanism is seen primarily in adults, the neurological consequences have not been as marked as for the congenital cofactor deficiency.Sony VAIO VGN-AW92JS Battery

Related diseases

A congenital molybdenum cofactor deficiency disease, seen in infants, results in interference with the ability of the body to use molybdenum in enzymes. It causes high levels of sulphite and urate, and neurological damage.Sony VAIO VGN-AW92YS Battery

The cause is the inability of the body to synthesize molybdenum cofactor, a heterocyclic molecule which binds molybdenum at the active site in all known human enzymes which use molybdenum.

Copper-molybdenum antagonism

High levels of molybdenum can interfere with the body's uptake of copper, producing copper deficiency.Sony VAIO VGN-AW93FS Battery

Molybdenum prevents plasma proteins from binding to copper, and it also increases the amount of copper that is excreted in urine. Ruminants that consume high amounts of molybdenum develop symptoms including diarrhea, stunted growth, anemia and achromotrichia (loss of hair pigment).Sony VAIO VGN-AW93GS Battery

These symptoms can be alleviated by the administration of more copper into the system, both in dietary form and by injection. The condition, as an effective copper deficiency, can be aggravated by excess sulfur.

Copper reduction or deficiency can also be deliberately induced for therapeutic purposes by the compound ammonium tetrathiomolybdate,Sony VAIO VGN-AW93HS Battery

in which the bright red aniontetrathiomolybdate is the copper-chelating agent. Tetrathiomolybdate was first used therapeutically in the treatment of copper toxicosis in animals. It was then introduced as a treatment in Wilson's disease, a hereditary copper metabolism disorder in humans;Sony VAIO VGN-AW93ZFS Battery

it acts both by competing with copper absorption in the bowel and by increasing excretion. It has also been found to have an inhibitory effect on angiogenesis, potentially via the inhibition of copper ion dependent membrane translocation process invovling a non-classical secretion pathway.Sony VAIO VGN-AW93ZGS Battery

This makes it an interesting investigatory treatment for cancer, age-related macular degeneration, and other diseases featuring excessive blood vessel deposition.

Precautions

Molybdenum dusts and fumes, which can be generated by mining or metalworking, can be toxic, especially if ingested (including dust trapped in the sinuses and later swallowed).Sony VAIO VGN-AW93ZHS Battery

Low levels of prolonged exposure can cause irritation to the eyes and skin. Direct inhalation or ingestion of molybdenum and its oxides should be avoided. OSHA regulations specify the maximum permissible molybdenum exposure in an 8-hour day as 5 mg/m3. Chronic exposure to 60 to 600 mg/m3 can cause symptoms including fatigue, headaches and joint pains.Sony VAIO VGN-BZ11EN Battery

Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowestatomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature.Sony VAIO VGN-BZ11MN Battery

Naturally occurring technetium occurs as a spontaneous fission product in uranium ore or by neutron capture in molybdenum ores. The chemical properties of this silvery gray, crystallinetransition metal are intermediate between rhenium and manganese.

Many of technetium's properties were predicted by Dmitri Mendeleev before the element was discovered.Sony VAIO VGN-BZ11XN Battery

Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional name ekamanganese (Em). In 1937 technetium (specifically the technetium-97 isotope) became the first predominantly artificial element to be produced, hence its name (from the Greek ????????, meaning "artificial").Sony VAIO VGN-BZ12VN Battery

Its short-lived gamma ray-emitting nuclear isomer—technetium-99m—is used in nuclear medicine for a wide variety of diagnostic tests. Technetium-99 is used as a gamma ray-free source of beta particles. Long-lived technetium isotopes produced commercially are by-products of fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods.Sony VAIO VGN-BZ12XN Battery

Because no isotope of technetium has a half-life longer than 4.2 million years (technetium-98), its detection in red giants in 1952, which are billions of years old, helped bolster the theory that stars can produce heavier elements.

