Прометий: Сирек кездесетін радиоактивті элемент

Promethium is a chemical element; it has symbol Pm and atomic number 61. All of its isotopes are radioactive; it is extremely rare, with only about 500–600 grams naturally occurring in Earth's crust at any given time. Promethium is one of only two radioactive elements that are followed in the periodic table by elements with stable forms, the other being technetium. Chemically, promethium is a lanthanide. Promethium shows only one stable oxidation state of +3. In 1902 Bohuslav Brauner suggested that there was a then unknown element with properties intermediate between those of the known elements neodymium (60) and samarium (62); this was confirmed in 1914 by Henry Moseley, who, having measured the atomic numbers of all the elements then known, found that atomic number 61 was missing. In 1926, two groups (one Italian and one American) claimed to have isolated a sample of element 61; both "discoveries" were soon proven to be false. In 1938, during a nuclear experiment conducted at Ohio State University, a few radioactive nuclides were produced that certainly were not radioisotopes of neodymium or samarium, but there was a lack of chemical proof that element 61 was produced, and the discovery was not generally recognized. Promethium was first produced and characterized at Oak Ridge National Laboratory in 1945 by the separation and analysis of the fission products of uranium fuel irradiated in a graphite reactor. The discoverers proposed the name "prometheum" (the spelling was subsequently changed), derived from Prometheus, the Titan in Greek mythology who stole fire from Mount Olympus and brought it down to humans, to symbolize "both the daring and the possible misuse of mankind's intellect". However, a sample of the metal was made only in 1963. The two sources of natural promethium are rare alpha decays of natural europium 151 (producing promethium 147) and spontaneous fission of uranium (various isotopes). Promethium 145 is the most stable promethium isotope, but the only isotope with practical applications is promethium 147, chemical compounds of which are used in luminous paint, atomic batteries and thickness measurement devices. Because natural promethium is exceedingly scarce, it is typically synthesized by bombarding uranium 235 (enriched uranium) with thermal neutrons to produce promethium 147 as a fission product.

Promethium is the only lanthanide and one of only two elements among the first 82 that has no stable or long lived (primordial) isotopes. This is a result of a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes; it is also the least stable element of the first 84. The primary decay products are neodymium and samarium isotopes (promethium 146 decays to both, the lighter isotopes generally to neodymium via positron decay and electron capture, and the heavier isotopes to samarium via beta decay). Promethium nuclear isomers may decay to other promethium isotopes and one isotope (145Pm) has a very rare alpha decay mode to stable praseodymium 141. The most stable isotope of the element is promethium 145, which has a specific activity of and a half life of 17.7 years via electron capture. The element also has 18 nuclear isomers, with mass numbers of 133 to 142, 144, 148, 149, 152, and 154 (some mass numbers have more than one isomer). The most stable of them is promethium 148m, with a half life of 43.1 days; this is longer than the half lives of the ground states of all promethium isotopes, except for promethium 143 to 147. In fact, promethium 148m has a longer half life than its ground state, promethium 148.

In 1934, Willard Libby reported that he had found weak beta activity in pure neodymium, which was attributed to a half life over 1012 years. Almost 20 years later, it was claimed that the element occurs in natural neodymium in equilibrium in quantities below 10−20 grams of promethium per one gram of neodymium. However, these observations were disproved by newer investigations, because for all seven naturally occurring neodymium isotopes, any single beta decays (which can produce promethium isotopes) are forbidden by energy conservation. In particular, careful measurements of atomic masses show that the mass difference between 150Nd and 150Pm is negative (−87 keV), which absolutely prevents the single beta decay of 150Nd to 150Pm. In 1965, Olavi Erämetsä separated out traces of 147Pm from a rare earth concentrate purified from apatite, resulting in an upper limit of 10−21 for the abundance of promethium in nature; this may have been produced by the natural nuclear fission of uranium, or by cosmic ray spallation of 146Nd. Both isotopes of natural europium have larger mass excesses than sums of those of their potential alpha daughters plus that of an alpha particle; therefore, they (stable in practice) may alpha decay to promethium. Uranium is thus "responsible" for 560 g of promethium in Earth's crust. Promethium has also been identified in the spectrum of the star HR 465 in Andromeda; it also has been found in HD 101065 (Przybylski's star) and HD 965. Because of the short half life of promethium isotopes, they should be formed near the surface of those stars.

