NUCLEAR PHYSICS:
Berkeley Crew Bags Element 118

Robert F. Service


 Step aside, element 114; there's a new heavyweight champ. Physicists at the Lawrence Berkeley National Laboratory in California announced earlier this week that they have created two new superheavy elements, tipping the scales at 118 and 116 protons. The new heavyweights come as something of a surprise, as standard theories had suggested that the technique used to create them--fusing two medium-weight nuclei at a relatively low energy--should top out at 112. The team's success suggests that the method may produce weightier champs, with atomic numbers 119 and beyond. "It's a very exciting result," says Ron Lougheed, a heavy-element physicist at Lawrence Livermore National Laboratory in California. "I suspect it will lead to a flurry of new isotopes in this region."

 Ever since the early 1940s, when Glenn Seaborg and his Berkeley colleagues created the first handful of artificial elements beyond the 94 that exist in nature, physicists have vied to forge the next heaviest element. The most recent milestone came in January when researchers at the Joint Institute for Nuclear Research in Dubna, Russia, won a race to create the long-sought element 114 (Science, 22 January, p. 474). Element 114 was a special prize because its 30-second lifetime seemed to confirm predictions of an island of stability--a realm of superheavy elements including 114 and its neighbors, whose nuclei have an internal structure that makes them more stable than heavier and lighter isotopes.

 Although the 114 work has yet to be duplicated, the success marked an unexpected renaissance for a previously successful technique known as hot fusion, in which a beam of light isotopes is smashed into a heavier target, such as plutonium. Prior to that success, the technique of choice had been cold fusion, a gentler collision of medium-sized isotopes. Researchers at the Institute for Heavy Ion Research (GSI) in Darmstadt, Germany, used the technique to lay claim to five elements from 107 to 112 since the early 1980s. Conventional theories suggested that neither technique would be able to form elements as big as 118 without them instantly breaking apart, or fissioning.

 The Berkeley team's big break came at the prodding of Robert Smola«nczuk, a visiting theorist from the Soltan Institute for Nuclear Studies in Poland, who suggested that there may still be a little warmth left in cold fusion. His calculations suggested that bombarding a lead target with krypton ions would have reasonable odds of producing a few atoms of 118 after all: The compound nucleus, he found, was less likely to fission than previously thought. "We didn't really believe it," says Ken Gregorich, who led the 15-member Berkeley team. "But it was one of those experiments where there was little to lose and a big upside. We tried it and were surprised to see something."

 They saw a lot of somethings. After an accelerator flung the krypton ions into the lead, the impact debris was swept into another machine that separated detritus from atoms of potential interest, which were channeled to a radiation detector. The detector measured a distinct pattern of alpha-particle emissions as the sought-after heavyweight shed pieces of itself in search of a more stable configuration. "During 11 days of experiments, three such alpha-decay chains were observed indicating production of three atoms of element 118," says Gregorich. As an added bonus, the first alpha decay in each case produced an atom of element 116--also never before seen. And the time course of the decays lent support to the theory of the island of stability around element 114.

 Next up, Gregorich says, his team plans to switch the lead target for one of bismuth atoms, which harbor an extra proton. If they fuse with krypton, the group will have yet another champ at 119.

Volume 284, Number 5421 Issue of 11 Jun 1999, p 1751 
©1999 by The American Association for the Advancement of Science
.