Search for element 43

From the 1860s through 1871, early forms of the periodic table proposed by Dimitri Mendeleev contained a gap betweenmolybdenum (element 42) and ruthenium (element 44).Sony VAIO VGN-BZ31VT Battery

In 1871, Mendeleev predicted this missing element would occupy the empty place below manganese and therefore have similar chemical properties. Mendeleev gave it the provisional name ekamanganese(from eka-, the Sanskrit word for one), because the predicted element was one place down from the known element manganese.Sony VAIO VGN-BZ31XT Battery

Many early researchers, both before and after the periodic table was published, were eager to be the first to discover and name the missing element; its location in the table suggested that it should be easier to find than other undiscovered elements. It was first thought to have been found in platinum ores in 1828 and was given the name polinium, but turned out to be impure iridium.Sony VAIO VGN-BZ560 Battery

Then, in 1846, the element ilmenium was claimed to have been discovered, but later was determined to be impure niobium. This mistake was repeated in 1847 with the "discovery" of pelopium.

In 1877, the Russian chemist Serge Kern reported discovering the missing element in platinum ore.Sony VAIO VGN-BZ560N24 Battery

Kern named what he thought was the new element davyum (after the noted English chemist Sir Humphry Davy), but it was eventually determined to be a mixture of iridium, rhodium and iron. Another candidate, lucium, followed in 1896, but it was determined to be yttrium.Sony VAIO VGN-BZ560N30 Battery

Then in 1908, the Japanese chemist Masataka Ogawa found evidence in the mineral thorianite, which he thought indicated the presence of element 43. Ogawa named the element nipponium, after Japan (which is Nippon in Japanese). In 2004, H. K Yoshihara used "a record of X-ray spectrum of Ogawa's nipponium sample from thorianite [which] was contained in a photographic plate preserved by his family.Sony VAIO VGN-BZ560P Battery

The spectrum was read and indicated the absence of the element 43 and the presence of the element 75 (rhenium)."

German chemists Walter Noddack, Otto Berg, and Ida Tacke reported the discovery of element 75 and element 43 in 1925, and named element 43 masurium Sony VAIO VGN-BZ560P20 Battery

(after Masuria in eastern Prussia, now in Poland, the region where Walter Noddack's family originated).[6] The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-raydiffraction spectrograms.

Sony VAIO VGN-BZ560P22 Battery

The wavelength of the X-rays produced is related to the atomic number by a formula derived byHenry Moseley in 1913. The team claimed to detect a faint X-ray signal at a wavelength produced by element 43. Later experimenters could not replicate the discovery, and it was dismissed as an error for many years.Sony VAIO VGN-BZ560P28 Battery

Still, in 1933, a series of articles on the discovery of elements quoted the name masurium for element 43. Debate still exists as to whether the 1925 team actually did discover element 43.

Official discovery and later history

The discovery of element 43 was finally confirmed in a December 1936 experiment at the University of Palermo in Sicily conducted by Carlo Perrier and Emilio Segrè.Sony VAIO VGN-BZ560P30 Battery

In mid-1936, Segrè visited the United States, first Columbia University in New York and then the Lawrence Berkeley National Laboratory in California. He persuaded cyclotron inventor Ernest Lawrence to let him take back some discarded cyclotron parts that had become radioactive.Sony VAIO VGN-BZ560P34 Battery

Lawrence mailed him a molybdenum foil that had been part of the deflector in the cyclotron.

Segrè enlisted his colleague Perrier to attempt to prove, through comparative chemistry, that the molybdenum activity was indeed Z = 43. They succeeded in isolating the isotopes technetium-95 and technetium-97.Sony VAIO VGN-BZ561 Battery

University of Palermo officials wanted them to name their discovery "panormium", after the Latin name for Palermo, Panormus. In 1947 element 43 was named after the Greek word ????????, meaning "artificial", since it was the first element to be artificially produced.Sony VAIO VGN-BZ561N20 Battery

Segrè returned to Berkeley and met Glenn T. Seaborg. They isolated the metastable isotope technetium-99m, which is now used in some ten million medical diagnostic procedures annually.

