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Technetium is a chemical element in the periodic table that has the symbol Tc and atomic number 43. The chemical properties of this silvery gray, radioactive, crystalline transition metal are intermediate between rhenium and manganese and it is rarely found in nature. Its short-lived isotope Tc-99m is used in nuclear medicine to diagnose certain cancers, Tc-99 is used as a gamma ray-free source of beta rays, and its pertechnate ion could find use as a corrosion preventer for steel (this possible use is hindered by technetium's radioactivity).

Dmitri Mendeleev predicted many of the properties of element 43, which he called ekamanganese, well before its actual discovery (see Mendeleev's predicted elements). In 1937 its isotope Tc-97 became the first element to be artificially produced, hence its name (from the Greek technètos, meaning "artificial"). Most technetium produced on Earth is a by-product of fission of uranium-235 in nuclear reactors and is extracted from nuclear fuel rods. No isotope of technetium has a half life longer than 4.2 million years (Tc-98), so its detection in red giants in 1952 helped bolster the theory that stars can produce heavier elements.

Notable characteristics
Technetium is a silvery-gray radioactive metal with an appearance similar to platinum. However, it is commonly obtained as a gray powder. Its position in the periodic table is between rhenium and manganese and as predicted by the periodic law its properties are intermediate between those two elements. This element is unusual among the lighter elements because it has no stable isotopes and is therefore extremely rare on Earth.

The metal form of technetium slowly tarnishes in moist air. Its oxides are TcO2 and Tc2O7. Under oxidizing conditions technetium (VII) will exist as the pertechnetate ion, TcO4-. Common oxidation states of technetium include 0, +2, +4, +5, +6 and +7. When in powder form technetium will burn in oxygen. It dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid. It has characteristic spectral lines at 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.

The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields even though technetium is not normally magnetic. The crystal structure of the metal is hexagonal close-packed. Pure metallic single-crystal technetium becomes a type II superconductor at 7.46 K; irregular crystals and trace impurities raise this temperature to 11.2 K for 99.9% pure technetium powder. Below this temperature technetium has a very high magnetic penetration depth, the largest among the elements apart from niobium.

Nuclear medicine
Tc-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 body. It well suited to the role because it emits readily detectable 140 keV gamma rays, it does not emit beta radiation, and it has a short half-life of 6.01 hours (meaning it has almost completely decayed to Tc-99 in 24 hours). In the book Technetium by Klaus Schwochau, 31 different radiopharmaceuticals based on Tc-99m are listed for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gall bladder, kidneys, skeleton, blood and tumors.

Immunoscintigraphy incorporates Tc-99m into a monoclonal antibody (a type of immune system protein) capable of binding to cancer cells. A few hours after injection, medical equipment is used to detect the gamma rays emitted by the Tc-99m; higher concentrations indicate where the cancer is. This technique is particularly useful for detecting hard to find cancers, such as those affecting the intestine. These modified antibodies are sold by the German company Hoechst under the name Scintium.

When Tc-99m is combined with a tin compound it binds to red blood cells and is therefore used to map circulatory system disorders. A pyrophosphate ion with Tc-99m adheres to calcium deposits in damaged heart muscle, making it useful to gauge damage done after a heart attack. The sulfur colloid of Tc-99m is scavenged by the spleen, making it possible to image the structure of that organ.

Radiation exposure due to diagnostic treatment involving Tc-99m can be kept low. While Tc-99m is quite radioactive (allowing small amounts to be easily detected) it has a short half life, after which it decays into the less radioactive Tc-99. In the form administered in these medical tests (usually pertechnate) both isotopes are quickly eliminated from the body (generally within a few days ).

Industrial
Under certain circumstances, a small concentration (5&times;10-5 M) of the pertechnate ion in water can protect iron and carbon steels from corrosion. While (for example) CrO42- can also inhibit corrosion, it requires a concentration ten times as high. In one experiment, a test specimen was kept in an aqueous solution of pertechnate for 20 years and was still uncorroded. The mechanism by which pertechnate prevents corrosion is not well-understood, but seems to involve the reversible formation of a thin surface layer. The effect disappears rapidly if the concentration of pertechnate falls below the minimum concentration or if too high a concentration of other ions is added. The radioactive nature of technetium (3 MBq per liter at the concentrations required) makes this corrosion protection impractical in almost all situations. Nevertheless, corrosion protection by pertechnate ions was proposed (but never adopted) for use in boiling-water reactors.

Like rhenium and palladium, technetium can serve as a catalyst. For certain reactions, for example the dehydrogenation of isopropyl alcohol, it is a far more effective catalyst than either rhenium or palladium. Of course, its radioactivity is a major problem in finding safe applications.

Technetium-99 decays almost entirely by beta decay, emitting beta particles with very consistent low energies and no accompanying gamma rays. Moreover, its very 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 NIST standard beta emitter, used for equipment calibration.

Tc-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.

