Syntheses of transuranium elements

The uses of radioisotopes in medicine are extremely important. Certain elements are readily absorbed by particular organs in a human body, and this is capitalized upon in the use of radiotracers (introduced by food or drug intake) to probe the function of human organs. An advantage of the technique is that it is non-invasive. 

Although the uptake of by the thyroid gland was a health risk after the Chernobyl disaster (Box 2.2), controlled uptake has medical applications. If a patient ingests l (e.g. as a solution of l-labelled Nal), the isotope is quickly absorbed by the th5Toid gland and the size and state of the gland can be detected by monitoring the radioactivity emitted. For l, ri 8 days, and the dose administered soon decays. 

In addition to using radioisotopes to examine patients, the y-radiation emitted may be used in cancer treatment. Cobalt 

Hawthorne (1993) Angewandte Chemie, International Edition in English, vol. 32, p. 950 - The role of chemistry in the development of boron neutron capture therapy . 

Elder and K. Tepperman (1994) Metal-based drugs imaging agents in Encyclopedia of Inorganic Chemistry, ed. R.B. King, Wiley, Chichester, vol. 4, p. 2165. 

The transuranium elements are shown in Table 3.2 and have all been discovered since 1940. By 1955, the table extended to mendelevium and, by 1997, to meitnerium (Z = 109). In mid-2004, the number of elements in the periodic table stood at 112, although the lUPAC has formally to authenticate element 112. In 2003 and 2004, the lUPAC approved the names darmstadtium and roentgenium for elements 110 and 111, respectively. Element 112 is currently known as ununbium ( one-one-two ). This method of naming newly 

Nuclear power is now used in a number of countries as a source of electrical power. At the heart of a nuclear power plant is a nuclear reactor. The fuel in all commercial nuclear reactors is uranium, but of naturally occurring uranium only 0.7% is 92U, the radionuclide required for the fission process. Enrichment of the uranium is usually carried out but, even then, constitutes only a few per cent of the uranium used as the fuel source. 

The PWR has three main water circuits. The first is the primary circuit that carries heat energy from the nuclear reactor to the steam generators. The water is maintained at a pressure of sl50bar and an operating temperature of s573 K. Since the primary circuit is a closed loop, it is the only water circuit in the power station that contains radioactivity. The second water circuit is the water-steam cycle, and the final circuit is the water cooling system, which dissipates excess heat. 

The nuclear reactor at Sizewell B power station. David Parker I Science Photo Library 

Estimates of the total radiation released from the Chernobyl disaster vary but it may have been as great as 178 MCi 1 Ci is roughly equal to the activity of 1 g of radium. Thirty-one people died on the night of the explosion from radiation or burns, and there were 200 known casualties from radiation sickness. In the longer term, Chernobyl has left the world with a number of long-lived radioisotopes distributed in the atmosphere. The main health risks come from 53I (t = 8.02 days), 55CS = 2.06yr) and 

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The syntheses of transuranium elements have passed through three distinct stages of evolution. Starting in the 1940s, relatively stable, neutron-rich metals such as uranium and plutonium were bombarded... 

This changing pattern is hy no means random, all the breakthroughs and failures had their quite ohjective causes. They will be apparent when we discuss syntheses of transuranium elements one by one starting from the first one, neptunium. 

More scientists were involved in the discoveries of synthesized elements (more than 30). It is not surprising because many experimenters and theorists (both physicists and chemists) as well as technicians are involved in the work on syntheses of transuranium elements, particularly those with large Z values. For instance, th6 report on the synthesis of element 106 was signed by eleven Dubna scientists and each made a significant contribution to the work. 

Seaborgium - the atomic number is 106 and the chemical symbol is Sg. The name derives from the American chemist Glenn Theodore Seaborg , who led a team that first synthesized a number of transuranium elements. The element Seaborgium was first synthesized by American scientists from the University of California lab in Berkeley, California imder Albert Ghiorso, who used the nuclear reaction Cf ( 0,4n) Sg. The longest half-life associated with this unstable element is 21 second Sg. 

Ah lawrencium isotopes of masses 255 to 260 have been synthesized by bombardment of transuranium elements with heavy ions. 

If you read through the names of the transuranium elements, you ll notice that many of them have been named in honor of their discoverers or the laboratories at which they were created. There are ongoing efforts throughout the world s major scientific research centers to synthesize new transuranium elements and study their properties. 

