The discovery of element 117, a synthetic element that does not occur in nature, required many years of collaborative research by dozens of scientists from several international institutions.
The experiment that produced the first evidence of element 117 could not have happened without multiple world-class scientific facilities, including a unique accelerator complex in Russia and the nuclear research reactor and processing facilities at the Department of Energy’s Oak Ridge National Laboratory.
ORNL is the only place in the world that can produce sufficient quantities of the radioactive element berkelium, a synthetic element that is essential for the creation of element 117. Berkelium is a byproduct of californium production at ORNL’s High Flux Isotope Reactor (HFIR).
Vanderbilt University professor Joe Hamilton noted when californium production resumed at ORNL in 2008 and introduced ORNL’s Jim Roberto to Yuri Oganessian, who successfully pioneered the “hot fusion” approach to synthesize superheavy elements by bombarding actinide targets with heavy ions accelerated in a cyclotron at the Joint Institute for Nuclear Research in Dubna, Russia. This meeting formed the basis of the ongoing collaboration among the three institutions. Oganessian’s earlier work led to the discovery of elements 114, 116 and 118 in 2000-2004 in collaboration with Lawrence Livermore National Laboratory. Actinide target materials for all of these discoveries came from ORNL and Dimitrovgrad in Russia. But element 117 identification was not possible without berkelium.
ORNL and JINR formally agreed to collaborate on superheavy element research in December 2008, including production of berkelium target material at ORNL and participation in the accelerator experiments at JINR. Lawrence Livermore National Laboratory, which had collaborated with JINR on previous superheavy element research, also joined the team in late 2008, adding nuclear data analysis capabilities.
The berkelium was produced through 250 days of irradiation at ORNL’s HFIR and 90 days of processing at the adjoining Radiochemical Engineering and Development Center (REDC) to separate and purify the berkelium material.
On June 15, 2009, ORNL sent 22 milligrams of berkelium-249, with the clock ticking away on its 327-day half-life, to JINR. This material was forwarded to the Russian Research Institute of Atomic Reactors in Dimitrovgrad, which fabricated a target by applying the berkelium radioisotope to a thin film of titanium.
The target was sent from RIAR to JINR at Dubna, where the experiment began on July 28, 2009.
The 249Bk target was bombarded for 150 days with an intense beam of 7 trillion calcium-48 ions per second at one of the world's most powerful heavy ion accelerators. Under very rare conditions, the calcium nuclei (containing 20 protons) interacted with the berkelium nuclei (containing 97 protons) to create a few compound nuclei with atomic number 117. These compound nuclei were separated from the calcium beam by a set of strong magnets at the Dubna Gas Filled Recoil Separator. The element 117 nuclei were implanted into silicon detectors, where distinctive radioactive decays were measured.
Eventually, the detectors turned up six short-lived but history-making atoms of element 117, which then decayed into elements 115, 113, 111, 109, 107 and 105.
The international research team announced its discovery of element 117 in April 2010 in a Physical Review Letters publication. The discovery paper included 33 authors from six institutions: JINR (15), ORNL (7), LLNL (6), Vanderbilt (2), RIAR (1), and University of Nevada Las Vegas (2).
A group of 72 scientists from 16 institutions in Australia, Finland, Germany, India, Japan, Norway, Poland, Sweden, Switzerland, the United Kingdom and the United States conducted confirmation experiments to independently verify the discovery of element 117. This research involved the production of berkelium at ORNL and bombardment with high-power calcium-ion beams in an accelerator at GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. This independent observation of element 117 was presented in a Physical Review Letters study published in May 2014.
Additional confirmation studies were conducted by JINR, ORNL, LLNL, Vanderbilt and the University of Tennessee, Knoxville (UTK) at JINR in 2012 using berkelium from ORNL (Phys. Rev. Lett. 109, 162501 (2012)). ORNL’s Krzysztof Rykaczewski and UTK’s Robert Grzywacz led the development of a new detector and digital data acquisition system that was used in the follow-up experiments on super heavy nuclei at JINR.
A joint committee of the International Union of Pure and Applied Physics and International Union of Pure and Applied Chemistry assessed the evidence and announced on December 30, 2015, that the criteria for the discovery of element 117 had been met. IUPAC also confirmed the discovery of element 115, a decay product of element 117, in this announcement. The committee then invited the discovering team from JINR, LLNL, ORNL, and Vanderbilt to propose permanent names and symbols for elements 117 and 115.
On June 8, 2015, the International Union of Pure and Applied Chemistry (IUPAC) Inorganic Chemistry Division published a Provisional Recommendation for the name tennessine and symbol Ts for element 117, in recognition of the contributions of Oak Ridge National Laboratory, Vanderbilt University, and the University of Tennessee at Knoxville to superheavy element research, including the production and chemical separation of unique actinide target materials for superheavy element synthesis at ORNL’s HFIR-REDC facilities. Recognizing Tennessee in the naming of element 117 had been a topic of discussion since originally suggested by Hamilton. The name tennessine was formally proposed by the discovery team during a video conference on March 23, 2016, with the ending “ine” chosen to be consistent with the required convention for elements in group 17 of the periodic table. The provisional name moscovium was proposed for element 115, honoring the Russian region that is home to JINR.
Significance of element 117
In the current periodic table, elements beyond uranium (atomic number of 92) are increasingly unstable and decay rapidly into other elements.
Nuclear physicists theorize that an “island of stability” exists beyond the current periodic table where new superheavy elements would exhibit longer lifetimes. Such an island would extend the periodic table to even heavier elements, and the increased lifetimes would enable chemistry experiments and potential applications for these elements.
Element 117 was the only missing element in row seven of the periodic table. On course to the island of stability, researchers initially skipped element 117 due to the difficulty in obtaining the berkelium target material. Element 117 and the resulting new isotopes from the radioactive decay of element 117 bring scientists closer to the island of stability and support a general trend of increasing stability for superheavy elements with increasing numbers of neutrons in the nucleus. The discovery of two isotopes of element 117 and their 11 decay products provides strong evidence for the existence of the island of stability.
- April 9, 2010 – Discovery of element 117 announced in Physical Review Letters http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.104.142502
- October 19, 2012 – 2nd experiment at JINR confirms element 117 - http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.109.162501
- May 1, 2014 – Experiment at GSI independently confirms element 117 - http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.172501
- December 30, 2015 – IUPAC announces verification of four new elements - http://iupac.org/discovery-and-assignment-of-elements-with-atomic-numbers-113-115-117-and-118/
- June 8, 2016 – IUPAC issues provisional recommendation on proposed names - http://iupac.org/recommendation/names-and-symbols-of-the-elements-with-atomic-numbers-113-115-117-and-118/