Technetium

Technetium is a chemical elementthat has the symbol Tc and the atomic number43. Pronounced tek-nee-s(h)ee-um, the chemical properties of this silvery grey, radioactive, crystalline transition metalare intermediate between rheniumand manganese. Its short-lived isotopeTc-99mis used in nuclear medicinefor a wide variety of diagnostic tests. Tc-99 is used as a gamma ray-free source of beta particles, and its pertechnetate ion(TcO₄^-) could find use as an anodic corrosioninhibitor for steel.

Dmitri Mendeleevpredicted many of the properties of element 43, which he called ekamanganese, well before its actual discovery (see Mendeleev's predicted elements). In 1937its isotope Tc-97 became the first element to be artificially produced, hence its name (from the Greekτεχνητος, meaning "artificial"). Most technetium produced on Earth is a by-product of fissionof uranium-235in nuclear reactorsand is extracted from nuclear fuel rods. No isotope of technetium has a half-lifelonger than 4.2 million years (Tc-98), so its detection in red giantsin 1952helped bolster the theory that stars can produce heavier elements. On earth, technetium occurs naturally only in uranium ores as a product of spontaneous fission; the quantities are minute but have been measured.

Notable characteristics

Technetium is a silvery-grey radioactive metalwith an appearance similar to platinum. However, it is commonly obtained as a grey powder. Its position in the periodic table is between rheniumand manganeseand as predicted by the periodic lawits properties are intermediate between those two elements. This element is unusual among the lighter elements because it has no stable isotopesand is therefore extremely rare on Earth.

The metal form of technetium slowly tarnishesin moist air. Its oxidesare TcO₂ and Tc₂O₇. Under oxidizing conditions technetium (VII) will exist as the pertechnetate ion, TcO₄^-. Common oxidation statesof 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 linesat 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.

The metal form is slightly paramagnetic, meaning its magnetic dipolesalign with external magnetic fieldseven though technetium is not normally magnetic. The crystal structureof the metal is hexagonal close-packed. Pure metallic single-crystal technetium becomes a type II superconductorat 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.

Applications

Nuclear medicine

Tc-99m ("m" indicates that this is a metastablenuclear isomer) is used in radioactive isotope medical tests, for example as a radioactive tracerthat medical equipment can detect in the body. It is well suited to the role because it emits readily detectable 140 keVgamma rays, 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 radiopharmaceuticalsbased on Tc-99m are listed for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, bloodand tumors.

Immunoscintigraphyincorporates Tc-99m into a monoclonal antibody, an immune systemproteincapable of binding to cancercells. A few hours after injection, medical equipment is used to detect the gamma rays emitted by the Tc-99m; higher concentrations indicate where the tumor 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 Hoechstunder the name Scintium.

When Tc-99m is combined with a tincompound it binds to red blood cellsand can therefore be used to map circulatory systemdisorders. A pyrophosphateion with Tc-99m adheres to calciumdeposits in damaged heartmuscle, making it useful to gauge damage after a heart attack. The sulfurcolloid of Tc-99m is scavenged by the spleen, making it possible to image the structure of that organ.

Radiation exposuredue 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 pertechnetate) both isotopes are quickly eliminated from the body (generally within a few days ).

Industrial

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 NISTstandard beta emitter, used for equipment calibration.

Tc-95m, with a half-life of 61 days, is used as a radioactive tracerto study the movement of technetium in the environment and in plant and animal systems.

Like rheniumand palladium, technetium can serve as a catalyst. For certain reactions, for example the dehydrogenationof 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.

Under certain circumstances, a small concentration (5×10^?5 mol/L) of the pertechnetate ion in water can protect iron and carbon steels from corrosion. While (for example) CrO₄^2? 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 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. 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. The radioactive nature of technetium (3 MBqper liter at the concentrations required) makes this corrosion protection impractical in almost all situations. Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.

