(10-16) 12:36 PDT LIVERMORE -- Scientists at the Lawrence Livermore National Laboratory and their Russian colleagues have discovered a long-sought and controversial "superheavy" element -- one that exists for only a tiny fraction of a second -- in experiments at a nuclear research center near Moscow, the Livermore team announced today.

By bombarding the radioactive element Californium with an electrically charged beam of calcium in Russia's Dubna nuclear reactor, the two teams of chemists and physicists detected a mere 3 atoms of the heaviest element ever discovered -- number 118 on the periodic table -- before the fleeting substance decayed into a series of somewhat lighter elements.

The American scientists said they are supremely confident their discovery is no mere random event -- an important claim because only four years ago, scientists at the Lawrence Berkeley National Laboratory had to officially retract their 1999 published claim that they had in fact discovered element 118, and had fired the renowned Russian-born nuclear chemist who led their group.

The new team of American and Russian scientists is calling the new element Eka-radon, because on the periodic table of elements, familiar to every high school chemistry student, it would be placed just below radon, a well known radioactive element.

The new element, said the scientists, would most likely be a "noble gas" -- related on the periodic table to the better-known gases helium, neon, argon, krypton and xenon. But it is highly unstable, and within a fleeting instant of less than a thousandth of a second it emits sub-nuclear particles from its heart and decays swiftly and in succession to element 116 and then element 114, the Livermore scientists reported.

The new element, said Kenneth Moody, leader of the Livermore team, is the sixth "superheavy" that the American and Russian teams have discovered in their quest to understand the basic structure of all the atoms that make up the known universe.

There is an unknown region on the periodic table known as the "Island of Stability" where elements might be far more stable than this one, Moody said in a telephone conference with reporters.

"The decay products of all the isotopes we have made so far paint a picture of a large, sort of flat Island of Stability," Moody said, "and indicate that we may have luck if we try to go even heavier."

E-mail David Perlman at dperlman@sfchronicle.com.