Elements 116 and 118 Are Discovered

At the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, physicists (including collaborators from Lawrence Livermore National Lab in the United States) have sent a beam of calcium-48 ions into a target of californium-249 atoms to create temporarily a handful of atoms representing element 118. The nucleus for these atoms have a total atomic mass of 294 units.

In fact, only three of these atoms, the heaviest ever produced in a controlled experiment, were observed. After sending 2 x 10¹⁹ calcium projectiles into the target, one atom of element 118 was discovered in the year 2002 and two more atoms in 2005. The researchers held up publication after seeing their first specimen in order to find more events. According to Livermore physicist Ken Moody, speaking at a press conference today from Livermore, the three events have been well studied and the odds of a statistical fluke at work here are less than a part in 100 thousand.

Caution would naturally be on the minds of anyone announcing a new element; Evidence for element 118 was offered once before, by a team at the Lawrence Berkeley National Laboratory (see PNU 432), but this claim was later retracted (PNU 550) when it was discovered that some of the data had been falsified.

In searching through 10¹⁹ collision events, how do you know you have found a new element? Because of the clear and unique decay sequence involving the offloading of alpha particles, nuclear parcels consisting of two protons and two neutrons. In this case, nuclei of element 118 decay to become element 116 (hereby itself discovered for the first time), and then element 114, and then element 112 by emitting detectable alphas. The 112 nucleus subsequently fissions into roughly equal-sized daughter particles.

The average lifetime observed for the three examples of element 118 was about one millisecond, not long enough to perform any kind of chemical tests (you'd need an hour's time for that). Element 118 lies just beneath radon in the periodic table and is therefore a kind of noble gas.

The Dubna-Livermore team previously announced the discovery of elements 113 and 115 (see PNU 672) and next hope to produce element 120 by crashing a beam of iron atoms into a plutonium target. To build nuclei much heavier than this you would need a beam of neutron-rich radioactive nuclei; the proposal to build an accelerator in the United States for doing just this has been stalled.

Oganessian et al., Physical Review C, October 2006
Livermore press release