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Gadolinium is a silvery-white, malleable and ductile rare-earth metal with a metallic luster. It crystallizes in hexagonal, close-packed alpha form at room temperature, but, when heated to 1508 K or more, it transforms into its beta form, which has a body-centered cubic structure.
Unlike other rare earth elements, gadolinium is relatively stable in dry air. However, it tarnishes quickly in moist air and forms a loosely-adhering oxide that spalls off, and then exposes more surface to oxidation. Gadolinium reacts slowly with water, and it is soluble in dilute acids.
Gadolinium-157 has the highest thermal neutron capture cross-section of any known nuclide with the exception of Xenon-135, 49,000 barns, but it also has a fast burn-out rate, limiting its usefulness as a nuclear control rod material.
Gadolinium demonstrates a magnetocaloric effect whereby its temperature increases when it enters a magnetic field and decreases when it leaves the magnetic field. The effect is considerably stronger for the gadolinium alloy Gd5(Si2Ge2) .
Gadolinium is used for making gadolinium yttrium garnets, which have microwave applications, and gadolinium compounds are used for making phosphors for colour TV tubes. Gadolinium is also used for manufacturing compact discs and computer memory.
Gadolinium is used in nuclear marine propulsion systems as a burnable poison. The gadolinium slows the initial reaction rate, but, as it decays, other neutron poisons accumulate, allowing for long-running cores. Gadolinium is also used as a secondary, emergency shut-down measure in some nuclear reactors, particularly of the CANDU type.
Gadolinium also possesses unusual metallurgic properties, with as little as 1% of gadolinium improving the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation.
Because of their paramagnetic properties, solutions of organic gadolinium complexes and gadolinium compounds are used as intravenous radiocontrast agents to enhance images in medical magnetic resonance imaging. Magnevist is the most widespread example.
Besides MRI, gadolinium (Gd) is also used in other imaging. In X-ray, gadolinium is contained in the phosphor layer, suspending in a polymer matrix at the detector. Terbium-doped gadolinium oxysulfide (Gd2O2S: Tb) at the phosphor layer is to convert the X-rays releasing from the source into light. Gd can emit at 540nm (green light spectrum = 520 – 570nm), which is very useful for enhancing the imaging quality of the X-ray that is exposed to the photographic film. Beside Gd's spectrum range, the compound also has a K-edge at 50 kiloelectron volt (keV), which means its absorption of X-ray through photoelectric interactions is great. The energy conversion of Gd is up to 20%, which means, one-fifth of the X-ray striking on the phosphor layer can be converted into light photons.
Gadolinium oxyorthosilicate (Gd2SiO5, GSO; usually doped by 0.1-1% of Ce) is a single crystal that is used as a scintillator in medical imaging equipment like as Positron Emission Tomography (PET), and for detecting neutrons.
Gadolinium gallium garnet (Gd3Ga5O12) is a material with good optical properties, and is used in fabrication of various optical components and as substrate material for magneto–optical films.
In the future, gadolinium ethyl sulfate, which has extremely low noise characteristics, may be used in masers. Furthermore, gadolinium's high magnetic moment and low Curie temperature (which lies just at room temperature) suggest applications as a magnetic component for sensing hot and cold.
Due to extremely high neutron cross-section of gadolinium, this element is very effective for use with neutron radiography.
In 1880, Swiss chemist Jean Charles Galissard de Marignac observed spectroscopic lines due to gadolinium in samples of didymium and gadolinite; French chemist Paul Émile Lecoq de Boisbaudran separated gadolinia, the oxide of Gadolinium, from Mosander's yttria in 1886. The element itself was isolated only recently.
In older literature, the natural form of the element is often called an earth, meaning that the element came from Earth. In fact, gadolinium is the element that comes from the earth, gadolinia. Earths are compounds of the element and one or more other elements. The two most common combining-elements are oxygen and sulfur. For example, gadolinia contains gadolinium oxide (Gd2O3).
Gadolinium has no known native biological role, but in research on biological systems it has a few roles. It is used as a component of MRI contrast agents, as, in the 3+ oxidation state, the metal has 7 unpaired f electrons. This causes water around the contrast agent to relax quickly, enhancing the quality of the MRI scan. Second, as a member of the lanthanides, it is used in various Ion Channel electrophysiology experiments, where it is used to block sodium leak channels, as well as to stretch activated ion channels.
Gadolinium is never found in nature as the free element, but is contained in many rare minerals such as monazite and bastnäsite. It occurs only in trace amounts in the mineral gadolinite, which was also named after Johan Gadolin. Today, it is prepared by ion exchange and solvent extraction techniques, or by the reduction of its anhydrous fluoride with metallic calcium.
Compounds of gadolinium include:
See also gadolinium compounds.
Naturally-occurring gadolinium is composed of 5 stable isotopes, 154Gd, 155Gd, 156Gd, 157Gd and 158Gd, and 2 radioisotopes, 152Gd and 160Gd, with 158Gd being the most abundant (24.84% natural abundance).
Thirty radioisotopes have been characterized, with the most stable being 160Gd with a half-life of more than 1.3×1021 years (the decay has not been observed - only the lower limit on the half-life is known), alpha-decaying 152Gd with a half-life of 1.08×1014 years, and 150Gd with a half-life of 1.79×106 years. All of the remaining isotopes are radioactive, having half-lives less than 74.7 years. The majority of these have half-lives less than 24.6 seconds. Gadolinium isotopes have 4 metastable isomers, with the most stable being 143mGd (t½ 110 seconds), 145mGd (t½ 85 seconds) and 141mGd (t½ 24.5 seconds).
The primary decay mode at atomic weights lower than the most abundant stable isotope, 158Gd, is electron capture, and the primary mode at higher atomic weights is beta decay. The primary decay products for isotopes of weights lower than 158Gd are the element Eu (europium) isotopes and the primary products at higher weights are the element Tb (terbium) isotopes.
Gadolinium-153 has a half-life of 240.4 ±10 days and emits gamma radiation with strong peaks at 41keV and 102keV. It is used as a gamma ray source in x-ray absorptiometry or bone density gauges for osteoporosis screening, and in the Lixiscope portable x-ray imaging system.
As with the other lanthanides, gadolinium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail. Also, in patients on dialysis, there are data suggesting that it may cause nephrogenic fibrosing dermopathy as a side effect of MRI investigations that require the use of a Gadolinium based contrast agent.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gadolinium". A list of authors is available in Wikipedia.|