Technetium-99 has also been proposed for use in optolectric nuclear batteries. Tc-99's beta decay electrons would stimulate an excimermixture, and the light would power a photocell. The battery would consist of an excimer mixture of argon/xenonin a pressure vessel with an internal mirrored surface, finely divided Tc-99, and an intermittent ultrasonicstirrer, illuminating a photocell with a bandgap tuned for the excimer. If the pressure-vessel is carbon fiber/epoxy, the weight to power ratio is said to be comparable to an air-breathing engine with fuel tanks.

History

Pre-discovery search

Image:Dmendeleev.jpg 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 platinumores in 1828. It was given the name polinium but it turned out to be impure iridium. Then in 1846the element ilmenium was claimed to have been discovered but was determined to be impure niobium. This mistake was repeated in 1847with the 'discovery' of pelopium. Dmitri Mendeleevpredicted that this missing element would be chemically similar to manganeseand gave it the name ekamanganese (see Mendeleev's predicted elements).

In 1877Russian chemist Serge Kernreported 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 1896but it was determined to be yttrium. Then in 1908the Japanese chemist Masataka Ogawafound evidence in the mineral thorianitefor 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 Bergand Ida Tacke(later Mrs. Noddack) reported the discovery of element 43 in 1925and named it masurium(after Masuriain eastern Prussia). The group bombarded columbitewith a beam of electronsand deduced element 43 was present by examining X-raydiffraction spectrograms. The wavelengthof the X-rays produced is related to the atomic number by a formula derived by Henry Moseleyin 1913. 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 1998that this dismissal began to be questioned. John T. Armstrongof the National Institute of Standards and Technologyran 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 Curtisof the Los Alamos National Laboratorymeasuring 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

Image:Emilio G. Segre.jpg Discoveryof element 43 has traditionally been assigned to a 1937experiment in Sicily conducted by Carlo Perrierand Emilio Segrè. The University of Palermoresearchers found the technetium isotope Tc-97 in a sample of molybdenumgiven to Segrè by Ernest Lawrencethe year before (Segrè visited Berkeley in the summer of 1936). The sample had previously been bombarded by deuteriumnuclei in the University of California, Berkeleycyclotronfor several months. University of Palermo officials tried unsuccessfully to force them to name their discovery panormium, after the Latinname for Palermo, Panormus. The researchers instead named element 43 after the Greekword technètos, meaning "artificial", since it was the first element to be artificially produced.

In 1952astronomer Paul W. Merrillin Californiadetected the spectral signatureof technetium (in particular, light at 403.1 nm, 423.8 nm, 426.8 nm, and 429.7 nm) in light from S-typered giants. These massive starsnear the end of their lives were rich in this short-lived element, meaning nuclear reactionswithin the stars must be producing it. This evidence was used to bolster the then unproven theorythat 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 pitchblendefrom the Belgian Congoin very small quantities (about 0.2 ng/kg); there it originates as a spontaneous fissionproduct of uranium-238. This discovery was made by B.T. Kenna and P.K. Kuroda. There is also evidence that the Oklonatural nuclear fission reactorproduced significant amounts of technetium-99, which has since decayed to ruthenium-99.

Occurrence and production

Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a spontaneous fission productof uranium. In 1999 David Curtis (see above) estimated that a kilogram of uranium contains 1 nanogram (1×10^?9 g) of technetium. Extraterrestrial technetium was found in some red giantstars (S-, M-, and N-types) that contain an absorption line in their spectrum indicating the presence of this element.

Image:Technetium Generator.jpg In contrast with the rare natural occurrence, bulk quantities of technetium-99 are produced each year from spent nuclear fuelrods, which contain various fission products. The fission of a gram of the rare isotope uranium-235in nuclear reactorsyields 27 mg of Tc-99, giving technetium a fission yieldof 6.1%. (Other fissionableisotopes also produce similar yields of technetium.) It is estimated that up to 1994, about 49,000 TBq(78 metric tons) 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 ORNLpermit for US$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.