Pre-discovery search
For a number of years there was a gap in the periodic table between molybdenum (element 42) and ruthenium (element 44). Many early researchers 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. It was given the name polinium but it turned out to be impure iridium. Then in 1846 the element ilmenium was claimed to have been discovered but was determined to be impure niobium. This mistake was repeated in 1847 with the 'discovery' of pelopium. Dmitri Mendeleev predicted that this missing element would be chemically similar to manganese and gave it the name ekamanganese (see Mendeleev's predicted elements).

In 1877 Russian chemist Serge Kern reported discovery of the missing element in platinum ore. Kern named what he thought was the new element davyum, after the noted English chemist Sir Humphry Davy, but it was determined to be a mixture of iridium, rhodium and iron. Another candidate, lucium, followed in 1896 but it was determined to be yttrium. Then in 1908 the Japanese chemist Masataka Ogawa found evidence in the mineral thorianite for what he thought indicated the presence of element 43. Ogawa named the element nipponium, after Japan (which is Nippon in Japanese). Later analysis indicated the presence of rhenium (element 75), not element 43.

Disputed 1925 discovery
German chemists Walter Noddack, Otto Berg and Ida Tacke (later Mrs. Noddack) reported the discovery of element 43 in 1925 and named it masurium (after Masuria in eastern Prussia). The group bombarded columbite with a beam of electrons and deduced element 43 was present by examining X-ray diffraction spectrograms. The wavelength of the X-rays produced is related to the atomic number by a formula derived by Henry Moseley. The team claimed to detect a faint X-ray signal at a wavelength produced by element 43. Contemporary experimenters could not replicate the discovery, and in fact it was dismissed as an error for many years.

It was not until 1998 that this dismissal began to be questioned. John T. Armstrong of the National Institute of Standards and Technology ran computer simulations of the experiments and obtained results very close to those reported by the 1925 team; the claim was further supported by work published by David Curtis of the Los Alamos National Laboratory measuring the (tiny) natural occurrence of technetium. Debate still exists as to whether the 1925 team actually did discover element 43.

Official discovery and later history
Discovery of element 43 has traditionally been assigned to a 1937 experiment in Sicily conducted by Carlo Perrier and Emilio Segrè. The University of Palermo researchers found the technetium isotope Tc-97 in a sample of molybdenum given to Segrè by Ernest Lawrence the year before (Segrè visited Berkeley in the summer of 1936). The sample had previously been bombarded by deuterium nuclei in the University of California, Berkeley cyclotron for several months. University of Palermo officials tried unsuccessfully to force them to name their discovery panormium, after the Latin name for Palermo, Panormus. The researchers instead named element 43 after the Greek word technètos, meaning "artificial", since it was the first element to be artificially produced.

In 1952 astronomer Paul Merril in California detected the spectral signature of technetium (in particular, light at 403.1 nm, 423.8 nm, 426.8 nm, and 429.7 nm) in light from S-type red giants. These massive stars near the end of their lives 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 heavier elements are produced. More recently, it 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. In 1962, technetium-99 was isolated and identified in pitchblende from the Belgian Congo in very small quantities (about 0.2 ng/kg); there it originates as a spontaneous fission product of uranium-238. This discovery was made by B.T. Kenna and P.K. Kuroda. There is also evidence that the Oklo natural nuclear reactor produced significant amounts of technetium-99, which has since decayed to ruthenium-99.

Occurrence
Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a decay product of uranium. In 1999 David Curtis (see above) estimated that a kilogram of uranium contains 1 nanogram (1&times;10-9 g) of technetium. Extraterrestrial technetium was found in some red giant stars (S-, M-, and N-types) that contain an emission line in their spectrum indicating the presence of this element.

Production
In contrast with the rare natural occurrence, bulk quantities of technetium-99 are produced each year from spent nuclear fuel rods, which contain various fission products. The fission of a gram of the rare isotope uranium-235 in nuclear reactors yields 27 mg of Tc-99, giving technetium a fission yield of 6.1%. (Other fissionable isotopes also produce similar yields of technetium.) It is estimated that up to 1994, about 49,000 TBq (78 metric ton) of technetium was produced in nuclear reactors, which is by far the dominant source of terrestrial technetium. However, only a fraction of the production is used commercially. As of 2005, technetium-99 is available to holders of an ORNL permit for USD $83/g plus packing charges.

The actual production of technetium-99 from spent nuclear fuel is a long process. During fuel reprocessing, it appears in the waste liquid, which is highly radioactive. After sitting for several years, the radioactivity has fallen to a point where extraction of the long-lived isotopes, including technetium-99, becomes feasible. Several chemical extraction processes are used yielding technetium-99 metal of high purity.

The meta stable (a state where the nucleus is in an excited state) isotope Tc-99m is produced as a byproduct from the fission of uranium in nuclear reactors. Molybdenum (Mo) containing a large quantity of the Tc-99m parent isotope Mo-99 is extracted from the reactor's radioactive waste and shipped to hospitals. 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 hospital then chemically extracts the technetium from the solution by using a technetium-99m generator ("technetium cow").

Other technetium isotopes are not produced in significant quantities by fission; when needed, they are manufactured by neutron irradiation of parent isotopes (for example, Tc-97 can be made by neutron irradiation of Ru-96).

