Element 118 Bows Out

In the spring of 1999, scientists at the Lawrence Berkeley National Laboratory were excited to find ¹ three events whose decay chains bore the imprint of the heaviest artificially created isotope to date, with atomic number Z = 118 (see Physics Today, August 1999, page 17 ). In subsequent runs, however, the Berkeley team has not seen any more events like those on which it based its claim, and searches for element 118 have also come up dry at the Laboratory for Heavy Ion Research (GSI) in Darmstadt, Germany; at the RIKEN Accelerator Research Facility in Wako, Japan; and at the GANIL accelerator in Caen, France. The Berkeley experimenters have now submitted a comment to Physical Review Letters retracting their published claim.

Group leader Kenneth Gregorich told us that he and his coworkers still don’t understand what happened, and they are working to ferret out the problem. In the meantime, Gregorich said, “There’s been quite a bit of experimental and theoretical work based on our 1999 data, so that we felt we needed to get the word out.” The heavy-ion researchers to whom we spoke congratulate the Berkeley researchers for being straightforward about the misidentification, unfortunate though it was.

The experiment that produced the errant three events was one of the first to be conducted on a gas-filled separator that had been newly installed at the Lawrence Berkeley laboratory. By now, the experimenters there have had two years of experience on the facility, and their procedures and computer codes have naturally evolved. When the subsequent experiments failed to turn up evidence of the earlier decay chains, the group went back to the original data. Analyzing the old data in several independent ways, they did not see the three chains. They are now trying to figure out why the chains showed up in the earlier analysis.

The Berkeley researchers were trying to form the isotope ²⁹³118 by the fusion reaction of krypton-86 nuclei impinging on a lead-208 target. Many heavy-ion researchers had discounted the possibility of seeing such a heavy isotope; the synthesis of elements with Z up to 112 had suggested that the fusion cross sections would decrease with atomic number to the point where element 118 would be undetectable. But a theoretical calculation at the time projected a higher production rate for this particular reaction, with a magic projectile, ⁸⁶Kr, incident on a doubly magic ²⁰⁸ Pb target.

Experiments at Berkeley, GSI, RIKEN, and GANIL suggest that the production rates for element 118 on their machines are less than one atom per week. To continue to hunt, Gregorich said, “will take either lots more patience or more sensitive technology.”