Almost 20 years after it was first created and identified at LBL's Heavy Ion Linear Accelerator, the heavy man-made element 106 has been re-created and confirmed in an experiment at the Lab's 88-Inch Cyclotron.
The experiment was designed by Ken Gregorich, a divisional fellow in LBL's Nuclear Science Division, and carried out by a team including Gregorich, faculty senior scientist Darleane Hoffman, and postdocs and students from LBL and the UC Berkeley Chemistry Department. The confirmation will be reported at the Actinides '93 conference to be held Sept. 20-24 in Santa Fe, N.M.
The isotope of element 106 produced and identified in the experiment consisted of 106 protons and 157 neutrons. Isotopes are forms of the same element that vary in the number of neutrons they contain. Although the chemical properties of different isotopes are identical, their nuclear properties may differ.
The nuclear properties and half-life (0.9 second) of the isotope observed at the 88-Inch Cyclotron are the same as those reported in the original discovery of element 106 by Al Ghiorso and his co-workers at the HILAC in 1974. Thus, this experiment constitutes direct confirmation of the original discovery, and paves the way for the long-delayed naming of element 106.
According to criteria proposed by nuclear science researchers in the 1970s, naming of a new element is the prerogative of the original discovery team, but proposal of a name should await independent confirmation of the discovery. Because of the extreme difficulty of creating and identifying elements as heavy as 106, such confirmation can be a very long time in coming. According to Ghiorso, members of the original discovery team are elated about the confirmation and are discussing possible names.
The heaviest element whose discovery has been previously confirmed is element 105, hahnium. Three more heavy elements, 107 through 109, have been discovered at Germany's GSI laboratory, but have not yet been confirmed by other researchers.
[Coincidentally, a joint working party set up by the International Union of Pure and Applied Chemistry (IUPAC) and the equivalent physics body (IUPAP) has just issued its report on credit for discovery and naming of the reported transfermium elements, a long-time subject of dispute between the U.S. team at LBL and similar teams at the Dubna accelerator in Russia. The report, published in the August 1993 issue of IUPAC's journal Pure and Applied Chemistry, was meant to cast light on the question of priority and credit for heavy-element discoveries, but turns out to generate as much heat as light. In a response published in the same journal, LBL's Glenn Seaborg and Ghiorso take issue with some of the working party's findings, saying that credit for some discoveries has been assigned on the basis of "less than definitive" results.]
The confirmation of element 106 at the 88-Inch Cyclotron depended on finding and identifying correlated "parent-daughter" alpha decays in the energy range predicted for isotope 263 of element 106 and its daughter element rutherfordium-259 (element 104).
Target nuclei of californium-249 were bombarded with oxygen-18 ions. The reaction products -- containing a few nuclei of element 106 -- recoiled out of the target, were swept away by a jet of helium gas, and came to rest in one of 80 plastic foils positioned around the periphery of the rotating horizontal wheel of the MGA (Merry Go Around system). There, an atom of 106 underwent an alpha decay, which was recorded by one of the detectors positioned in pairs above and below the wheel. The daughter isotope, rutherfordium-259 (an isotope of element 104), recoiled into an opposing detector. The foil was then moved into a new position and the decay of the rutherfordium-259 was recorded. Final identification of element 106-263 involved the detection of nine such correlated pairs of parent-daughter alpha decays during a run of about 80 hours.
In addition to Gregorich and Hoffman, the research team included staff scientist Diana Lee, postdoctoral fellow Mike Mohar, and UCB Chemistry Department graduate students Mike Lane, Chris Kacher, and Eric Sylwester.
In a related but independent development, a collaborative group working at Russia's Dubna accelerator is reporting two new isotopes of element 106: element 106-256 with 159 neutrons, and element 106-266 with 160 neutrons. This work is a collaboration between the Dubna group, headed by Y.A. Lazarev, and a group from the Lawrence Livermore National Laboratory (LLNL), including Ron Lougheed, K. J. Moody, John Wild, Ken Hulet, and J. H. McQuaid. The work is also being described at the Actinides '93 conference in Santa Fe.