Transuranium elements synthesis

The effects of a rather distinct deformed shell at = 152 were clearly seen as early as 1954 in the alpha-decay energies of isotopes of californium, einsteinium, and fermium. In fact, a number of authors have suggested that the entire transuranium region is stabilized by shell effects with an influence that increases markedly with atomic number. Thus the effects of shell substmcture lead to an increase in spontaneous fission half-Hves of up to about 15 orders of magnitude for the heavy transuranium elements, the heaviest of which would otherwise have half-Hves of the order of those for a compound nucleus (lO " s or less) and not of milliseconds or longer, as found experimentally. This gives hope for the synthesis and identification of several elements beyond the present heaviest (element 109) and suggest that the peninsula of nuclei with measurable half-Hves may extend up to the island of stabiHty at Z = 114 andA = 184. 

Since the radioactive half-lives of the known transuranium elements and their resistance to spontaneous fission decrease with increase in atomic number, the outlook for the synthesis of further elements might appear increasingly bleak. However, theoretical calculations of nuclear stabilities, based on the concept of closed nucleon shells (p. 13) suggest the existence of an island of stability around Z= 114 and N= 184. Attention has therefore been directed towards the synthesis of element 114 (a congenor of Pb in Group 14 and adjacent superheavy elements, by bombardment of heavy nuclides with a wide range of heavy ions, but so far without success. 

The complexity of the reactions involved in the bombardment of plutonium and the production of higher transuranium elements can be seen from the following scheme which indicates the method of synthesis of einsteinium and fermium ... 

The chemical elements are the building blocks of nature. All substances are combinations of these elements. There are (as of 2005) 113 known chemical elements with the heaviest naturally occurring element being uranium (Z = 92). The 22 heaviest chemical elements, the transuranium elements, are manmade. The story of their synthesis, their properties, their impact on chemistry and physics, and their importance to society is fascinating. This story is of particular importance to nuclear chemistry because most of our knowledge of these elements and their properties comes from the work of nuclear chemists, and such work continues to be a major area of nuclear chemical research. One of us (GTS) has been intimately involved in the discovery and characterization of these transuranium elements. 

The synthesis reactions used to discover the transuranium elements are given in Table 15.2. All these reactions are complete fusion reactions in which the reacting nuclei fuse, equilibrate, and deexcite in a manner independent of their mode of formation. Other production reactions involving a partial capture of the projectile nucleus are also possible. 

Other nuclear transmutations can lead to the synthesis of entirely new elements never before seen on Earth. In fact, all the transuranium elements—those elements with atomic numbers greater than 92—have been produced by bombardment reactions. Plutonium, for example, can be made by bombarding uranium-238 with a particles ... 

With this, the goals of those who first sought artificial elements beyond uranium were realized. The understanding of nuclear behavior was deepened by the discovery of nuclear fission, and the periodic system was extended and clarified by the synthesis of transuranium elements. 

Actinides served already as targets, when neutron capture and subsequent P decay were used for the first synthesis of transuranium elements. Later, up to the synthesis of seaborgium, actinides were irradiated with light-ion beams from accelerators. At that time it was already known that cold fusion reactions yield higher cross sections for heavy element production. 

The most important transmutations by a particles are of the (a,p) and (a,n) types. The (a,p) processes (for example, Na23(a,p)Mg26) are common with targets of low atomic weights (Z > 25) as has been seen, these were the first artificial transmutations to be studied. The (a,n) reactions (and the closely related reactions in which two, three, or more neutrons are ejected by a particles of high energy) are of considerable interest in connection with the synthesis of the transuranium elements and of astatine (element 85). The following are typical and important examples ... 

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. 

The first transuranium elements were discovered at Berkeley, California, by G. T. Seaborg and his group, first reports about elements 104 to 106 came from Dubna, Russia, synthesis of elements 107 to 112 was first accomplished at Darmstadt, that of element 114 at Dubna, and that of elements 116 and 118 at Berkeley. With increasing atomic number the stability decreases appreciably to values of the order of milliseconds, and the question whether an island of higher stability may be reached at atomic munbers of about 114 (or 120 or 126) is still open. 

Unlike most of the naturally occurring elements, which can be handled and studied, the transuranium elements are all radioactive and break down incredibly fast. The synthesis and detection of transuranium elements takes great technical expertise. In addition, the experimental machinery needed to do this work is extremely expensive as well as complicated, therefore only a few research centers in the world are involved in this area of study. 

