DARMSTADTIUM AND BEYOND

DARLEANE C. HOFFMAN, UNIVERSITY OF CALIFORNIA, BERKELEY

More than 150 years elapsed between the discovery of uranium (92) in 1789 and neptunium (Np, 93), the first transuranium element, in 1940. Now, 110 elements have been identified, named, and placed in the periodic table. No longer are there four "missing" elements in the periodic table, as there were when the first "artificial" element, technetium (43), was produced and identified in 1937, just before World War II. Uranium was then the heaviest element known, and the elements astatine (85, discovered in 1940) and francium (87, 1939) were missing from the body of the table and promethium (61, 1945) was missing from the lanthanides. It is awe-inspiring to realize that those four missing elements, plus the 11 actinides from Np through lawrencium (Lr) and the first six transactinides from rutherfordium (Rf) through meitnerium (Mt), have been added to the periodic table since 1937 when Tc was produced! These 21 elements have increased the number of known elements by nearly 25% in 66 years.

Discoveries of three transactinides--elements 110, 111, and 112--were reported between 1995 and 1996. A Joint Working Party (JWP) of the International Union of Pure & Applied Chemistry (IUPAC)/International Union of Pure & Applied Physics was appointed to consider the discovery claims; the evidence has been deemed sufficient for elements 110 and 111, and the GSI discoverers have been invited to propose "real" names to replace the three-letter symbols and "systematic names" based on 0 = nil, 1 = un, 2 = bi, 3 = tri, and so mandated by IUPAC in 1979. Many of you will no doubt recall hearing Glenn T. Seaborg's sonorous and humorous drawn-out pronunciation of the provisional designations for elements 110 (Uun) and 111(Uuu) as "oon-oon' NIL-i-em" and "oon-oon' OON-i-em," much to the amusement of most of us working in this field who simply used the atomic numbers, thus avoiding these designations. Darmstadtium was approved by IUPAC as the official name for element 110 on Aug. 16, 2003.

The production and identification of Np and the rest of the actinides were accomplished rather quickly, as shown in the timeline. However, all the elements after mendelevium (Md, 101) were first positively identified using "physical" rather than chemical-separation techniques, ushering in a period of controversy. Methods other than the classical chemical-separation techniques had to be devised for positive determination of the atomic number (Z) of these short-lived elements, which were produced in quantities of only a few atoms by bombardment of heavy targets with high-intensity projectiles.

Positive identification of the first transactinides, Rf and dubnium (Db) [or Ha; many publications of chemical studies prior to 1997 use hahnium (Ha) for element 105], was delayed until 1969–70 as scientists worked to develop new techniques to produce and identify these elements. New methods based on - correlation to link the new element to known daughter nuclides were developed. Spontaneous fission rather than -decay is often the dominant decay mode and, although relatively easy to detect, it effectively destroys information about the atomic number and mass number of the original nucleus and has led to much controversy concerning identification of the atomic number of the fissioning nuclide.

Element 106 was soon produced at the Heavy Ion Linear Accelator (HILAC) at Berkeley (1974) by a joint Berkeley/Livermore group, but a long time elapsed before elements 107 through 109 were reported in 1981–84 by researchers using new production methods and the velocity separator SHIP at the Universal Linear Accelerator (UNILAC) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany. More than 10 years passed while the GSI groups made further improvements in SHIP and UNILAC before production of elements 110 through 112 was reported in 1995 and 1996.

Darmstadtium. Four different isotopes of element 110 were initially reported by three different groups. A single event of ²⁶⁷110 produced in the ²⁰⁹Bi(⁵⁹Co,n) reaction was reported in 1995 by a group led by Albert Ghiorso at the Lawrence Berkeley National Laboratory (LBNL). Evidence for isotopes ²⁶⁹110 and ²⁷¹110 produced in reactions of lead (Pb) and bismuth (Bi) targets with ^62,64Ni projectiles was reported in 1995 by a group led by Sigurd Hofmann at GSI. A single event of ²⁷³110 produced in the ²⁴⁴Pu (³⁴S,5n) reaction was reported in 1996 by a joint Dubna/Livermore group led by Yu. A. Lazarev at the Joint Institutes for Nuclear Research in Russia. Each of these groups used different production reactions, so none constituted confirmation of the others.