In 1952, astronomer Paul W. Merrill in California detected the spectral signature of technetium (in particular, light withwavelength of 403.1 nm, 423.8 nm, 426.2 nm, and 429.7 nm) in light from S-type red giants.Sony VAIO VGN-BZ561P20 Battery

The stars were near the end of their lives, yet were rich in this short-lived element, meaning nuclear reactions within the stars must be producing it. This evidence was used to bolster the then-unproven theory that stars are where nucleosynthesis of the heavier elements occurs.Sony VAIO VGN-BZ562P Battery

More recently, such observations provided evidence that elements were being formed by neutron capture in the s-process.

Since its discovery, there have been many searches in terrestrial materials for natural sources of technetium.Sony VAIO VGN-BZ563P Battery

In 1962, technetium-99 was isolated and identified in pitchblende from the Belgian Congo in extremely small quantities (about 0.2 ng/kg); there it originates as a spontaneous fission product of uranium-238. There is also evidence that the Oklonatural nuclear fission reactor produced significant amounts of technetium-99, which has since decayed into ruthenium-99.Sony VAIO VGN-BZAAFS Battery

Physical properties

Technetium is a silvery-gray radioactive metal with an appearance similar to that of platinum. It is commonly obtained as a gray powder. The crystal structure of the pure metal is hexagonal close-packed. Atomic technetium has characteristicemission lines at these wavelengths of light: 363.3 nm, 403.1 nm, 426.2 nm, 429.7 nm, and 485.3 nm.Sony VAIO VGN-BZAAHS Battery

The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields, but will assume random orientations once the field is removed. Pure, metallic, single-crystal technetium becomes a type-II superconductorat temperatures below 7.46 K.Sony VAIO VGN-BZAANS Battery

Below this temperature, technetium has a very high magnetic penetration depth, the largest among the elements apart from niobium.

Chemical properties

Technetium is placed in the seventh group of the periodic table, between rhenium and manganese. Sony VAIO VGN-BZAAPS Battery

As predicted by periodic law, its chemical properties are therefore intermediate between those two elements. Of the two, technetium more closely resembles rhenium, particularly in its chemical inertness and tendency to form covalent bonds. Unlike manganese, technetium does not readily form cations (ions with a net positive charge).Sony VAIO VGN-CR11H/B Battery

Common oxidation states of technetium include +4, +5, and +7. Technetium dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid of any concentration.

Hydride and oxides

Reaction of technetium with hydrogen produces the negatively charged hydride [TcH9]2? ion, which has the same type of crystal structure as (isostructural with) [ReH9]2?.Sony VAIO VGN-CR11S/L Battery

It consists of a trigonal prism with a technetium atom in the center and six hydrogen atoms at the corners. Three more hydrogens make a triangle lying parallel to the base and crossing the prism in its center. Although those hydrogen atoms are not equivalent geometrically, their electronic structure is almost the same.Sony VAIO VGN-CR11S/P Battery

This complex has a coordination number of 9 (meaning that the Tc atom has nine neighbors), which is the highest for a technetium complex. Two hydrogen atoms in the complex can be replaced by sodium (Na+) or potassium (K+) ions.

The metal form of technetium slowly tarnishes in moist air, and in powder form will burn in oxygen.Sony VAIO VGN-CR11S/W Battery

Two oxides have been observed: TcO2 and Tc2O7. Under oxidizing conditions, which tend to strip electrons from atoms, technetium(VII) will exist as the pertechnetate ion, TcO?

4.[20][24]

At temperatures of 400–450 °C, technetium oxidizes to form pale-yellow heptoxide:Sony VAIO VGN-CR11Z/R Battery

4 Tc + 7 O2 ? 2 Tc2O7

It adopts a centrosymmetric structure with two types of Tc-O bonds; their bond lengths are 167 and 184 pm, and the O-Tc-O angle is 180°.

