Superheavy Elements

In 2010, the Physics Division of Oak Ridge National Laboratory played a major role in the discovery of two new elements, element 117 and its decay product element 115. The identification of element Z=117, officially named tennessine, completed the 7th row of the Periodic Table of Elements and provided evidence for the existence of the predicted Island of Stability for superheavy elements. More than 200 news articles worldwide chronicled the discovery, and the American Physical Society designated the formal recognition of tennessine as one of the ten top physics news of the year.

The synthesis of new elements 117Ts and 115Mc

The main overarching goals of the Superheavy Element (SHE) research are related to the understanding of physics defining the extent of the nuclear and atomic worlds. Superheavy elements, a term coined by John Wheeler in 1955, refers to the elements with atomic numbers greater than 103. It is expected that a set of isotopes of superheavy elements may have considerably longer half-lives than known isotopes of these elements, predicted to appear as an “Island of Stability” in the chart of nuclides. Today we have reached the beachhead of this long-sought Island of Stability in the heaviest elements [1]. There are new fascinating phenomena, absent in lighter nuclei, but present at the Island and beyond the currently anticipated landscape of nuclei and atoms [2,3]. The strong Coulomb fields at the highest charge numbers are trying to break apart the nuclei, while the nuclear interactions are stabilizing these multinucleon objects. The Coulomb force overcoming the attractive nuclear forces and triggering fission of the heaviest nuclei and atoms may terminate the Periodic Table and Chart of Nuclei. This potential effect of the competition between fundamental forces can be verified experimentally during future experiments involving the discoveries of new superheavy elements and nuclei. The structure of these heaviest nuclei and atoms must be understood before we can reliably predict the limits of our nuclear and atomic worlds.

ORNL chemists and physicists, Julie Ezold, Kevin Felker, Robin Taylor, Dennis Benker, Frank Riley, Porter Bailey, Krzysztof Rykaczewski, Chuck Alexander, Rose Boll,  Robert Grzywacz and Jim Roberto, co-authors of publications related to the synthesis of elements 117 and 115 are standing near the updated Periodic Table during celebrations of the discoveries at ORNL in January 2017. The copies of this Periodic Table, co-signed by Tennessee Governor Bill Haslam and by ORNL director Thom Mason, were donated and distributed by UT-Battelle to all public middle and high schools in Tennessee.

In 2010, Oak Ridge National Laboratory (ORNL) played a major role in the discovery of two new elements, element 117 [4,5] and its decay product element 115 [6,7]. The identification of element Z=117 (now officially designated as tennessine) completed the 7th row of the Periodic Table of Elements and provided evidence for the existence of the predicted Island of Stability for superheavy elements [8]. More than 200 news articles worldwide chronicled the discovery, and the American Physical Society designated the formal recognition of tennessine as one of the ten top physics news of the year.

Overall, ORNL has discovered three new elements, promethium (Z=61), moscovium (Z=115) and tennessine (Z=117), as well as made major contributions to the identification of six more. The element promethium was discovered by ORNL chemists Jakob Marinsky, Larry Glendenin and Charles Coryell  [9] at the Oak Ridge Graphite Reactor in 1945. The confirmation of elements 104-106 by atomic number Z identification was achieved by Pete Dittner and Curt Bemis  [10] using accelerator-produced nuclei in the Physics Division in the 1970s.  Unique actinide target materials from ORNL enabled the discovery of elements 114 (flerovium), 116 (livermorium), and 118 (oganesson) in hot fusion reactions at the Joint Institute for Nuclear Research (JINR, Dubna, Russia) in the 2000s [11]. The recent discovery of the new elements 117 and 115 was officially credited in December 2015 to an international collaboration involving the Joint Institute for Nuclear Research, Oak Ridge National Laboratory, Lawrence Livermore National Laboratory, Vanderbilt University, and the University of Tennessee at Knoxville by the International Union of Pure and Applied Chemistry and International Union of Pure and Applied Physics [12]. With the exception of promethium, all of these new elements were synthesized using actinide materials available from ORNL’s High Flux Isotope Reactor (HFIR) and the adjacent Radiochemical Engineering Development Center (REDC), see [5,11,13]. The latest discoveries were complemented by state-of-the-art digital detection technologies developed in the Physics Division for the discovery of new short-lived proton and alpha emitters at ORNL’s Holifield Radioactive Ion Beam Facility by Krzysztof Rykaczewski and Robert Grzywacz [14,15]. These technologies, developed further at the Digital Pulse Processing Laboratory led by Robert Grzywacz at the University of Tennessee at Knoxville [16], were later implemented at JINR to enable detection of sub-microsecond activities that have become increasingly important in superheavy element studies  [17,18,19].