For the (n,y) jS case the upper horizontal row of Figure 15.2 rqrresoits the successive formation of higher isotopes of the target element (the constant Z-chain) and the vertical rows the isobaric decay chains of each of these isotopes (the constant A-chains). The first of these two rows is indicated by heavy arrows. Chains which involve both induced transformations and radioactive decay play a central role in theories about the formation of the elements in the universe, in the thermonuclear reactions in the stars (Ch. 17), and in the synthesis of transuranium elements (Ch. 16). 

Recoil techniques have been used extensively in the synthesis of higher transuranium elements. As an example, consider the formation of element 103, lawrencium (Fig. 15.3). The nuclides, which were formed in the reaction in the target between Cf and projectiles of recoiled out of the thin target into the helium gas where they were stopped by atomic collisions with the He-atoms. These recoil species became electrically charged cations as they lost electrons in the collision with the gas atoms. They could, therefore, be attracted to a moving, negatively charged, metal coated plastic band, which... 

A recently reported synthesis of the transuranium element bohrium (Bh) involved the bombardment of berkelium-249 with neon-22 to produce bohrium-267. Write a nuclear reaction for this synthesis. The half-life of bohrium-267 is 15.0 seconds. If... 

The actinides are a row of radioactive elements from thorium to lawrencium. They were not always separated into their own row in the periodic table. Originally, the actinides were located within the d-block following actinium. In 1944, Glenn Seaborg proposed a reorganization of the periodic chart to reflect what he knew about the chemistry of the actinide elements. He placed the actinide series elements in their own row directly below the lanthanide series. Seaborg had played a major role in the discovery of plutonium in 1941. His reorganization of the periodic table made it possible for him and his coworkers to predict the properties of possible new elements and facilitated the synthesis of nine additional 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. 

Now we come to the point in time when the words transuranium elements started to be linked with the word synthesis. 

The history of syntheses saw its periods of breakthroughs and slack periods. The first breakthrough period was from 1940 to 1945 when four transuranium elements were synthesized, namely, neptunium (Z = 93), plutonium (Z = 94), americium (Z = 95), and curium (Z = 96). The period till 1949 was a slack time and no new elements were discovered. In the next breakthrough period from 1949 to 1952 four more transuranium elements were added to the periodic system, namely berklium (Z = 97), californium (Z = 98), einsteinium (Z = 99), and fermium (Z = 100). In 1955, fifteen years after the synthesis of the first transuranium element, one more element, mendelevium (Z = 101), was synthesized. The next 25 years saw much less syntheses and only six new elements appeared in the periodic system. Here scientists encountered an entirely new situation and many former criteria for evaluating discoveries of elements proved inapplicable. 

The discovery of Hahn and Strassmann decisively stimulated actual synthesis of transuranium elements. To start, a reliable technique was needed for detection of the atoms of element 93 in a mass of fission fragments. As the masses of these fragments were comparatively small they had to travel longer distances (had longer paths) than the atoms of element 93 with a large mass. 

Synthesis of neptunium exhibited a significant feature which was to prove typical for syntheses of all transuranium elements (and other synthesized elements, too). First, one isotope with a certain mass number was synthesized. For neptunium this was neptunium-239. From that time it became a rule to date a discovery of a new transuranium element by the time of reliable synthesis of its first isotope. But sometimes this isotope proved to be so short-lived that it was difficult to subject it to physical and chemical analyses let alone find a useful application for it. A study of a new element would best be conducted with its longest-lived isotope. In the case of neptunium this was iieptunium-237 synthesized in 1942 in the following reaction ... 

The work on the synthesis of element 94 was headed by the famous American scientist G. Seaborg whose group discovered many transuranium elements. During the winter of 1940-1941 they studied the nuclear reaction U(d, 2n) which gave rise to the isotope neptunium-238. An alpha-active substance accumulated with time in the reaction product. The scientists extracted this substance and found... 

After synthesis of californium scientists in America (and in other countries) started a serious reassessment of their plans. They asked whether it was reasonable to plan for syntheses of heavier transuranium elements in the foreseeable future. 

Most probably, the controversies on the syntheses of the transuranium elements with Z > 102 are quite understandable. Each such synthesis is a heroic feat of science and technology. In this complicated work errors and inaccuracies are inescapable. It has long been an accepted view that strict criteria of reliability should be worked out for syntheses... 

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. 

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