||Editor's note: On July 27, 2001, the results reported below were retracted
through a correspondence with Physical Review Letters.
BERKELEY, CA — Discovery of two new "superheavy" elements has been announced by scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. Element 118 and its immediate decay product, element 116, were discovered at Berkeley Lab's 88-Inch Cyclotron by bombarding targets of lead with an intense beam of high-energy krypton ions. Although both new elements almost instantly decay into other elements, the sequence of decay events is consistent with theories that have long predicted an "island of stability" for nuclei with approximately 114 protons and 184 neutrons.
"We jumped over a sea of instability onto an island of stability that theories have been predicting since the 1970s," said nuclear physicist Victor Ninov who was first author of a paper that has been submitted to Physical Review Letters.
Said Ken Gregorich, a nuclear chemist who led the discovery team, "We were able to produce these superheavies using a reaction that, until a few months ago, we had not considered using. However, theoretician Robert Smolanczuk (a visiting Fulbright scholar from the Soltan Institute for Nuclear Studies in Poland) calculated that this reaction should have particularly favorable production rates. Our unexpected success in producing these superheavy elements opens up a whole world of possibilities using similar reactions: new elements and isotopes, tests of nuclear stability and mass models, and a new understanding of nuclear reactions for the production of heavy elements."
Gregorich and Ninov are members of Berkeley Lab's Nuclear Science Division (NSD). Walter Loveland, on sabbatical from Oregon State University, also made major contributions to this work. Other participants from the NSD included long-time leaders in the search for superheavy elements Albert Ghiorso and Darleane Hoffman, plus Diana Lee, Heino Nitsche, Wladyslaw Swiatecki, Uwe Kirbach, Carola Laue, and graduate students from the University of California at Berkeley Jeb Adams, Joshua Patin, Dawn Shaughnessy, Dan Strellis, and Philip Wilk. Hoffman and Nitsche are also professors of chemistry at UC Berkeley.
Noting that four members of the discovery team are German citizens, U.S. Secretary of Energy Bill Richardson, whose department funded this work said, "This stunning discovery which opens the door to further insights into the structure of the atomic nucleus also underscores the value of foreign visitors and what the country would lose if there were a moratorium on foreign visitors at our national labs. Scientific excellence doesn't recognize national boundaries, and we will damage our ability to perform world-class science if we cut off our laboratories from the rest of the world."
The isotope of element 118 with mass number 293 identified at Berkeley Lab contains 118 protons and 175 neutrons in its nucleus. By comparison, the heaviest element found in nature in sizeable quantities is uranium which, in its most common form, contains 92 protons and 146 neutrons. Transuranic elements in the periodic table can only be synthesized in nuclear reactors or particle accelerators. Though often short-lived, these artificial elements provide scientists with valuable insights into the structure of atomic nuclei and offer opportunities to study the chemical properties of the heaviest elements beyond uranium.
Within less than a millisecond after its creation, the element 118 nucleus decays by emitting an alpha particle, leaving behind an isotope of element 116 with mass number 289, containing 116 protons and 173 neutrons. This daughter, element 116, is also radioactive, alpha-decaying to an isotope of element 114. The chain of successive alpha decays continues until at least element 106.
"In these experiments, observation of a chain of six high-energy alpha decays within about one second unambiguously signaled the production and decay of element 118," says Gregorich. "During 11 days of experiments, three such alpha-decay chains were observed indicating production of three atoms of element 118. The decay energies and lifetimes measured for these new isotopes of elements 118, 116, 114, 112, 110, 108, and 106 provide strong support for the existence of the predicted island of stability."
Referring to these results, discovery-team member Hoffman said, "After a 30-year search, this discovery is extremely gratifying. I only wish Glenn Seaborg had been alive to see these results." Seaborg, the recently deceased Nobel laureate chemist and co-discoverer of plutonium and nine other transuranic elements, was one of the earliest and most outspoken advocates of experiments to reach the predicted island of stability.
Elements 118 and 116 were discovered by accelerating a beam of krypton-86 ions to an energy of 449 million electron volts and directing the beam into targets of lead-208. This yielded heavy compound nuclei at low excitation energies.
During the last several years, low excitation energy reactions failed to take scientists beyond element 112, and it was assumed that production rates for heavier elements were too small to extend the periodic table further using this approach. However, the recent calculations of Smolanczuk indicating increased production rates for the Kr-86 + Pb-208 reaction prompted the experimental search for element 118 at Berkeley Lab.
The key to the success of this experiment was the newly constructed Berkeley Gas-filled Separator (BGS). Said Gregorich, "The innovative BGS design has resulted in a separator with unsurpassed efficiency and background suppression which allows us to investigate nuclear reactions with production rates smaller than one atom per week. For these experiments, the strong magnetic fields in the BGS focused the element 118 ions and separated them from all of the interfering reaction products which were produced in much larger quantities."
Another important factor for the experiment's success was the unique ability of the 88-Inch Cyclotron to accelerate neutron-rich isotopes such as krypton-86 to high-energy and high-intensity beams with an average current of approximately 2 trillion ions per second.
"The 88-inch Cyclotron is the only accelerator in the United States at this time that can provide krypton beams at the intensities that this experiment demanded," said Claude Lyneis, the NSD physicist who heads the accelerator facility for Berkeley Lab.
In operation since 1961, the 88-inch Cyclotron has been upgraded with the addition of a high-performance ion sources and can now accelerate beams of ions as light as hydrogen or as heavy as uranium. The 88-Inch Cyclotron is a national user facility serving researchers from around the world for basic and applied studies.
Said I-Yang Lee, scientific director at the 88-Inch Cyclotron, "From the discovery of these two new superheavy elements, it is now clear that the island of stability can be reached. Additionally, similar reactions can be used to produce other elements and isotopes, providing a rich new region for the study of nuclear and even chemical properties."
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.