Technetium

Characteristics

Technetium is a silvery-grey radioactive promethium have no stable isotopes, but are followed by elements which do.

Technetium is therefore extremely rare on Earth. Technetium plays no natural biological role and is not normally found in the human body.

The metal form of technetium slowly tarnishes in moist air. Its spectral lines at 363 nm, 403 nm, 410 nm, 426 nm, 430 nm, and 485 nm.^[8]

The metal form is slightly niobium.^[11]

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. Its radiological toxicity (per unit of mass) is a function of compound, type of radiation for the isotope in question, and the isotope half-life. sieverts) for a typical Tc-99m based nuclear scan (see more on this subject below).^[10]

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 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.^[10]

Applications

Nuclear medicine

^99mTc ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope radiopharmaceuticals based on ^99mTc for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood and tumors.^[10]

Immunoscintigraphy incorporates ^99mTc into a Sanofi-Aventis) under the name "Scintium".^[14]

When ^99mTc is combined with a sulfur colloid of ^99mTc is scavenged by the spleen, making it possible to image the structure of the spleen.^[16]

Radiation exposure due to diagnostic treatment involving Tc-99m can be kept low. Because ^99mTc has a short half-life and high energy gamma (allowing small amounts to be easily detected), its quick decay into the far-less radioactive ⁹⁹Tc results in relatively less total radiation dose to the patient, per unit of initial activity after administration. In the form administered in these medical tests (usually pertechnetate) both isotopes are quickly eliminated from the body, generally within a few days.^[15]

Technetium for nuclear medicine purposes is usually extracted from technetium-99m generators. ^95mTc, 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.^[10]

Industrial

Technetium-99 decays almost entirely by gamma rays. 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 NIST standard beta emitter, used for equipment calibration.^[10]

Technetium-99 has also been proposed for use in optoelectric and nuclear batteries.^[17]

Chemical

Like 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.^[10]

Under certain circumstances, a small concentration (5×10⁻⁵ Activated carbon can also be used for the same effect.) 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. Nevertheless, corrosion protection by pertechnetate ions was proposed (but never adopted) for use in boiling water reactors.^[10]

History

Search for element 43

  For a number of years there was a gap in the periodic table between manganese and gave it the name ekamanganese.

In 1877, the Russian chemist Serge Kern reported discovering the missing element in 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 utilized "a record of X-ray spectrum of Ogawa's nipponium sample from thorianite [which] was contained in a photographic plate preserved by his family. The spectrum was read and indicated the absence of the element 43 and the presence of the element 75 (rhenium)."^[19]

German chemists X-ray diffraction spectrograms. The wavelength of the X-rays produced is related to the atomic number by a formula derived by Henry Moseley in 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.^[21][22]

In 1998 John T. Armstrong of the National Institute of Standards and Technology ran "computer simulations" of the 1925 experiments and obtained results quite close to those reported by the Noddack team. He claimed that this was further supported by work published by David Curtis of the Los Alamos National Laboratory measuring the (tiny) natural occurrence of technetium.^[21][23] However, the Noddack's experimental results have never been reproduced, and they were unable to isolate any element 43. Debate still exists as to whether the 1925 team actually did discover element 43.

Official discovery and later history

Glenn T. Seaborg. They isolated the technetium-99m isotope which is now used in some 10,000,000 medical diagnostic procedures annually.^[25]

In 1952 astronomer Paul W. Merrill in California detected the s-process.^[10]

Since its discovery, there have been many searches in terrestrial materials for natural sources. In 1962, technetium-99 was isolated and identified in natural nuclear fission reactor produced significant amounts of technetium-99, which has since decayed to ruthenium-99.^[10]

Occurrence and production

Natural production

Since technetium is unstable, only minute traces occur naturally in the Earth's crust as a spontaneous uranium. In 1999 David Curtis (see above) estimated that a kilogram of uranium contains 1 nanogram (1×10⁻⁹ g) of technetium.^[28] Extraterrestrial technetium was found in some red giant stars (S-, M-, and N-types) that contain an absorption line in their spectrum indicating the presence of this element.^[29]

Long-lived fission products
t½(my)		Yield%	KeV	β
99Tc	.211	6.0507	294
126Sn	.230	.0236	4050	γ
79Se	.295	.0508	151
93Zr	1.53	6.2956	91	γ
135Cs	2.3	6.3333	269
107Pd	6.5	.1629	33
129I	15.7	.6576	194	γ

Byproduct production of Tc-99 in fission wastes

In contrast with the rare natural occurrence, bulk quantities of technetium-99 are produced each year from plutonium-239.