However, in 2002 the GSI group reported confirmation of their own results and the production of additional isotopes of 110, ^269,271110. Groups at LBNL and at the Japan Atomic Energy Research Institute have also confirmed the earlier GSI results. The LBNL group was unable to confirm their initial result because the SuperHILAC was shut down permanently after the experiment was finished, and the Dubna/LLNL group did not conduct additional experiments to confirm its report. After consideration of all the results, the JWP said that clearly the GSI group should be acknowledged as the discoverers. The GSI group was invited by IUPAC to propose a name for element 110 and chose darmstadtium (Ds) after Darmstadt, near the site where the research was conducted. The Inorganic Chemistry Division of IUPAC submitted the proposed name to the IUPAC Bureau and Council for final approval at the IUPAC General Assembly in Ottawa last month.

Elements 111 and 112. The GSI group initially reported discovery of elements 111 and 112 in 1995 and 1996, respectively, but JWP found that the data were insufficient to constitute discovery. Additional results designed to confirm the initial investigations were published by the GSI group in 2002, and JWP has now completed its examination of the additional data. The findings have been reviewed, and the summary should soon be published on the IUPAC Web page (http://www.iupac.org) for comment. JWP found that the additional evidence presented for element 111 is sufficient to constitute confirmation of the discovery by the GSI group, but deemed that the data for element 112 are still not sufficient.

Attempts by the GSI group to produce element 113 using similar methods were unsuccessful, and extrapolation from their previous experiments convinced them that production rates for the elements beyond 112 using Pb and Bi targets had dropped so low that further increases in the efficiency of their systems or perhaps use of different target-projectile combinations would be required. Production of element 113 has not yet been reported.

Now that production of the elements 110 and 111 has been confirmed and confirmation of 112 may soon follow, what about their chemistry? The half-lives of the longest confirmed isotopes are only 55 milliseconds (ms) for 110, 2 ms for 111, and 0.6 ms for the reported isotope of 112, hardly promising candidates for chemical studies. A similar situation exists for Mt, whose longest known isotope is only about 40 ms. Some longer lived isotopes of elements 110 and 112 have been reported by researchers at Dubna in 2000–02, but none of these has yet been confirmed.

An isotope with a half-life of at least a second is needed for chemical studies, and its decay characteristics must be well established in order to furnish a positive "signature" to prove that the desired element is actually being studied. Because the production rates are so low--often only decay of a few atoms per week can be detected--the results of many separate identical experiments must be combined. Very efficient chemical separations must be devised that reach equilibrium rapidly, can be conducted in a short time compared to the half-life, and give the same results on an "atom-at-a-time" basis as for macroquantities.

Typically, the isotope with the longest half-life and largest production rate is chosen for chemical studies; this is not necessarily the first isotope discovered. For example, the half-lives of the isotopes used in the first definitive chemical studies of the transactinides are 75 seconds (²⁶¹Rf), 34 seconds (²⁶²Db), 21 seconds (²⁶⁶Sg), 17 seconds (²⁶⁷Bh), and about 14 seconds (²⁶⁹Hs). [The predictions of deformed shells at Z = 108–110 and number of neutrons (N) = 162–164 spurred experimentalists to search for longer lived isotopes of seaborgium (Sg), bohrium (Bh), and hassium (Hs) for chemical studies; these were discovered and used in the first chemical studies.] Their well-known a-decay properties were used for positive identification.

The improvement in experimental techniques for atom-at-a-time studies of elements with both short half-lives and small production rates has permitted chemical studies of both aqueous- and gas-phase chemistry of the transactinides through Sg. In general, these studies have confirmed that their chemical properties are similar to those of their lighter homologs in groups 4, 5, and 6, respectively; however, unexpected deviations from simple extrapolation of known trends within the groups were found. Theoretical investigations based on molecular relativistic calculations helped provide guidance for experimentalists in designing these experiments.

Bh and Hs have only been studied in the gas phase. Studies of the oxychloride of Bh reported in 2000 showed that it behaved similarly to rhenium and technetium, and in 2002 separation of Hs as a volatile oxide similar to that of osmium tetroxide was reported. Solution chemistry of Bh and Hs has not been conducted because the aqueous chemistry and preparation of samples suitable for measuring a-particles or fission fragments is too slow. Very fast liquid-liquid extraction systems followed by direct incorporation of the activity in a flowing-liquid scintillation detection system have been developed for Rf and Db and may prove applicable in future studies of solution chemistry of short-lived species. Another promising technique is the use of a "preseparator" such as the Berkeley Gas-filled Separator (BGS) to separate and furnish the desired heavy-element isotope to the chemical system so that studies of its chemistry do not first require decontamination from other recoiling products.