Technetium heptoxide is the precursor to sodium pertechnetate:

Tc2O7 + 2 NaOH ? 2 NaTcO4 + H2O

Black-colored technetium dioxide (TcO2) can be produced by reduction of heptoxide with technetium or hydrogen.Sony VAIO VGN-CR120E/L Battery

Pertechnetic acid (HTcO4) is produced by reacting Tc2O7 with water or oxidizing acids, such as nitric acid, concentrated sulfuric acid, aqua regia, or a mixture of nitric and hydrochloric acids. The resulting dark red, hygroscopic (water absorbing) substance is a strong acid and easily donates protons. In concentrated sulfuric acid Tc(VII) tetraoxidotechnetate anion converts to the octahedral form of technetic(VII) acid TcO3(OH)(H2O)2.Sony VAIO VGN-CR120E/P Battery

The pertechnate (tetroxidotechnetate) anion TcO4? consists of a tetrahedron with oxygens in the corners and Tc atom in the center. Unlike permanganate (MnO4?), it is only a weak oxidizing agent. Pertechnate is often used as a convenient water-soluble source of Tc isotopes, such as 99mTc, and as a catalyst.Sony VAIO VGN-CR120E/R Battery

Sulfides, selenides, tellurides

Technetium forms various sulfides. TcS2 is obtained by direct reaction of technetium and elemental sulfur, while Tc2S7 is formed from the pertechnic acid as follows:

2 HTcO4 + 7 H2S ? Tc2S7 + 8 H2O

In this reaction technetium is reduced to Tc(IV) while exess sulfur forms a disulfide ligand.Sony VAIO VGN-CR120E/W Battery

The produced technetium heptasulfide has a polymeric structure (Tc33–S)(S2)3S6)n with a core similar to Mo33–S)(S2)62–.

Upon heating, technetium heptasulfide decomposes into disulfide and elementary sulfur:

Tc2S7 ? 2 TcS2 + 3 S

Analogous reactions occur with selenium and tellurium.Sony VAIO VGN-CR125E/B Battery

Clusters and organic complexes

Several technetium clusters are known, including Tc4, Tc6, Tc8 and Tc13. The more stable Tc6 and Tc8 clusters have prism shapes where vertical pairs of Tc atoms are connected by triple bonds and the planar atoms by single bonds.Sony VAIO VGN-CR13/B Battery

Every Tc atom makes six bonds, and the remaining valence electrons can be saturated by one axial and two bridging ligand halogen atoms such aschlorine or bromine.

Technetium forms numerous organic complexes, which are relatively well-investigated because of their importance for nuclear medicine.Sony VAIO VGN-CR13/L Battery

Technetium carbonyl (Tc2(CO)10) is a white solid. In this molecule, two technetium atoms are weakly bound to each other; each atom is surrounded by octahedra of five carbonyl ligands. The bond length between Tc atoms, 303 pm, is significantly larger than the distance between two atoms in metallic technetium (272 pm).Sony VAIO VGN-CR13/P Battery

Similar carbonyls are formed by manganese and rhenium.

A technetium complex[note 3] with an organic ligand (shown in the figure on right) is commonly used in nuclear medicine. It has a unique Tc-O functional group (moiety) oriented perpendicularly to the plane of the molecule, where the oxygen atom can be replaced by a nitrogen atom.Sony VAIO VGN-CR13/R Battery

Technetium, atomic number (Z) 43, is the lowest-numbered element in the periodic table that is exclusively radioactive. The second-lightest radioactive element, promethium, has an atomic number of 61. Atomic nuclei with an odd number of protons are less stable than those with even numbers, even when the total number of nucleons (protons + neutrons) are even.Sony VAIO VGN-CR13/W Battery

Odd numbered elements therefore have fewer stable isotopes.

The most stable radioactive isotopes are technetium-98 with a half-life of 4.2 million years (Ma), technetium-97 (half-life: 2.6 Ma) and technetium-99 (half-life: 211,000 years). Thirty other radioisotopes have been characterized with mass numbers ranging from 85 to 118.Sony VAIO VGN-CR131E/L Battery

Most of these have half-lives that are less than an hour; the exceptions are technetium-93 (half-life: 2.73 hours), technetium-94 (half-life: 4.88 hours), technetium-95 (half-life: 20 hours), and technetium-96 (half-life: 4.3 days).

