BERKELEY, CA -- Discovery of two new "superheavy" elements hasbeen announced by scientists at the U.S. Department of Energy'sLawrence Berkeley National Laboratory. Element 118 and itsimmediate decay product, element 116, were discovered at BerkeleyLab's 88-Inch Cyclotron by bombarding targets of lead with anintense beam of high-energy krypton ions. Although both newelements almost instantly decay into other elements, the sequenceof decay events is consistent with theories that have longpredicted an "island of stability" for nuclei with approximately114 protons and 184 neutrons.
"We jumped over a sea of instability onto an island of stabilitythat theories have been predicting since the 1970s," said nuclearphysicist Victor Ninov who was first author of a paper that hasbeen 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 reactionthat, until a few months ago, we had not considered using.However, theoretician Robert Smolanczuk (a visiting Fulbrightscholar from the Soltan Institute for Nuclear Studies in Poland)calculated that this reaction should have particularly favorableproduction rates. Our unexpected success in producing thesesuperheavy elements opens up a whole world of possibilities usingsimilar reactions: new elements and isotopes, tests of nuclearstability and mass models, and a new understanding of nuclearreactions for the production of heavy elements."
Gregorich and Ninov are members of Berkeley Lab's NuclearScience Division (NSD). Walter Loveland, on sabbatical fromOregon State University, also made major contributions to thiswork. Other participants from the NSD included long-time leadersin the search for superheavy elements Albert Ghiorso and DarleaneHoffman, plus Diana Lee, Heino Nitsche, Wladyslaw Swiatecki, UweKirbach, Carola Laue, and graduate students from the Universityof California at Berkeley Jeb Adams, Joshua Patin, DawnShaughnessy, Dan Strellis, and Philip Wilk. Hoffman and Nitscheare also professors of chemistry at UC Berkeley.
Noting that four members of the discovery team are Germancitizens, U.S. Secretary of Energy Bill Richardson, whosedepartment funded this work said, "This stunning discovery whichopens the door to further insights into the structure of theatomic nucleus also underscores the value of foreign visitors andwhat the country would lose if there were a moratorium on foreignvisitors at our national labs. Scientific excellence doesn'trecognize national boundaries, and we will damage our ability toperform world-class science if we cut off our laboratories fromthe rest of the world."
The isotope of element 118 with mass number 293 identified atBerkeley Lab contains 118 protons and 175 neutrons in itsnucleus. By comparison, the heaviest element found in Nature insizeable quantities is uranium which, in its most common form,contains 92 protons and 146 neutrons. Transuranic elements inthe periodic table can only be synthesized in nuclear reactors orparticle accelerators. Though often short-lived, theseartificial elements provide scientists with valuable insightsinto the structure of atomic nuclei and offer opportunities tostudy the chemical properties of the heaviest elements beyonduranium.
Within less than a millisecond after its creation, the element118 nucleus decays by emitting an alpha particle, leaving behindan isotope of element 116 with mass number 289, containing 116protons and 173 neutrons. This daughter, element-116, is alsoradioactive, alpha-decaying to an isotope of element 114. Thechain of successive alpha decays continues until at least element106.
"In these experiments, observation of a chain of six high-energyalpha decays within about one second unambiguously signaled theproduction and decay of element 118," says Gregorich. "During 11days of experiments, three such alpha-decay chains were observedindicating production of three atoms of element 118. The decayenergies and lifetimes measured for these new isotopes ofelements 118, 116, 114, 112, 110, 108, and 106 provide strongsupport 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 andco-discoverer of plutonium and nine other transuranic elements,was one of the earliest and most outspoken advocates ofexperiments to reach the predicted island of stability.
Elements 118 and 116 were discovered by accelerating a beam ofkrypton-86 ions to an energy of 449 million electron volts anddirecting the beam into targets of lead-208. This yielded heavycompound nuclei at low excitation energies.
During the last several years, low excitation energy reactionsfailed to take scientists beyond element 112, and it was assumedthat production rates for heavier elements were too small toextend the periodic table further using this approach. However,the recent calculations of Smolanczuk indicating increasedproduction rates for the Kr-86 + Pb-208 reaction prompted theexperimental search for element 118 at Berkeley Lab.
The key to the success of this experiment was the newlyconstructed Berkeley Gas-filled Separator (BGS). Said Gregorich,"The innovative BGS design has resulted in a separator withunsurpassed efficiency and background suppression which allows usto investigate nuclear reactions with production rates smallerthan one atom per week. For these experiments, the strongmagnetic fields in the BGS focused the element 118 ions andseparated them from all of the interfering reaction productswhich were produced in much larger quantities."
Another important factor for the experiment's success was theunique ability of the 88-Inch Cyclotron to accelerateneutron-rich isotopes such as krypton-86 to high-energy andhigh-intensity beams with an average current of approximately 2trillion ions per second.
"The 88-inch Cyclotron is the only accelerator in the UnitedStates at this time that can provide krypton beams at theintensities that this experiment demanded," said Claude Lyneis,the NSD physicist who heads the accelerator facility for BerkeleyLab.
In operation since 1961, the 88-inch Cyclotron has been upgradedwith the addition of a high-performance ion sources and can nowaccelerate beams of ions as light as hydrogen or as heavy asuranium. The 88-Inch Cyclotron is a national user facilityserving researchers from around the world for basic and appliedstudies.
Said I-Yang Lee, scientific director at the 88-Inch Cyclotron,"From the discovery of these two new superheavy elements, it isnow clear that the island of stability can be reached.Additionally, similar reactions can be used to produce otherelements and isotopes, providing a rich new region for the studyof nuclear and even chemical properties."
Berkeley Lab is a U.S. Department of Energy national laboratorylocated in Berkeley, California. It conducts unclassifiedscientific research and is managed by the University ofCalifornia.
The above story is based on materials provided by Lawrence Berkeley National Laboratory. Note: Materials may be edited for content and length.
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