These results for the lighter transactinides suggest that Mt, Ds, and elements 111 and 112 should be placed under iridium, platinum, gold, and mercury, respectively, as members of the 6d transition series that began with Rf and is expected to end with element 112. Mt and Ds have received little recent attention from chemical theorists, as 111 and 112 have been deemed more interesting. Relativistic calculations indicate that in element 111, the +3 and +5 states will be more stable than in gold and that the +1 exhibited by gold may be very difficult to prepare. Element 112 is the most interesting, as the calculations predict that the strong relativistic contraction and stabilization of the 7s orbitals and its closed-shell configuration should make element 112 rather inert and that it may actually behave more like a rare gas and have properties more similar to radon than to mercury. Experiments have been designed to try to determine whether 112 behaves more like mercury or radon, but definitive results must await discovery of a longer lived a-decaying isotope in order to establish that element 112 is actually being investigated.

Superheavy Elements. The transactinides are defined simply as all those beyond Lr, so, of course, this includes the long-sought "island" of superheavy elements (SHEs) predicted in the 1970s to be near the spherical nuclear shells at Z = 112–114 and N = 184. Half-lives as long as billions of years were calculated, and the island was believed to be separated by a "sea of instability" from the peninsula of known nuclei.

Production of element ²⁸⁸114 (174 neutrons) with a half-life of about three seconds and element ²⁹²116 (176 neutrons) with a half-life of ~50 µs was reported by a Dubna/LLNL collaboration working at Dubna in 2000–02 using ²⁴⁴Pu and ²⁴⁸Ca targets with ⁴⁸Ca projectiles, but the results have yet to be confirmed. They have proposed that these should be called SHEs, although they are still far from the 184-neutron shell. The element 112 and 110 daughters of these -decay chains were reported to have half-lives on the order of 10 seconds. It is extremely important to confirm these results, because investigations of their chemical properties could then be considered if the production rates can be increased.

Some nuclear theorists predict that element 110 with N = 182–184 might be the longest lived SHE, with a half-life of about 100 years. Others propose that the strongest spherical shell might be at Z = 124 or 126, while still others suggest Z = 120 and N = 172. At any rate, it now seems clear that species with half-lives long enough for chemical studies can exist all along the way to an "island of stability"; the critical problem is how to synthesize them.

Reactions of Pb or Bi with ⁸⁷Rb or ⁸⁶Kr projectiles could produce ²⁹⁴119, whose half-life would only be microseconds, but it would decay to new longer lived odd-Z elements 117, 115, and 113 via successive a-emission and end with a known Lr isotope. Some theorists have suggested that so-called unshielded reactions in which the Coulomb barrier is below the bombarding energy might result in enhanced production yields of very neutron-rich nuclei. For example, the reaction ¹⁷⁰Er(¹³⁶Xe, n) would make ³⁰⁵122 with 183 neutrons. Reactions using radioactive beams such as ⁴⁷K or ⁴⁶Ar with radioactive actinide targets have been suggested, but even these still don't reach the Z = 114, N = 184 region. Another possibility is to use accelerated, neutron-rich fission product beams of krypton, rubidium, and strontium with Pb and Bi targets to reach the Z = 120 and 126 region. Not only must new reactions be investigated, but imaginative methods to increase beam intensities and production rates and even to develop new kinds of accelerators and projectiles must be found if we are to produce these longer lived isotopes that lie just beyond our reach.

New automated systems with preseparators may be envisioned to facilitate both aqueous- and gas-phase studies of short-lived isotopes, and techniques to "stockpile" possible longer lived species at the accelerators where they are produced for later off-line separation must be devised if this tantalizing new region of elements is to be explored.

Darleane C. Hoffman is a professor of the graduate school of chemistry at the University of California, Berkeley, and faculty senior scientist in the Heavy Element Group of the Nuclear Science Division of Lawrence Berkeley National Laboratory. She was awarded the National Medal of Science in 1997, the ACS Priestley Medal in 2000, and the 2003 Sigma Xi William Procter Prize.

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