Technetium

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 very rarely found in nature. Its short-lived isotope Tc-99m is used in nuclear medicine to diagnose certain cancers.

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 teknetos, 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 (the age of the universe is about 14 billion years so any technetium made at or shortly after the Big Bang has long since disappeared).

Notable characteristics

Technetium is a silvery-gray radioactive metal, with an appearance that is similar to that of platinum. However, it is commonly obtained as a gray powder. This element is unusual 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^-.Template:Inote. Common oxidation states of technetium include 0, +2, +4, +5, +6 and +7.Template:Inote

When in powder form technetium will burn in oxygen.Template:Inote It dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid. The metal form is slightly paramagnetic, meaning its magnetic dipoles align with external magnetic fields even though technetium is not normally magnetic.Template:Inote The crystal structure of the metal is hexagonal close-packed.

Metallic technetium becomes a type II superconductor at 7.80 K; Template:Inote below this temperature it has a very high magnetic penetration depth; the largest among the elements apart from niobium.Template:Inote It has characteristic spectral lines at 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.Template:Inote

Applications

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).Template:Inote

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.Template:Inote
The sulfur colloid of Tc-99m is scavenged by the spleen, making it possible to image the structure of the spleen.Template:Inote

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 Template:Inote). Template:Inote

Other uses;

Tc-95m, with a half-life of 61 days, is used as a source of gamma rays in radioactive tracer studies.
TcO₄^- concentrates in the thyroid and is thus used to image that organ.Template:Inote
Organic technetium compounds are used in bone imaging.
Technetium is also a valuable source of beta rays.

History

For a number of years there was a gap in the periodic table at element 43. Dmitri Mendeleev predicted that this missing element would be chemically similar to manganese and gave it the name ekamanganese (see Mendeleev's predicted elements). Over the years would-be discoverers gave what they thought was element 43 various names, such as davyum, lucium, and nipponium.

In 1925 German chemists Walter Noddack, Otto Berg and Ida Tacke reported the discovery of element 43 and named it masurium. 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. Other expermenters did not replicate the discovery, and in fact it was dismissed as an error for many years.Template:Inote 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.Template:Inote Debate still exists as to whether the 1925 team actually did discover element 43.

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 sent to them by Ernest Lawrence. The sample had previously been bombarded by deuterium nuclei in the University of California, Berkeley cyclotron for several months.Template:Inote Element 43 was named by them after the Greek word teknetos, meaning "artificial", since it was the first element to be artificially produced.Template:Inote

In 1952 astronomer Paul Merril in California detected the spectral signature of technetium in light from 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.Template:Inote

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 Africa in very small quantities; there it originates as a spontaneous fission product of uranium-238. This discovery was made by B.T. Kenna and P.K. Kuroda. Template:Inote

Occurrence

File:Technetium Generator.jpg

The first "technetium cow" (a device for separating Tc-99m from its parent isotope, Mo-99) at Brookhaven national laboratory.

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 (1×10^-9 g) of technetium.Template:Inote 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 technetium. Template:Inote

In contrast with the rare natural occurrence, bulk quantities of technetium 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%.Template:Inote 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. Template:Inote 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.Template:Inote

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 manmade 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 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.Template:Inote

An estimated 160 TBq (about 250 kg) of technetium-99 was released into the environment up to 1994 by atmospheric nuclear tests. Template:Inote 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. 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.Template:Inote

The meta stable (a state where extra energy is in the nucleus) isotope Tc-99m is produced as a byproduct from the fission of uranium in nuclear reactors. It is prepared in a technetium-99m generator ("technetium cow") by chemically extracting molybdenum containing a large quantity of the isotope molybdenum-99 from the reactor's radioactive waste and shipping it 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.Template:Inote

Isotopes

Technetium is one of two elements in the first 83 that have no stable isotopes (the other such element is promethium).Template:Inote 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). Template:Inote

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).Template:Inote

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.Template:Inote

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.Template:Inote Template:Inote

Precautions

All isotopes of technetium are radioactive. Neither the element nor its compounds are found in nature, for practical purposes, and the element and its compounds are encountered extremely rarely by most people. Tc-99 is a radioactive contamination hazard; one gram has 6.2×10⁸ disintegrations a second (that is, 0.62 GBq/g), and it should therefore be handled in a glove box despite its relatively long half life and weak 0.29 MeV beta emission.Template:Inote

Technetium has no natural biological role and is not normally found in the human body. However, technetium is a component of the high-level radioactive waste produced by nuclear reactors, so when this waste is improperly disposed of, technetium is released into the environment (along with uranium, plutonium, and other transuranics). As a result it is found in small quantities in some seafood from certain areas. For example, lobster from west Cumbria contains small amounts of technetium,Template:Inote probably from radioactive waste dumped into the sea by the Sellafield nuclear plant from the 1940s to the 1960s. Technetium is poorly absorbed by the gut.Template:Inote

References

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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
Los Alamos National Laboratory – Technetium (viewed 1 December 2002 and 22 April 2005)
WebElements.com "Technetium" Uses (viewed 1 December 2002 and 22 April 2005)
EnvironmentalChemistry.com Nuclides / Isotopes (viewed 1 December 2002 and 22 April 2005)
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 Apr 2005)
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 Apr 2005)
TECHNETIUM by John T. Armstrong (viewed 23 Apr 2005)
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 Apr 2005)
The CRC Handbook of Chemistry and Physics, 85th edition, 2004-2005, CRC Press

Table

WebElements.com – Technetium, and EnvironmentalChemistry.com – Technetium per the guidelines at Wikipedia's WikiProject Elements (all viewed 1 December 2002)

External links