In 2005, Yuri Oganessian of JINR approached ORNL to join JINR in a US-Russia effort to discover element 117 based on bombarding a 249Bk target available only from ORNL with the world’s most intense 48Ca beam at JINR. A proposed schedule for production of 249Bk at ORNL, followed by six months of dedicated accelerator time at JINR, was developed in 2008 and agreed to by the DOE Office of Nuclear Physics later that year. Krzysztof Rykaczewski was the Principal Investigator for ORNL with overall coordination by ORNL Associate Laboratory Director Jim Roberto. The production and separation of the unique 249Bk target material (T1/2=327 days), a byproduct of 252Cf production, required a year of intense neutron irradiation of Cm and Am seed material in ORNL’s world’s leading High Flux Isotope Reactor( HFIR), followed by several months of chemical separation and purification at the adjacent Radiochemical Engineering Development Center (REDC). The effort resulted in 22 mg of pure 249Bk having an activity of about 1.3*1012 Becquerel (35 Ci). This effort required the participation of dozens of ORNL scientists and engineers [5,11,13].

In June 2009, the 249Bk was shipped to Russia but was immediately returned due to confusion related to the required paperwork. With the precious cargo slowly decaying, it ultimately required five transatlantic flights before the material was formally accepted at Moscow’s Sheremetyevo airport. The 249Bk was then transported by small plane to Dmitrovgrad, where the Bk material was hand-painted onto Ti-foils at the Research Institute of Atomic Reactors. More than 150 days of round-the-clock irradiations with an intense 48Ca beam at the Flerov Laboratory for Nuclear Reactions at JINR resulted in the observation of six decay chains of the new element 117, including the 293117 and 294117 isotopes [4]. Physics Division scientists traveled to Russia to participate in the experiment which also produced nine additional new superheavy nuclei with odd atomic numbers between 115 and 105 in the decay chains of element 117 [4,5]. The increasing lifetimes of these nuclei with increasing neutron number provides direct evidence for the existence of an “Island of Stability” for the heaviest elements and nuclei. These longer lifetimes result from quantum shell effects originally predicted in the 1960s [20,21,22]. The increased stability provides opportunities for physics and chemistry studies of these superheavy nuclei and points to the possibility of even heavier elements.

Confirmation experiments for element 117 were performed in 2012 at the Gesellschaft fuer Schwer-Ionen Forschung (GSI) in Germany [23,24] and also at JINR using the next batch of 249Bk material produced at ORNL [25,26]. About 24 mg of 249Bk was equally split between two projects using 48Ca beams for the irradiation. In addition to the confirmation of the discovery of element 117, a joint study using a 50Ti beam was undertaken at GSI involving ORNL scientists aimed at the synthesis of another new element 119 [27].

To date, 24 decay chains of 293Ts and 294Ts have been observed by the international collaborations working at JINR Dubna and GSI Darmstadt [28,24]. Additional important confirmation of the identification of Z=117 isotopes was obtained through so-called cross bombardment. The 289115 isotope, which is an alpha decay product of 293117, was directly produced and identified by the US-Russia collaboration among the products of the 48Ca + 243Am reaction, which was investigated from late 2010 to early 2012 [6,7,28]. It provided further evidence for the new elements 117 and 115. In 2015, a joint working group of the International Union of Pure and Applied Chemistry and International Union of Pure and Applied Physics (IUPAC) recognized the discovery of element 117 and its decay product, element 115 [11], giving the naming priority to the Russian-American collaboration [29].

