October 16, 2006
Scientists from California's Lawrence Livermore National Laboratory (LLNL) and the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, have discovered element 118.
"From the three atoms that we found, the average lifetime of 118 is 0.9 milliseconds," says LLNL team member Dawn Shaughnessy. That's too short for scientists to study the new element's chemical properties, but the superheavy atom sits directly beneath radon on the periodic table. It's in the same family as helium and neon.
The researchers first saw a single element 118 atom in 2002, but the evidence wasn't conclusive. Between February and June 2005, in experiments conducted at the JINR U400 cyclotron, the researchers observed a tell-tale chain of decaying atoms that establishes the element's existence. "There is only a small probability that what we've seen is due to random events," says LLNL team member Nancy Stoyer. "We're very confident."
The 2005 experiments bombarded a rotating californium target with 10 million trillion calcium ions; this produced only two element 118 nuclei. The Livermore-Dubna team observed these nuclei decay first to element 116, then to element 114, then to element 112, after which they underwent fission and split apart. The scientists published their results in the October 2006 issue of Physical Review C.
Scientists think certain combinations of protons and neutrons — so-called magic numbers — will allow some very heavy elements to be far more stable than others. Using such experiments, they're trying to map out an "island of stability" at the fringes of the periodic table.
"The decay properties of all the isotopes that we have made so far paint the picture of a large, sort of flat island of stability and indicate that we may have luck if we try to go even heavier," says Ken Moody, Livermore's team leader. "Anything more you can learn about the periodic table is exciting. It can tell us why the world is here and what it is made of."
"The heavy element community will continue to search for new elements until the limit of nuclear stability is found," says LLNL team member Mark Stoyer. "It is expected that limit will be found."
"We're already beyond where the periodic table would have ended if it hadn't been for this extra stability effect," adds Moody. In 2007, the scientists plan to look for element 120 by bombarding a plutonium target with iron isotopes.
In 1999 and 2001, the LLNL scientists announced the discovery of elements 114 and 116, respectively. In 2004, the Livermore-Dubna team observed elements 113 and 115. Also in 1999, researchers at California's Lawrence Berkeley National Laboratory reported synthesizing element 118, but, in 2001, they retracted their claim when subsequent experiments failed to confirm the discovery.