The primary decay mode for isotopes lighter than technetium-98 (9843Tc) is electron capture, giving molybdenum (Z=42).Sony VAIO VGN-CR13G Battery

For heavier isotopes, the primary mode isbeta emission (the loss of an electron or positron), giving ruthenium (Z=44), with the exception that technetium-100 can decay both by beta emission and electron capture.

Technetium also has numerous nuclear isomers, which are isotopes with one or more excited nucleons. Technetium-97m (97mTc ; 'm' stands for metastability) is the most stable, with a half-life of 91 days (0.0965 MeV).Sony VAIO VGN-CR13G/B Battery

This is followed by technetium-95m (half-life: 61 days, 0.03 MeV), and technetium-99m (half-life: 6.01 hours, 0.142 MeV). Technetium-99m only emits gamma rays and decays to technetium-99.

Technetium-99 (9943Tc) is a major product of the fission of uranium-235 (23592U), making it the most common and most readily available Tc isotope.Sony VAIO VGN-CR13G/L Battery

One gram of technetium-99 produces 6.2×108 disintegrations a second (that is, 0.62 GBq/g).

Occurrence and production

Only minute traces occur naturally in the Earth's crust as a spontaneous fission product in uranium ores. A kilogram of uranium contains an estimated 1 nanogram (10?9 g) of technetium.Sony VAIO VGN-CR13G/P Battery

Some red giant stars with the spectral types S-, M-, and N contain an absorption line in their spectrum indicating the presence of technetium. These red-giants are known informally as technetium stars.

Fission waste product

In contrast with its rare natural occurrence, bulk quantities of technetium-99 are produced each year from spent nuclear fuel rods, which contain various fission products.Sony VAIO VGN-CR13G/R Battery

The fission of a gram of uranium-235 in nuclear reactors yields 27 mg of technetium-99, giving technetium a fission product yield of 6.1%. Other fissile isotopes also produce similar yields of technetium, such as 4.9% from uranium-233 and 6.21% from plutonium-239.Sony VAIO VGN-CR13G/W Battery

About 49,000 TBq (78 metric tons) of technetium is estimated to have been produced in nuclear reactors between 1983 and 1994, which is by far the dominant source of terrestrial technetium. Only a fraction of the production is used commercially.

Technetium-99 is produced by the nuclear fission of both uranium-235 and plutonium-239.Sony VAIO VGN-CR13T/L Battery

It is therefore present in radioactive waste and in the nuclear fallout offission bomb explosions. Its decay, measured in becquerels per amount of spent fuel, is dominant after about 104 to 106 years after the creation of the nuclear waste. From 1945 to 1994, an estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment by atmospheric nuclear tests.Sony VAIO VGN-CR13T/P Battery

The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. Reprocessing methods have reduced emissions since then,Sony VAIO VGN-CR13T/R Battery

but as of 2005 the primary release of technetium-99 into the environment is by the Sellafield plant, which released an estimated 550 TBq (about 900 kg) from 1995–1999 into the Irish Sea. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.Sony VAIO VGN-CR13T/W Battery

Discharge of technetium into the sea has resulted in some seafood containing minuscule quantities of this element. For example, European lobster and fish from west Cumbria contain about 1 Bq/kg of technetium.Sony VAIO VGN-CR15/B Battery

Fission product for commercial use

The vast majority of the technetium-99m used in medical work is produced by irradiating highly enriched uranium targets in a reactor, extracting molybdenum-99 from the targets, and recovering the technetium-99m that is produced upon decay of molybdenum-99.Sony VAIO VGN-CR150E/B Battery

Almost two-thirds of the world's supply comes from two reactors; the National Research Universal Reactor at Chalk River Laboratories reactor in Ontario, Canada, and the Petten nuclear reactor of the Netherlands. All major technetium-99m producing reactors were built in the 1960s and are close to the end of their lifetime.Sony VAIO VGN-CR190 Battery

The two new Canadian Multipurpose Applied Physics Lattice Experiment reactors planned and built to produce 200% of the demand of technetium-99m relieved all other producers from building their own reactors. With the cancellation of the already tested reactors in 2008 the future supply of technetium-99m became very problematic.Sony VAIO VGN-CR190E/L Battery

