Poor Element 118. It tried to make a grand entrance back in 1999, when researchers from Lawrence Berkeley Laboratory announced the first experimental observation for this rare "superheavy" element, along with its immediate decay product, Element 116. Two years later, the researchers retracted their claim, after they were unable to reproduce those results in subsequent experiments. A re-analysis of the original data uncovered evidence not of Element 118, but of scientific misconduct on the part of Bulgarian physicist Victor Ninov.
Berkeley Lab did what any prestigious scientific institution would do in those circumstances: it launched an investigation. A panel of experts concluded that Ninov was the only person responsible for translating the raw computer data into readable results, and that he'd exploited his position to insert false data into his analysis. Ninov denied the charge, apparently claiming to be the victim of an international scientific conspiracy intent on blaming him for the lab's mistakes. He was summarily fired despite such protestations, and last we heard, he was teaching physics courses and quietly going about his business at the University of the Pacific. Element 118 retreated into the wings to await its cue to step back into the limelight.
That long-awaited cue arrived this week with the publication in Physical Review C of a scientific paper, and an accompanying press release from Lawrence Livermore National Laboratory announcing that this time, really and truly, Element 118 had indeed made its presence known on the (increasingly crowded) stage of the periodic table.
Alas, the critical reception was a bit, well, subdued, to say the least. Sure, there were minor mentions in science news feeds, the odd snarky blog post, and the obligatory article in the science section of the New York Times, but by and large -- thanks in large part to its tarnished history -- Element 118 made its debut to polite clapping rather than a standing ovation. (Such a disappointment for any aspiring elemental diva: a walk-on cameo appearance instead of the expected starring role. Jen-Luc Piquant feels its pain, but also points out that "Element 118" isn't the catchiest stage name.) And that's too bad, because it really is kind of cool when you put the discovery into a broader context, particularly for those of us who aren't "physics insiders."
See, our entire observable world is made up of the first 90 or so elements in the periodic table, and most of those were formed in stars. It takes explosive energies on a par with large supernovae to produce the heaviest elements, up to, say, Fermium (Element 100). Elements above that simply don't exist on their own in nature, but nonetheless, scientists have managed to produce them in the laboratory, beginning in the 1940s with neptunium and plutonium (Elements 93 and 94, respectively). They're really unstable, though, decaying almost as quickly as they're born. Still, in the 1960s, a handful of physicists -- led by chemical physicist Glenn Seaborg, among others -- predicted that some elements around the 114 mark would be able to survive much longer, forming what has become known as an "island of stability" in the sea of superheavy unstable elements.
There's a certain amount of scientific debate about how stable particular superheavy elements might be, but everyone pretty much agrees that the protons and neutrons that make up atomic nuclei can only occupy discrete energy levels, just like the electrons that circle those nuclei in well-defined orbitals. They're known as "shells" among physical chemists, and each shell can only contain a certain number of particles. (Protons and neutrons fill separate shells.) Move up the periodic table, and one finds that the shells fill from the inside outward, going from low to high energies.
There are certain so-called "magic numbers" that appear in the process -- 2, 8, 20, 50, and 82, for example -- in which the outermost shell is tightly packed (filled to capacity), such that any new particle would have to go into a new shell. And some atomic nuclei are "doubly magic": the shells of both the protons and neutrons are filled, rather than one or the other. So the most stable atoms have shells that are all filled to capacity, making them far less likely to decay. For instance, oxygen has 8 protons and 8 neutrons, making it one of the most stable elements, because both protons and neutrons have "magic numbers." Ditto for lead (82 protons and 126 neutrons). And prevailing nuclear theory therefore allows for such a shell structure to create accessible islands of stability.
Enter the current team of researchers hailing both from Livermore and from Dubna, the Joint Institute for Nuclear Research (JINR) in Russia. They've been on a bit of a roll the past few years, announcing experimental evidence for Elements 113 and 115, and creating Element 114 in 1998. The latter lasted a total of 34 minutes before the final decay product fissioned, providing conclusive evidence that there really was such a thing as an "island of stability," and verifying the prediction from the 1960s that a likely "peak" of that expected island should be an atom with a doubly magic nucleus of 114 protons and 184 neutrons. They've been looking for further stable isotopes ever since.
This time around, they bombarded a target of californium-249 atoms with a bunch of calcium-48 ions, and over the course of three years, out of the resulting shower of atoms, they found evidence for three -- count 'em! (the researchers did) -- atoms of Element 118 (each with a total atomic mass of 294): one in 2002, and two in 2005. The trio of superheavyweights didn't hang around very long, however: they lasted about one millisecond. Since any meaningful chemical analysis would require at least an hour or so, scientists weren't able to make any definitive direct measurements of its properties. But they know that Element 118 should lie just beneath radon in the periodic table, so it's got to be some kind of noble gas. It's a start, right?
If Element 118 has such a short lifetime, how the hell did the researchers know for sure they'd found it? Pretty much the same way Fermilab's particle physicists can tell if they've produced a top quark in the lab's flagship atom smasher, the Tevatron: by telltale signature decay patterns. Nuclear decay is what happens when an atom's nucleus turns into a different element with a lower number on the periodic table. These decay patterns provide a useful kind of genealogical chart showing the branchings of the elemental family tree. Nuclei of Element 118 decay into Element 116 (itself a new element co-discovered by this particular collaboration), then into Element 114, and finally into Element 112, before it fissions into two daughter particles of roughly the same size.
So the scientists essentially traced the genealogy of those two final daughter particles backwards through time to identify Element 118 as the primary ancestor. It's a painstaking, time-consuming task to analyze all that raw data, made more difficult by the need to be doubly diligent in the wake of the Ninov scandal, so the researchers held off publishing their results until they'd combed through everything so thoroughly, there is a less than one in in ten thousand chance that their findings are due to a statistical fluke. And I'd wager Livermore made damn sure there was an even less chance of falsified data before they agreed to make the announcement.
Not content to rest on their laurels, the Livermore/Dubna collaborators will next search for an expected Element 120 by bombarding a plutonium target with iron isotopes, and so forth until they hit the theoretical limits of the periodic table. And who knows? Maybe they'll get better at producing isotopes that lie within the periodic "island of stability," so they'll stick around long enough for scientists to study their unique properties. In the meantime, let's all at least give a nod of acknowledgment to the belated stage debut of Element 118. It might seem a bit anticlimactic in light of its checkered history, but it's still a noteworthy achievement.