Production of Superheavy Elements

Superheavy elements are defined as the transactinide elements—elements with an atomic number greater than 103. The current production of new superheavy elements takes place mostly at four locations around the world the Lawrence Berkeley National Laboratory (LBNL) at Berkeley, California the GSI Helmholtz Centre for Heavy Ion Research (GSI) at Darmstadt, Germany the Joint Institute for Nuclear Research (JINR) at Dubna, Russia and the Superheavy Element Laboratory of the Institute of Physical and Chemical Research (RIKEN) at Nishina, Japan. As of 2012, the discoveries of elements 113-118 have all been reported. The discoveries of four of these elements, however, are still awaiting independent confirmation from the other laboratories. 

As an example of new element production, just three atoms of element 118 were synthesized by a joint team of American and German scientists working together at Darmstadt. As shovm in the following equation, a californium-249 target was bombarded by a beam of calcium-48 ions  

Superheavy elements are elements with an atomic number greater than 103. Currently, the discoveries of fifteen such elements have been reported. 

Although element 118 s position in the periodic table places it in the family of noble gases, the boiling point of element 118 is predicted to be above room temperature. If that proves to be true, then element 118 will be a noble liquid instead of a noble gas. As of early 2012, research was continuing into producing elements heavier than element 118. Presumably, element 119 will be an alkali metal, and element 120 will be an alkaline earth. 

Nuclear processes provide humankind with a double-edged sword. On one hand, there are many useful applications of radioactive substances in science and medicine. Nuclear power is, and will continue to be, an important soiuce of energy. On the other hand, there is always the danger of radioactive or fissile materials being used to threaten people s lives. No one can make radioactive or fissile materials just go away. Hopefully, wisdom will prevail, and peaceful applications of nuclear materials will dominate their use. 

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There are a number of excellent reviews of the history of the production of the actinide elements beyond uranium e.g., [73-75]. The focus of this section will be on those aspects relevant to the production of superheavy elements. 

Table 1 Production of superheavy element isotopes for chemistry experiments. Known nuclides with half-lives longer than 0.7 s are listed along with their decay modes, representative reactions of synthesis and production cross sections. If the element does not have a long-lived isotope, the longest-lived isotope is given...

Denisov, V.Yu., Hofmann, S. Production of superheavy elements in cold fusion reactions. Acta Phys. Pol. B31, 479 84 (2000)... 

Veselsky, M. Competition of fusion and quasifission in reactions leading to production of superheavy elements. Phys. At. Nuclei 66, 1086-1094 (2003)... 

Superheavy elements are those with Z > 104 (transactinides). One may distinguish the d elements (Z=104-112) and the superheavy Ip elements (Z=113-118). Considerable progress has been made in the production of such elements. Element 104-106 have isotopes which are stable enough to perform chemical experiments [141, 142], while for elements such as element 114, both the production rate and the lifetimes are too small to allow for experimental chemical investigations. Besides experiments, relativistic density functional calculations have been performed for compounds of the 6d elements. A detailed account can be found in chapter 1 of this volume. 

In recent years, the number of elements has increased well beyond 100 as the result of the synthesis of artificial elements. At the time of writing, conclusive evidence has been reported for element 111. Such elements are typically very unstable, and only a few atoms are produced at any time. However, ingenious chemical techniques have been devised that permit the chemical properties of these so-called superheavy elements to be examined and allow one to check whether extrapolations of chemical properties are maintained for such highly massive atoms. On a more philosophical note, the production of these elements allows us to examine whether the periodic law is an exceptionless law, of the same kind as Newton s law of gravitation, or whether deviations to the expected recurrences in chemical properties might take place once a sufficiently high atomic number is reached. No surprises have been found so far, but the question of whether some of these superheavy elements have the expected chemical properties is far from being fully resolved. One important complication that arises in... 

Fermi irradiated uranium with slow neutrons, and observed a variety of radioactivities that he tentatively identified as being transuranium elements [1]. We now know that these radioactive species were the products of the fission of the in the sample. Study of the chemical properties of these new nuclides led to the subsequent discovery of fission in 1939 [2, 3], Explanation of the fission process was closely connected to the creation of the liquid-drop model [4—6], in which the nucleus is treated like an incompressible charged fluid with surface tension. See Nuclear Structure of Superheavy Elements for more information on nuclear structure and the stability of the heaviest nuclides. 