At first Fermi actually thought he had synthesized some of element number 93, but the results he obtained were confusing and led to something else far more dramatic, as will shortly be described. These other developments distracted attention for a few years from the possible formation of transuranium elements. 

The second part of the book comprising two chapters (Chapters 12 and 13) is devoted to synthesized elements. In Chapter 12 the reader will be introduced to the synthesis of new elements within the previous boundaries of the periodic system—from hydrogen to uranium (technetium, promethium, astatine, francium). Chapter 13 covers the history of transuranium elements and prospects of nuclear synthesis. 

Starting from neptunium the American scientists for a long time played a leading part in discoveries of transuranium elements. This can easily be explained by the fact that the USA hardly experienced the hardships of the World War II. It should be noted, however, that in 1942 element 93 was independently synthesized by the German physicist K. Starke. 

But there is a fundamental difference between the attitudes of the contemporaries of McMillan and Abelson and the scientists working now. The former knew too little, the latter know too much (paradoxical as it sounds) to find a definite solution to the problem. In the forty-year history of transuranium elements there were times when the end seemed to be near. As nuclei with increasingly high Z values were synthesized a regular decrease in the half-lives was observed, particularly with respect to spontaneous fission from billions of years to hours, to minutes, to seconds, and to fractions of a second. A simple extrapolation indicated that for Z equalling 108-110 the nuclei would be so short-lived that they would decay at the moment of formation. 

Synthesizing the Transuranium Elements Scientists use accelerators for many applications, from producing radioisotopes used in medical applications to studying the fundamental nature of matter. Perhaps the most specific application for chemists is the synthesis of transuranium elements, those with atomic numbers higher than uranium, the heaviest naturally occurring element. Some reactions that were used to form several of these elements appear in Table 23.5. 

The main source of transuranium elements is the high-flux reactor, in which or heavier nuclei get transformed into higher-Z elements by multiple neutron capture. In the USA, there is a national program for the production of transuranium elements utilizing the high-flux reactor (HFIR) at Oak Ridge. The heaviest nuclide produced in the reactor is Fm. Neutron-deficient nuclides are synthesized in charged-particle accelerators and very neutron-rich nuclides with short half-lives are produced in reactors. 

Dmitri Mendeleev) Mendelevium, the ninth transuranium element of the actinide series discovered, was first identified by Ghiorso, Harvey, Choppin, Thompson, and Seaborg in early in 1955 during the bombardment of the isotope 253Es with helium ions in the Berkeley 60-inch cyclotron. The isotope produced was 256Md, which has a half-life of 76 min. This first identification was notable in that 256Md was synthesized on a one-atom-at-a-time basis. 

A further group of elements, the transuranium elements, has been synthesized by artificial nuclear reactions in the period from 1940 onwards their relation to the periodic table is discussed fully in Chapter 31 and need not be repeated here. Perhaps even more striking today are the predictions, as yet unverified, for the properties of the currently non-existent superheavy elements.Elements up to lawrencium (Z = 103) are actinides (5f) and the 6d transition series starts with element 104. So far only elements 104-112 have been synthesized, ) and, because there is as yet no agreement on trivial names for some of these elements (see pp. 1280-1), they are here referred to by their atomic numbers. A systematic naming scheme was approved by lUPAC in 1977 but is not widely used by researchers in the field. It involves the use of three-letter symbols derived directly from the atomic number by using the... 

Prior to 1940 only the naturally occurring actinides (thorium, protactinium and uranium) were known the remainder have been produced artificially since then. The transactinides are still being synthesized and so far the nine elements with atomic numbers 104-112 have been reliably established. Indeed, the 20 manmade transuranium elements together with technetium and promethium now constitute one-fifth of all the known chemical elements. 

It was detected by Urey, Brickwedde and Murphy in 1932. It occurs in all natural compounds of hydrogen including water, as well as in free hydrogen molecules at the ratio of about one part per 6,000 parts hydrogen. The principal application of deuterium is in tracer studies for measuring rates and kinetics of chemical reactions. It also is used in thermonuclear reactions and as a projectile in cyclotrons for bombardment of atomic nuclei to synthesize isotopes of several transuranium elements. Deuterium oxide, D2O, or heavy water is used as a neutron moderator in nuclear reactors. 