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.^[31] However, only a fraction of the production is used commercially. As of 2005, technetium-99 is available to holders of an ORNL permit for US$83/g plus packing charges.^[32]

Since the yield of technetium-99 as a 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 becquerels per amount of spent fuel, is dominant at about 10⁴ to 10⁶ years after the creation of the nuclear waste.^[31]

An estimated 160 TSellafield 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.^[33]

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.^[34] The uranium, thereby affecting these elements' solubility in soil and sediments. Their ability to reduce technetium may determine a large part of Tc's mobility in industrial wastes and other subsurface environments.^[35]

The long half-life of technetium-99 and its ability to form an anionic species 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 cationic species like iodide are less able to absorb onto the surfaces of minerals so they are likely to be more mobile.

By comparison ruthenium can be used.

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.^[10]

Neutron activation of molybdenum or other pure elements

The meta stable (a state where the nucleus is in an excited state) isotope ^99mTc is produced as a technetium-99m generator ("technetium cow," also occasionally called a molybdenum cow).

The normal technetium cow is an fission product, then separated.^[38]

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

Isotopes

Main article: isotopes of technetium

Technetium is one of the two elements in the first 82 that have no stable half-life of 4.2 Ma), ⁹⁷Tc (half-life: 2.6 Ma) and ⁹⁹Tc (half-life: 211.1 ka).^[40]

Twenty-two other radioisotopes have been characterized with u (⁸⁸Tc) to 112.931 u (¹¹³Tc). Most of these have half-lives that are less than an hour; the exceptions are ⁹³Tc (half-life: 2.75 hours), ⁹⁴Tc (half-life: 4.883 hours), ⁹⁵Tc (half-life: 20 hours), and ⁹⁶Tc (half-life: 4.28 days).^[40]

Technetium also has numerous meta states. ^97mTc is the most stable, with a half-life of 90.1 days (0.097 MeV). This is followed by ^95mTc (half life: 61 days, 0.038 MeV), and ^99mTc (half-life: 6.01 hours, 0.143 MeV). ^99mTc only emits gamma rays, subsequently decaying to ⁹⁹Tc.^[40]

For isotopes lighter than the most stable isotope, ⁹⁸Tc, the primary ruthenium, with the exception that ¹⁰⁰Tc can decay both by beta emission and electron capture.^[40][41]

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 ⁹⁹Tc produces 6.2×10⁸ disintegrations a second (that is, 0.62 GBq/g).^[42]

Stability of technetium isotopes

Technetium and promethium are unusual light elements in that they have no stable isotopes. The reason for this is somewhat complicated. ^[43]

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 odd number of nucleons A, the graph of binding energies vs. atomic number (number of protons) is shaped like a parabola (U-shaped), with the most stable nuclide at the bottom. A single beta decay or electron captures then transforms one nuclide of mass A into the next or preceding one, if the product has a lower binding energy and the difference in energy is sufficient to drive the decay mode. When there is only one parabola, there can be only one stable isotope lying on that parabola.^{[citation needed]}

For a fixed even number of nucleons A, the graph is jagged and is better visualized as two separate parabolas for even and odd atomic numbers, 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.

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 there are only 4 truly stable examples as opposed to very long-lived: the light nuclei: ²H, ⁶Li, ¹⁰B, ¹⁴N). However, if this happens, there can be no stable isotope with an even number of neutrons and an even number of protons.^{[citation needed]}

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).^{[citation needed]} 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.^[44]

References

Works cited

Notes