In 1902, Czech chemist Bohuslav Brauner found out that the differences in properties between neodymium and samarium were the largest between any two consecutive lanthanides in the sequence then known; as a conclusion, he suggested there was an element with intermediate properties between them. This prediction was supported in 1914 by Henry Moseley who, having discovered that atomic number was an experimentally measurable property of elements, found that a few atomic numbers had no known corresponding elements: the gaps were 43, 61, 72, 75, 85, and 87. With the knowledge of a gap in the periodic table several groups started to search for the predicted element among other rare earths in the natural environment. The first claim of a discovery was published by Luigi Rolla and Lorenzo Fernandes of Florence, Italy. After separating a mixture of a few rare earth elements nitrate concentrate from the Brazilian mineral monazite by fractionated crystallization, they yielded a solution containing mostly samarium. This solution gave x ray spectra attributed to samarium and element 61. In honor of their city, they named element 61 "florentium". The results were published in 1926, but the scientists claimed that the experiments were done in 1924. Also in 1926, a group of scientists from the University of Illinois at Urbana–Champaign, Smith Hopkins and Len Yntema published the discovery of element 61. They named it "illinium", after the university. Both of these reported discoveries were shown to be erroneous because the spectrum line that "corresponded" to element 61 was identical to that of didymium; the lines thought to belong to element 61 turned out to belong to a few impurities (barium, chromium, and platinum). In 1934, Josef Mattauch finally formulated the isobar rule. One of the indirect consequences of this rule was that element 61 was unable to form stable isotopes. From 1938, a nuclear experiment was conducted by H. B. Law et al. at the Ohio State University. Nuclides were produced in 1941 which were not radioisotopes of neodymium or samarium, and the name "cyclonium" was proposed, but there was a lack of chemical proof that element 61 was produced and the discovery was not largely recognized.

The production methods for different isotopes vary, and only those for promethium 147 are given because it is the only isotope with industrial applications. Promethium 147 is produced in large quantities (compared to other isotopes) by bombarding uranium 235 with thermal neutrons. The output is relatively high, at 2.6% of the total product. Another way to produce promethium 147 is via neodymium 147, which decays to promethium 147 with a short half life. Neodymium 147 can be obtained either by bombarding enriched neodymium 146 with thermal neutrons Another method is to bombard uranium 238 with fast neutrons to cause fast fission, which, among multiple reaction products, creates promethium 147. As early as the 1960s, Oak Ridge National Laboratory could produce 650 grams of promethium per year and was the world's only large volume synthesis facility. Gram scale production of promethium has been discontinued in the U. S. in the early 1980s, but will possibly be resumed after 2010 at the High Flux Isotope Reactor. In 2010, Russia was the only country producing promethium 147 on a relatively large scale.

Most promethium is used only for research purposes, except for promethium 147, which can be found outside laboratories. It is obtained as the oxide or chloride, in milligram quantities. This isotope does not emit gamma rays, and its radiation has a relatively small penetration depth in matter and a relatively long half life. Some signal lights use a luminous paint, containing a phosphor that absorbs the beta radiation emitted by promethium 147 and emits light. Promethium may be favored over tritium for nuclear safety reasons. In atomic batteries, the beta particles emitted by promethium 147 are converted into electric current by sandwiching a small promethium source between two semiconductor plates. These batteries have a useful lifetime of about five years. Promethium is also used to measure the thickness of materials by evaluating the amount of radiation from a promethium source that passes through the sample. It has possible future uses in portable X ray sources, and as auxiliary heat or power sources for space probes and satellites (although the alpha emitter plutonium 238 has become standard for most space exploration related uses). Promethium 147 is also used, albeit in very small quantities (less than 330nCi), in some Philips CFL (Compact Fluorescent Lamp) glow switches in the PLC 22W/28W 15mm CFL range.

The element has no biological role. Promethium 147 can emit gamma rays during its beta decay, which are dangerous for all lifeforms. Interactions with tiny quantities of promethium 147 are not hazardous if certain precautions are observed. In general, gloves, footwear covers, safety glasses, and an outer layer of easily removed protective clothing should be used.