The discovery of the new elements was open to public comments for several months. There were many suggestions submitted to IUPAC for possible names, including a proposal to name element 117 after a famous rock star that was supported by more than two hundred thousand petitioners. However, according to IUPAC rules [30], only the discoverers of the element can propose a name, and the possibilities are restricted to certain categories such as a scientist’s name or a geographical location. The collaboration chose tennessine and moscovium for elements 117 and 115, respectively, recognizing the contributions from Tennessee (ORNL, UTK, Vanderbilt) and from the Moscow region. The discovery of these elements was the result of an international collaboration achieving what neither country could have accomplished alone and is a powerful example of how such collaborations can facilitate discoveries in science.

ORNL physicists are currently participating in the search for new elements 119 at RIKEN in Japan and 120 at JINR in Russia. These experiments are dependent on unique actinide target materials from ORNL and are benefiting from digital data acquisition systems modelled after the system developed by Grzywacz, Rykaczewski and their ORNL and UTK associates. These international experiments are also searching for new heavy nuclei in their decay chains, including new isotopes of Ts in the decay chain from the yet unknown element 119.

The ultimate experiment aiming at the synthesis of a nucleus with a potentially magic neutron number N=184 and of three new elements Z=124, 122 and 120 has been proposed by Rykaczewski et al [31]. The fusion-evaporation reaction of 58Fe beam impacting a target of 251Cf nuclei leads to an excited compound nucleus with the mass number A=309 and atomic number Z=124. After one-neutron evaporation, the N=184 isotone 308124 is created. Nuclear models predict a long alpha decay chain originating from 308124 nucleus and ending at known superheavy nuclei 292Lv, 288Fl and 284Cn. However, there is no theoretical consensus about the enhanced stability at N=184 for such high atomic numbers. Also, there are dramatically different theoretical predictions of the fission barriers in mass A~308 nuclei affecting the reliability of the fusion-1n evaporation cross section estimations for the synthesis of 308124. This high-risk high-reward experiment will likely benefit from information obtained from the synthesis of element 119 and 120. 

 

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M. Schaedel, B. Schausten, A. Semchenkov, D.A. Shaughnessy, P. Steinegger, J. Steiner, E.E. Tereshatov, P. Thoerle-Pospiech, 
K. Tinschert, T. Torres De Heidenreich, N. Trautmann, A. Tuerler, J. Uusitalo, M. Wegrzecki, N. Wiehl, S.M. Van Cleve, and V. Yakusheva
Physical Review C, in press

28.“Superheavy nuclei from 48Ca-induced reactions”
Yu. Ts. Oganessian and V.K. Utyonkov,
Nuclear Physics A 944, 62, 2015. 
https://doi.org/10.1016/j.nuclphysa.2015.07.003

29. “Names and symbols of the elements with atomic numbers 113, 115, 117 and 118 (IUPAC Recommendations 2016) “
Lars Öhrström and Jan Reedijk
Pure and Applied Chemistry, 88(12), 1225, 2016.
https://doi.org/10.1515/pac-2016-0501

30. “How to name new chemical elements (IUPAC Recommendations 2016)”
Willem H. Koppenol, John Corish, Javier García-Martínez, Juris Meija and Jan Reedijk
Pure and Applied Chemistry, 88(4), 401, 2016.
https://doi.org/10.1515/pac-2015-0802

31. “ORNL actinide materials and a new detection system for superheavy nuclei”
K.P. Rykaczewski, J.B. Roberto, N.T. Brewer and V. K. Utyonkov,
EPJ Web of Conferences 131, 05005, 2016.
Nobel Symposium NS 160 – Chemistry and Physics of Heavy and Superheavy Elements
Kristianstad, SWEDEN Date: ‏ MAY 29-JUN 03, 2016
NOBEL SYMPOSIUM NS 160 – CHEMISTRY AND PHYSICS OF HEAVY AND SUPERHEAVY 
https://doi.org/10.1051/epjconf/201613105005