The element first was made by Ghiorso, Harvey, Choppin, Thompson, and Seaborg in 1955 in Berkeley, California. It was synthesized by bombardment of einsteinium-253 with alpha particles of 41 MeV energy in a 60-inch cyclotron. The element was named Mendelevium in honor of Russian chemist Dimitri Mendeleev. Mendelevium —258 isotope with a half-life of 60 days was discovered in 1967. The element has no commercial use except in research to synthesize isotopes of other transuranium elements. 

Protactinium dipnictides (X = As, Sb) have been synthesized by reaction of As or Sb vapour with metal hydride at 400-700 Pa3As4 was obtained by thermal dissociation of PaAs2 at 840 °C, Pa3Sb4 from the corresponding dipnictide at 1200 °C. Monopnictides of protactinium were not obtained by thermal dissociation of higher compounds. The diantimonides of the transuranium elements Np, Pu, Am dissociate between 700-800 °C into the monocompounds. Monopnictides of the higher transuranium elements have been obtained at the pg scale with and 0 by thermal dissociation. 

The effort to synthesize artificial elements beyond uranium began in 1934, went on for several years with a number of apparent successes, and then came to an abrupt halt in 1938 when nuclear fission was discovered and scientists realized that they had not found a single new element in all that time - the entire four-year search for transuranium elements had in fact been the study of fission fragments. 

Among the elements known before transuranium elements started to be synthesized in 1940, uranium has a unique characteristic, the extreme stability of the triatomic uranyl ion OUO+z. Not only are the numbers of uranium(VI) compounds larger than of U(IV), and far larger than of the two other oxidation states U(V) and U(III) known from non-metallic compounds, but until the preparation of UOFj discussed below, the only two U(VI) compounds known to contain less than two oxygen atoms per uranium atom were the octahedral molecules UFe and UQ6. 

Glenn Seaborg was only twenty-eight when he discovered plutonium. Within the next few years he headed several groups of research workers who created seven more transuranium elements. In 1944 came elements Nos. 95 and 96 which were named americium, and curium after the Curies. Almost five more years passed before two new births were announced— elements No. 97, christened berkelium after the home of the cyclotron that Lawrence had given to science, and No. 98, named californium. Another four crowded years went by and element No. 99 was synthesized and was given the name einsteinium after the great scientist who had just died. 

Already many more elements have been synthesized than could be predicted in the early 1960s. Higher neutron-flux reactors could make the necessary amounts of the heavier transuranium elements needed for synthesis of the elements beyond 112. Much will depend upon whether the more neutron-rich isotopes with longer half-lives, essential for any study of chemical properties, can be made. The only safe prediction is the unpredictability of this area. 

In the years since 1940 the elements with atomic numbers 93 through 112 and 114, 116, and 118, called the transuranium elements, have been synthesized. Many of these elements have very short half-lives, as shown in Table 21.4. As a result, only a few atoms of some of the transuranium elements have ever been formed. This, of course, makes the chemical characterization of these elements extremely difficult. 

The transuranium elements are synthesized by colliding accelerated charged particles with heavy atoms (i.e., curium and lead). In certain collisions the nuclei of the accelerated charged particles and the stationary heavy atoms will fuse to produce a n transuranium element. The lifetimes of these n elements are so short they often break down into other elements within fractions of a second and are detected only by their breakdown products, referred to as daughter elements. 

The discovery that a transmutation had happened started a flood of research. Soon after Harkins and Blackett had observed a nitrogen atom forming oxygen, other transmutation reactions were discovered by bombarding various elements with alpha particles. As a result, chemists have synthesized, or created, more elements than the 93 that occur naturally. These are synthetic elements. All of the transuranium elements, or those with more than 92 protons in their nuclei, are synthetic elements. To make them, one must use special equipment, called particle accelerators, described below. 

The major difficulty with synthesizing heavy elements is the number of protons in their nuclei (Z > 92). The large amount of positive charge makes the nuclei unstable so that they tend to disintegrate either by radioactive decay or spontaneous fission. Therefore, with the exception of a few transuranium elements like plutonium (Pu) and americium (Am), most artificial elements are made only a few atoms at a time and so far have no practical or commercial uses. 

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