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Element 118 Discovered Again--for the First Time

bombardment



SABRINA FLETCHER AND THOMAS TEGGE, LLNL
After claims of its discovery were retracted in 2002, a new team of researchers says it has produced a few scant atoms of the heaviest element yet, called simply element 118 after the number of protons in its nucleus. The discovery came after months of relentlessly bombarding heavy atoms onto a radioactive target and searching for distinctive chains of radioactive decays, the group reports. Confirmation of the finding, however, will come only when other groups have reproduced it.

In chemistry, elements having certain numbers of electrons are particularly stable. These are the noble gases: helium, neon, argon, krypton, xenon and radon. Similarly, elements having certain so-called magic numbers of protons or neutrons should also be especially stable. Examples include lead and tin. Based on the magic number 114, researchers believe there also exists a group of relatively stable heavy elements, called the island of stability, having roughly that many protons each. Experiments performed at Russia's Dubna, the Joint Institute for Nuclear Research, had already turned up signs of elements 113 through 116.

To go further, the group began blasting a target of radioactive californium 249 (98 protons, 115 neutrons) with a beam of calcium 48 ions (20 protons, 28 neutrons). The collisions should occasionally fuse the two different elements into a single atom of 118 protons and 143 neutrons, which would lie in the periodic table column directly below radon. Element 118 would then decay into element 116 and lighter elements by emitting a series of alpha particles, each of which contains two protons. But even an element reasonably close to a magic number becomes much less stable at the high temperatures created by the collisions, explains team member Kenton Moody of Lawrence Livermore National Laboratory, so the group had to search through an enormous number of events.

Finally, after a total of four months spent in 2002 and 2005, the researchers observed three events that looked like decays of element 118, which showed a half-life of a millisecond, they report in the October Physical Review C. The signatures match up with prior studies of the decay of element 116, bolstering the claim for the heavier element, Moody says. "We're being as careful as we can," he notes. Four years ago physicist Victor Ninov was fired from Lawrence Berkeley National Laboratory for falsifying the data behind a previously reported discovery of element 118. "We worried about this a lot," Moody reflects. "We do what we can to minimize the problem. There is no one person who's responsible for the data analysis."

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