However the Chalk River reactor has been shut down for maintenance since August 2009, with an expected reopening in April 2010, and the Petten reactor had a 6-month scheduled maintenance shutdown beginning on Friday, February 19, 2010. With millions of procedures relying on technetium-99m every year,Sony VAIO VGN-CR190E/P Battery

the low supply has left a gap, leaving some practitioners to revert to techniques not used for 20 years. Somewhat allaying this issue is an announcement from a Polish research reactor, the Maria, that they have developed a technique to isolate technetium. The reactor at Chalk River Laboratory reopened in August 2010 and the Petten reactor reopened September 2010.Sony VAIO VGN-CR190E/R Battery

Waste disposal

The long half-life of technetium-99 and its ability to form an anionic species makes it a major concern for long-term disposal of radioactive waste. Many of the processes designed to remove fission products in reprocessing plants aim at cationic species like caesium (e.g., caesium-137) and strontium (e.g., strontium-90).Sony VAIO VGN-CR190E/W Battery

Hence the pertechnetate is able to escape through these treatment processes. Current disposal options favor burial in continental, geologically stable rock. The primary danger with such a course is that the waste is likely to come into contact with water, which could leach radioactive contamination into the environment.Sony VAIO VGN-CR19VN/B Battery

The anionic pertechnetate and iodide do not adsorb well onto the surfaces of minerals, so they are likely to be washed away. By comparison plutonium, uranium, andcaesium are much more able to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research.Sony VAIO VGN-CR19XN/B Battery

An alternative disposal method, transmutation, has been demonstrated at CERN for technetium-99. This transmutation process is one in which the technetium (technetium-99 as a metal target) is bombarded with neutrons to form the short-lived technetium-100 (half-life = 16 seconds) which decays by beta decay to ruthenium-100.Sony VAIO VGN-CR20 Battery

If recovery of usable ruthenium is a goal, an extremely pure technetium target is needed; if small traces of the minor actinides such as americium and curiumare present in the target, they are likely to undergo fission and form more fission products which increase the radioactivity of the irradiated target.Sony VAIO VGN-CR21/B Battery

The formation of ruthenium-106 (half-life 374 days) from the 'fresh fission' is likely to increase the activity of the final ruthenium metal, which will then require a longer cooling time after irradiation before the ruthenium can be used.

The actual production of technetium-99 from spent nuclear fuel is a long process.Sony VAIO VGN-CR21E/L Battery

During fuel reprocessing, it appears in the waste liquid, which is highly radioactive. After sitting for several years, the radioactivity falls to a point where extraction of the long-lived isotopes, including technetium-99, becomes feasible. Several chemical extraction processes are then used, yielding technetium-99 metal of high purity.Sony VAIO VGN-CR21E/P Battery

Neutron activation

The metastable isotope technetium-99m is produced as a fission product from the fission of uranium or plutonium in nuclear reactors. Because used fuel is allowed to stand for several years before reprocessing,Sony VAIO VGN-CR21E/W Battery

all molybdenum-99 and technetium-99m will have decayed by the time that the fission products are separated from the major actinides in conventional nuclear reprocessing. The liquid left after plutonium–uranium extraction (PUREX) contains a high concentration of technetium asTcO?4 but almost all of this is technetium-99, not technetium-99m.Sony VAIO VGN-CR21S/L Battery

Molybdenum-99 can be formed by the neutron activation of molybdenum-98. Molybdenum-99 has a half-life of 67 hours, so short-lived technetium-99m (half-life: 6 hours), which results from its decay, is being constantly produced. The technetium can then be chemically extracted from the solution by using a technetium-99m generator ("technetium cow", also occasionally called a "molybdenum cow"). Sony VAIO VGN-CR21S/P Battery

By irradiating a highly enriched uranium target to produce molybdenum-99, there is no need for the complex chemical steps which would be required to separate molybdenum from a fission product mixture. This method requires that an enriched uranium target be irradiated with neutrons to form molybdenum-99 as a fission product, then separated.Sony VAIO VGN-CR21S/W Battery

A drawback of this process is that it requires targets containing uranium-235, which are subject to the security precautions of fissile materials.

