FY2010 - Oak Ridge National Laboratory

Director’s R&D Fund—
General
05569
Identification of New, Super Heavy Element Z=117 Using HFIR-
Produced 249 Bk Target Material and Intense 48 Ca Beam at Dubna
Krzysztof P. Rykaczewski, Robert K. Grzywacz, Jeffrey L. Binder, Julie G. Ezold, and J. H. Hamilton
Project Description
The project aimed and succeeded in the identification of a new super-heavy chemical element of the
periodic table with atomic number Z=117. The short-lived radioactive 249 Bk target produced from the
curium and americium seed material irradiated during two years at the ORNL High Flux Isotope Reactor
(HFIR) was used in this experiment. Over 22 mg of 249 Bk, the radioactive isotope having Z=97 protons
and N=152 neutrons, were chemically separated at the ORNL Radiochemical Engineering Development
Center (REDC) in the first half of 2009. At the end of June 2009, the 249 Bk was transferred to the Institute
of Atomic Reactors at Dimitrovgrad (Russia) for target fabrication. The rotating target wheel containing
six 249 Bk target sectors was transferred to the Joint Institute for Nuclear Research (JINR) at Dubna
(Russia) in July 2009. The irradiation of 249 Bk with a 48 Ca beam of 252 MeV and 245 MeV energy,
started at the Flerov Laboratory of Nuclear Reactions (FLNR) of JINR at the end of July 2009 and lasted
150 days. Two isotopes of new chemical element Z=117 were identified among fusion-evaporation
reaction products. Five decay chains of 293(117) isotope and one long decay chain starting at 294(117)
isotope were observed, leading to the identification of 11 new super heavy nuclei.
The results were published by the Dubna–ORNL–Las Vegas–Livermore–Vanderbilt–Dimitrovgrad
collaboration in Physical Review Letters 104, 142502 in April 2010. An increased stability with larger
neutron number observed for newly identified super heavy nuclei represents an experimental verification
for the existence of the predicted Island of (Enhanced) Stability for Super Heavy Elements. The original
publication triggered over 200 articles in popular and scientific media, including New York Times,
Science, Physics Today, and others.
Mission Relevance
The discovery of a new chemical element is an increasingly difficult high-profile scientific achievement,
as could be seen from the media reaction to the announcement of the Z=117 discovery. The studies
aiming for super heavy elements help to establish and understand the limits of the periodic table of
elements and of atomic nuclei. The decay properties and production cross section of new isotopes
identified in this study helped us to understand the structure of the heaviest nuclei and the underlying
nuclear forces binding nucleons together. The decreasing alpha-decay energies and correspondingly
increasing half-lives with the increasing neutron number of odd-Z isotopes created suggestive evidence
for approaching the predicted magic neutron number N=184.
New ideas related to the search for even heavier new chemical elements have been developed during this
project. Upon success of the current experiment, the search for the new element Z=119 could be
performed using the next batch of HFIR-REDC–produced 249 Bk material and an intense 50 Ti Z=22 beam
either at JINR Dubna or GSI Darmstadt (Germany). Future studies of super heavy elements may use
Z=98 251 Cf targets extracted from old californium material present in the ORNL inventory of transactinides
isotopes. The amount of californium material of enhanced purity, at the level of 1 mg of 251 Cf to
1 ng of 252 Cf, will allow the search of new isotopes of elements Z=118 (with 48 Ca beam) and for the
isotopes of new element Z=120 (with 50 Ti beam). Since the alpha-decay half-lives of the isotopes of
elements heavier than Z=117 may be in the microsecond region, the use of a fast digital data acquisition
system designed at ORNL (at Holifield Radioactive Ion Beam Facility) will be essential in future studies
of new elements. The development of new detector and digital data acquisition systems, recently
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