Part of radioactive waste
Since technetium-99 is a major product of the nuclear fission of both uranium-235 and plutonium-239, it is present in radioactive waste of fission reactors and is produced when a fission bomb is detonated. The amount of artificially-produced technetium in the environment exceeds its natural occurrence to a large extent. This is due to release by atmospheric nuclear testing along with the disposal and processing of high-level radioactive waste. Due to its high fission yield and relatively high half-life, technetium-99 is one of the main components of nuclear waste. Its decay, measured in becquerel per amount of spent fuel, is dominant at about 104 to 106 years after the creation of the nuclear waste.

An estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment up to 1994 by atmospheric nuclear tests. The amount of technetium-99 from nuclear reactors released into the environment up to 1986 is estimated to be on the order of 1000 TBq (about 1600 kg), primarily by nuclear fuel reprocessing; most of this was discharged into the sea. In recent years, reprocessing methods have improved to reduce emissions, 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 into the Irish sea. From 2000 onwards the amount has been limited by regulation to 90 TBq (about 140 kg) per year.

As a result of nuclear fuel reprocessing, technetium has been discharged into the sea in a number of locations, and some seafood contains tiny but measurable quantities. For example, lobster from west Cumbria contains small amounts of technetium.

The long half-life of technetium-99 makes it a major concern when considering long-term disposal of high-level radioactive waste. Current disposal options favor burial in geologically stable rock. The primary danger with such a course is that the waste may come into contact with water, which could leach radioactive contamination into the environment. For this reason, the environmental chemistry of technetium is an active area of research. An alternative disposal method, transmutation, works well for technetium-99.

Isotopes
Technetium is one of two elements in the first 83 that have no stable isotopes (the other such element is promethium). The most stable radioisotopes are Tc-98 with a half-life of 4.2 million years, Tc-97 (half-life: 2.6 million years) and Tc-99 (half-life: 211,100 years).

Twenty-two other radioisotopes have been characterized with atomic masses ranging from 87.933 u (Tc-88) to 112.931 u (Tc-113). Most of these have half-lives that are less than an hour; the exceptions are Tc-93 (2.75 hours), Tc-94 (293 minutes), Tc-95 (20 hours), and Tc-96 (4.28 days).

Technetium also has numerous meta states. Tc-97m is the most stable, with a half-life of 90.1 days (0.097 MeV). This is followed by Tc-95m (half life: 61 days, 0.038 MeV), and Tc-99m (half-life: 6.01 hours, 0.143 MeV). Tc-99m only emits gamma rays, and it decays to Tc-99.

For isotopes lighter than the most stable isotope, Tc-98, the primary decay mode is electron capture, giving molybdenum. For the heavier isotopes, the primary mode is beta emission, giving ruthenium, with the exception that Tc-100 can decay both by beta emission and electron capture.

Technetium-99 is the most common and most readily available isotope, as it is a major product of the fission of uranium-235. One gram of Tc-99 produces 6.2&times;108 disintegrations a second (that is, 0.62 GBq/g).

Stability of technetium isotopes
Technetium and promethium are remarkable among the light elements in that they have no stable isotopes. The reason for this is somewhat complicated.

Using the liquid drop model for atomic nuclei, one can derive a semiempirical formula for the binding energy of a nucleus. This formula predicts a "valley of beta stability" along which nuclides do not undergo beta decay. Nuclides that lie "up the walls" of the valley tend to decay by beta decay towards the center (by emitting an electron, emitting a positron, or capturing an electron). For a fixed number of nucleons A, the binding energies lie on one or more parabolas, with the most stable nuclide at the bottom. One can have more than one parabola because isotopes with an even number of protons and an even number of neutrons are more stable than isotopes with an odd number of neutrons and an odd number of protons. A single beta decay then transforms one into the other. When there is only one parabola, there can be only one stable isotope lying on that parabola. When there are two parabolas, that is, when the number of nucleons is odd, it can happen (rarely) that there is a stable nucleus with an odd number of neutrons and an odd number of protons (although this happens only in three instances). However, if this happens, there canbe no stable isotope with an even number of neutrons and an even number of protons.

For technetium (Z=43), the valley of beta stability is centered at around 98 nucleons. However, for every number of nucleons from 95 to 102, there is already at least one stable nuclide of either molybdenum (Z=42) or ruthenium (Z=44). For the isotopes with odd numbers of nucleons, this immediately rules out a stable isotope of technetium, since there can be only one stable nuclide with a fixed odd number of nucleons. For the isotopes with an even number of nucleons, since technetium has an odd number of protons, any isotope must also have an odd number of neutrons. In such a case, the presence of a stable nuclide having the same number of nucleons and an even number of protons rules out the possibility of a stable nucleus.

Precautions
All isotopes of technetium are radioactive but the element and its compounds are extremely rarely found in nature. 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. In spite of the importance of understanding its toxicity in animals and humans, experimental evidence is scant. It appears to have low chemical toxicity, and even lower radiological toxicity.

When working in a laboratory context, 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. Soft X-rays are emitted when the beta particles are stopped, but as long as the body is kept more than 30 cm away these should pose no problem. The primary hazard when working with technetium is inhalation of dust; 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.