While the meta stable(a state where the nucleus is in an excited state) isotope Tc-99m is produced as a fission productfrom the fission of uraniumor plutoniumin nuclear reactors. Due to the fact that used fuel is allowed to stand for several years before reporcessing, all ^{99Mo and 99mTc will have decayed by the time that the fission products are separated from the major actinidesin conventional nuclear reprocessing. The PUREX raffinatewill contain a high concentration of technetium as TcO_{4- but alomst all of this will be 99Tc. The vast majority of the 99mTc used in medical work is formed from 99Mo which is formed by the neutronactivation of 98Mo. The normal technetium cow is a aluminacolumn which contains molybdenum, as alumnium has a small neutron cross sectional it would be likely that an alumina column bearing inactive 98Mo could be irradated with neutrons to make the radioactive column for the technetium cow. By working in this way, there is no need for the complex chemical steps which would be required to separate molybdenum from the fission product mixture. As an alternative method an enriched uraniumtarget can be irradated with neutronsto form 99Mo as a fission product[1].}}

^{99Mo has a half-life of 67 hours, so short-lived 99mTc (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 the yield of technetium-99 as a productof the nuclear fissionof both uranium-235 and plutonium-239 is moderate, it is present in radioactive wasteof fission reactors and is produced when a fission bombis 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 testingalong 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 2005the primary release of technetium-99 into the environment is by the Sellafieldplant, 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.

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, lobsterfrom west Cumbriacontains small amounts of technetium.

The long half-life of technetium-99 and its ability to form an anionicspecies makes it (along with ^{129I) a major concern when considering long-term disposal of high-level radioactive waste. In addition many of the processes designed to remove fission products from medium active process streams in reprocessing plants are designed to remove cationicspecies (for example cesium{eg 137Cs} and strontium{eg 90Sr}). Hence the pertechinate is able to escape through these treatment processes. Current disposal options favor burial in 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. The anionic pertechinate and iodideare less able to absorb onto the surfaces of minerals so they are likely to be more mobile. For comparison plutonium, uranium, and cesiumare much more able to bind to soil particles. For this reason, the environmental chemistry of technetium is an active area of research. An alternative disposal method, transmutation, has been demonstrated at CERNfor technetium-99. This transmutation process is one in which the technetium (99Tc as a metaltarget) is bombarded with neutronsto form the shortlived 100Tc (half life = 16 seconds) which decays by beta decayto ruthenium(100Ru). One disadvantage of this process is the need for a very pure technetium target, while small traces of other fission products are likely to slightly increase the activity of the irradated target if small traces of the minor actinides(such as americiumand curium) are present in the target then they are likely to undergo fission to form fission products. In this way a small activity and amount of minor actinides leads to a very high level of radioactivity in the irradated target. The formation of 106Ru (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 irradationbefore the rutheniumcan be used.}

Reductive immobilization

Chemical means

The pertechnetate ion(TcO₄^-) could find use as an anodic corrosioninhibitor for steel(this possible use is hindered by technetium's radioactivity). The pertechnetate reacts with the steel surface to form a layer of technetium dioxidewhich prevents further corrosion, this formation of technetium dioxideexplains how iron powder can be used to remove pertechnetate from water. As an alternative activated carboncan be used to remove pertechnetate from water.

Biological means

Arokiasamy J. Francis, Cleveland J. Dodge, G. E. Meinken in Radiochimica Acta, 2002, Volume 90, Issue 09-11, p. 791 reported that Clostridia is able to reduce Tc(VII) to Tc(IV).

Isotopes

Technetium is one of three elements in the first 82 that have no stable isotopes(the other such elements are promethiumand tungsten(although because the half-life of tungsten is so long (2.09E19 y), tungsten is usually classed as being stable). The most stable radioisotopesare Tc-98 with a half-lifeof 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 massesranging 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 (4.883 hours), 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 modeis 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 6.2×10⁸ disintegrations a second (that is, 0.62 GBq/g).