Other technetium isotopes are not produced in significant quantities by fission; when needed, they are manufactured by neutron irradiation of parent isotopes (for example, technetium-97 can be made by neutron irradiation of ruthenium-96).Sony VAIO VGN-CR21Z/N Battery

Nuclear medicine and biology

Technetium-99m ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the human body.Sony VAIO VGN-CR21Z/R Battery

It is well suited to the role because it emits readily detectable 140 keV gamma rays, and its half-life is 6.01 hours (meaning that about 94% of it decays to technetium-99 in 24 hours). There are at least 31 commonly used radiopharmaceuticals based on technetium-99m for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood, and tumors.Sony VAIO VGN-CR220E/R Battery

The longer-lived isotope technetium-95m, with a half-life of 61 days, is used as a radioactive tracer to study the movement of technetium in the environment and in plant and animal systems.

Industrial and chemical

Technetium-99 decays almost entirely by beta decay, emitting beta particles with consistent low energies and no accompanying gamma rays.Sony VAIO VGN-CR23/B Battery

Moreover, its long half-life means that this emission decreases very slowly with time. It can also be extracted to a high chemical and isotopic purity from radioactive waste. For these reasons, it is a National Institute of Standards and Technology(NIST) standard beta emitter, and is therefore used for equipment calibration.Sony VAIO VGN-CR23/L Battery

Technetium-99 has also been proposed for use in optoelectronic devices and nanoscale nuclear batteries.

Like rhenium and palladium, technetium can serve as a catalyst. For some reactions, for example the dehydrogenation of isopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. However, its radioactivity is a major problem in finding safe catalytic applications.Sony VAIO VGN-CR23/N Battery

When steel is immersed in water, adding a small concentration (55 ppm) of potassium pertechnetate(VII) to the water protects thesteel from corrosion, even if the temperature is raised to 250 °C. For this reason, pertechnetate has been used as a possible anodic corrosion inhibitor for steel, although technetium's radioactivity poses problems which limit this application to self-contained systems.Sony VAIO VGN-CR23/P Battery

While (for example) CrO42? can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a specimen of carbon steel was kept in an aqueous solution of pertechnetate for 20 years and was still uncorroded. The mechanism by which pertechnetate prevents corrosion is not well understood, but seems to involve the reversible formation of a thin surface layer.Sony VAIO VGN-CR23/R Battery

One theory holds that the pertechnetate reacts with the steel surface to form a layer of technetium dioxidewhich prevents further corrosion; the same effect explains how iron powder can be used to remove pertechnetate from water. (Activated carbon can also be used for the same effect.)Sony VAIO VGN-CR23/W Battery

The effect disappears rapidly if the concentration of pertechnetate falls below the minimum concentration or if too high a concentration of other ions is added.

As noted, the radioactive nature of technetium (3 MBq per liter at the concentrations required) makes this corrosion protection impractical in almost all situations.Sony VAIO VGN-CR240E/B Battery

Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.

Precautions

Technetium plays no natural biological role and is not normally found in the human body. Technetium is produced in quantity by nuclear fission, and spreads more readily than many radionuclides.Sony VAIO VGN-CR240N/B Battery

It appears to have low chemical toxicity. For example, no significant change in blood formula, body and organ weights, and food consumption could be detected for rats which ingested up to 15 µg of technetium-99 per gram of food for several weeks.Sony VAIO VGN-CR290EAL Battery

The radiological toxicity of technetium (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope's half-life.Sony VAIO VGN-CR25G/N Battery

All isotopes of technetium must be handled carefully. The most common isotope, technetium-99, is a weak beta emitter; such radiation is stopped by the walls of laboratory glassware. The primary hazard when working with technetium is inhalation of dust;Sony VAIO VGN-CR290EAN Battery

such radioactive contamination in the lungs can pose a significant cancer risk. For most work, careful handling in a fume hood is sufficient; a glove box is not needed.Sony VAIO VGN-CR290EAP Battery,Sony VAIO VGN-CR290EAR Battery,Sony VAIO VGN-CR290EAW Battery

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