Hot-fusion reactions were employed in the discoveries of the elements beyond mendelevium as far as element 106, producing the first three members of the domain of superheavy elements. Higher transactinides have also been synthesized in these reactions. As before, the general trends with increasing atomic number were shorter half-lives and smaller production cross sections, a consequence of decreased survival probability in the evaporation process [132, 133]. The probability of decay from the nuclear ground state by spontaneous fission became significant in these elements. The techniques used in the experiments still included radiochemistry and off-line radiation counting [134]. As half-lives dropped below minutes into seconds it became more common to use direct techniques like transportation in gas jets to mechanisms like wheels and tapes (see Sect. 3.3 and Experimental Techniques ). Detection of new nuclides resulted from the detailed... 

Bk( 0,5n) reactions, respectively, with corresponding cross sections of 5 nb and 6 nb [168-171]. The " Cm( F,5n) reaction has also been used to produce Db [170] and the Cf( N,4n) reaction has been used to produce Db [168]. The SF decay branch of the isotope Db (Ti/a = 27 s) was first observed in the products of the " Bk( 0,4n) reaction with a cross section of 6 nb [168], and was positively identified in subsequent radiochemical experiments [161]. Until recently, chemistry experiments were usually performed with Db (see Liquid-Phase Chemistry of Superheavy Elements and Gas-Phase Chemistry of Superheavy Elements ). 

Early efforts to produce superheavy nuclides in " Ca-induced reactions suffered from a lack of sensitivity [338-344]. Expectations for the production cross sections for evaporation residues in this work were optimistic, and experiments at that time had no chance of observing nuclides produced at the level of single-digit pico-barns. Expectations for nuclide half-lives were similarly optimistic, and some of the work relied on radiochemical separations following irradiations of extended length. See Historical Reminiscences The Pioneering Years of Superheavy Element Research for more details about these historical experiments. 

Hassium (Z = 108) is the lightest superheavy element that has been produced directly in " Ca-induced reactions. The 20-s isotope °Hs has been produced in the Ra(" Ca,4n) reaction with a cross section of 8 pb [133]. Difficulties in target handling and a limited cross section favor the production of this isotope via the hot-fusion reaction " Cm( Mg,4n) for radiochemistry experiments [179, 180] (see Sect. 2.2 and Gas-Phase Chemistry of Superheavy Elements ). A single... 

Under this constraint, it is not possible to produce new superheavy nuclides at greater neutron excess by cold fusion, or by hot fusion with heavy-ion beams with lower atomic numbers than argon. This is because of the neutron richness of the overshoot isotopes, daughters of the multiple emission of relatively proton-rich a particles in the decays of the " Ca-induced evaporation residues. Nevertheless, both reaction types offer advantages in the production rates of the known isotopes of superheavy elements with Z = 106-108 that are of interest to the radiochemist. As examples Direct production of the long-lived hassium isotope Hs is possible in the cold-fusion irradiation of ° Pb with radioactive Fe. From Fig. 2, the cross... 

These elements are the minimum requirement for superheavy element research a separator and a means to identify the reaction products. Alternatively a gas catcher is commonly used as a first stage for the transport to specialist detectors for chemical studies or transport to a trap. More details on gas-jet transport systems are given in Synthesis of Superheavy Elements and Experimental Techniques . 

The main aim of chemical research in the area of the heaviest elements is to assign a new element its proper place in the Periodic Table. Conceptually, it is the atomic number, Z, and electronic configuration of an element that define its position there. Since the latter cannot be measured for the very heavy elements, information on its chemical behavior is often used for this purpose. Unfortunately, with increasing nuclear charge cross-sections and production rates drop so rapidly that such chemical information can be accessed only for elements with a half-life of the order of at least few seconds and longer (see Synthesis of Superheavy Elements )- In this case, some fast chemistry techniques are used (see Experimental Techniques ). 

It was emphasized in [1] that the nuclear decay properties of the isotope to be used in these studies must be well known and have unique decay characteristics suitable for detection and positive identification on an atom-at-a-time basis in order to verify that it is from the element whose chemistry is to be studied It must have a half-life comparable to the proposed chemical separation procedure as well as a reasonable production and detection rate to permit statistically significant results to be obtained, and must give the same results for a few atoms as for macro amounts. For the transactinide elements, production rates range from a few atoms per minute for rutherfordium (Rf, Z = 104) to only about one atom per day in the case of elements 108 (hassium, Hs), 112, and 114, the heaviest elements studied to date with chemical techniques. Details of these chemical investigations are outlined in Liquid-Phase Chemistry of Superheavy Elements and Gas-Phase Chemistry of SuperheavyElements . 

Isotopes of heaviest actinides and transactinides are characterized by short half-lives and low production cross sections see Synthesis of Superheavy Elements . Study of their chemical properties is therefore a real challenge [1, 7]. It should be... 

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