Stability of technetium isotopes

Technetium and promethiumare remarkable among the light elements in that they have no stable isotopes. The reason for this is somewhat complicated.

Using the liquid drop modelfor 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 even, 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 can be 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 radioactivebut the element and its compoundsare extremely rarely found in nature. Technetium plays no natural biological role and is not normally found in the humanbody.

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 one is 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-raysare 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 contaminationin the lungs can pose a significant cancer risk. For most work, careful handling in a fume hoodis sufficient; a glove boxis not needed.

References

Prose contains specific citations in source text which may be viewed in edit mode.

Prose

• The Encyclopedia of the Chemical Elements, edited by Cifford A. Hampel, "Technetium" entry by S. J. Rimshaw (New York; Reinhold Book Corporation; 1968; pages 689-693) Library of Congress Catalog Card Number: 68-29938
• Nature's Building Blocks: An A-Z Guide to the Elements, by John Emsley (New York; Oxford University Press; 2001; pages 422-425) ISBN 0-19-850340-7
• The radiochemical Manual, 2nd Ed, edited by B.J. Wilson, 1966.
• Los Alamos National Laboratory – Technetium(viewed 1 December2002and 22 April2005)
• WebElements.com "Technetium" Uses(viewed 1 December2002and 22 April2005)
• EnvironmentalChemistry.com Nuclides / Isotopes(viewed 1 December2002and 22 April2005. JavaScript required, browser-restricted access)
• Elentymolgy and Elements Multidict by Peter van der Krogt, "Technetium"(viewed 30 April2005; Last updated 10 April2005)
• History of the Origin of the Chemical Elements and Their Discoverersby Norman E. Holden (viewed 30 April2005 ; last updated 12 March2004)
• Technetium as a Material for AC Superconductivity Applications by S. H. Autler, Proceedings of the 1968 Summer Study on Superconducting Devices and Accelerators
• Technetium heart scan, Dr. Joseph F. Smith Medical library (viewed 23 April2005)
• Gut transfer and doses from environmental technetium, J D Harrison et al 2001 J. Radiol. Prot. 21 9-11, Invited Editorial
• Ida Tacke and the warfare behind the discovery of fission, by Kevin A. Nies (viewed 23 April2005)
• TECHNETIUM by John T. Armstrong (viewed 23 April2005)
• Technetium-99 Behaviour in the Terrestrial Environment - Field Observations and Radiotracer Experiments, Keiko Tagami, Journal of Nuclear and Radiochemical Sciences, Vol. 4, No.1, pp. A1-A8, 2003
• Type 2 superconductors (viewed 23 April2005)
• The CRC Handbook of Chemistry and Physics, 85th edition, 2004-2005, CRC Press
• K. Yoshihara, "Technetium in the Environment" in "Topics in Current Chemistry: Technetium and Rhenium", vol. 176, K. Yoshihara and T. Omori (eds.), Springer-Verlag, Berlin Heidelberg, 1996.
• Schwochau, Klaus, Technetium, Wiley-VCH (2000), ISBN 3-527-29496-1
• RADIOCHEMISTRY and NUCLEAR CHEMISTRY, Gregory Choppin, Jan-Olov Liljenzin, and Jan Rydberg, 3rd Edition, 2002, the chapter on nuclear stability(PDF)

Table

• WebElements.com – Technetium, and EnvironmentalChemistry.com – Technetiumper the guidelines at Wikipedia's WikiProject Elements(all viewed 1 December2002)
• Nudat 2nuclide chart from the National Nuclear Data Center, Brookhaven National Laboratory
• Nuclides and IsotopesFourteenth Edition: Chart of the Nuclides, General Electric Company, 1989

External links

• WebElements.com – Technetium
• pubs.acs.org – ACS article on validity of Noddack and Tacke's discovery

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