Wednesday, November 07, 2012

Hunting number 113

Here’s the pre-edited form of an article on element 113 that appeared in the October issue of Prospect.


The periodic table of the elements just got a new member. At least, maybe it did – it’s hard to tell. Having run out of new elements to discover, scientists have over the past several decades been making ‘synthetic’ atoms too bloated to exist in nature. But this is increasingly difficult as the atoms get bigger, and the new element recently claimed by a Japanese group – currently known simply as element 113, its serial order in the periodic table – is frustratingly elusive. These artificial elements are made and detected literally an atom at a time, and the researchers claim only to have made three atoms in total of element 113, all of which undergo radioactive decay almost instantly.

That, and competition from teams in the United States and Russia, makes the claim controversial. The first group to sight a new element enjoys the privilege of naming it, an added spur to the desire to be first. Just as in the golden years of natural-element discovery in the nineteenth century, element-naming tends to be nationalistic and chauvinistic. No one could begrudge Marie and Pierre Curie their polonium, the element they discovered in 1989 after painstakingly sifting tonnes of uranium ore, which they named after Marie’s homeland. But the recent naming of element 114 ‘flerovium’ – after the founder of the Russian institute where it was made – and element 116 ‘livermorium’, after the Lawrence Livermore National Laboratory where it originated, display rather more concern for bragging than for euphony.

Perhaps this is inevitable, given that making new elements began in an atmosphere of torrid, even lethal, international confrontation. The first element heavier than uranium (element number 92, which is where the ‘natural’ periodic table stops) was identified in 1940 at the University of California at Berkeley. This was element 93, christened neptunium by analogy with uranium’s naming after the planet Uranus in 1789. It quickly decays into the next post-uranium element, number 94, the discovery of which was kept secret during wartime. By the time it was announced in 1946, enough had been made to obliterate a city: this was plutonium, the explosive of the Nagasaki atom bomb. The ensuing Cold War race to make new elements was thus much more than a matter of scientific priority.

To make new elements, extra nuclear particles – protons and neutrons – have to be crammed into an already replete nucleus. The sequential numbering of the elements, starting from hydrogen (element 1), is more than just a ranking: this so-called atomic number indicates how many protons there are in the nuclei of the element’s atoms. Differences in proton count are precisely what distinguish one element from another. All elements bar hydrogen also contain neutrons in their nuclei, which bind the protons together. There’s no unique number of neutrons for a given element: different neutron totals correspond to different isotopes of the element, which are all but chemically indistinguishable. If a nucleus has too few or too many neutrons, it is prone to radioactive decay, as is the case for example for carbon-14 (six protons, eight neutrons), which provides the basis for radiocarbon dating.

By element 92 (uranium), the nuclei are so swollen with particles that no isotopes can forestall decay. All the same, that process can be very slow: the most common isotope of uranium, uranium-238, has a half-life of about 4.5 billion years, so there’s plenty of it still lying around as uranium ore. Making nuclei more massive than uranium’s involves firing elementary particles at heavy atoms in the hope that some will temporarily stick. That was how Emilio Segrè and Edwin McMillan first made neptunium at Berkeley in 1939, by firing neutrons into uranium. (In the nucleus a neuron can split into a proton, raising the atomic number by 1, and an electron, which is spat out.) McMillan didn’t realise what he’d done until the following year, when chemist Philip Abelson helped him to separate the new element from the debris.

During the Second World War, both the Allies and the German physicists realised that an atomic bomb could be made from artificial elements 93 or 94, created by neutron bombardment of uranium inside a nuclear reactor. Only the Americans managed it, of course. The Soviet efforts in this direction began at the same time, thanks largely to the work of Georg Flerov. In 1957 he was appointed head of the Laboratory of Nuclear Reactions, a part of the Joint Institute of Nuclear Research in Dubna, north of Moscow. Dubna has been at the forefront of element-making ever since; in 1967 the lab claimed to have made element 105, now called dubnium.

That claim exemplifies the ill-tempered history of artificial elements. It was disputed by the rival team at Berkeley, who made 105 in the same year and argued furiously over naming rights. The Soviets wanted, magnanimously but awkwardly, to call it nielsbohrium, after Danish physicist Niels Bohr. The Americans preferred hahnium, after the German nuclear chemist Otto Hahn. Both dug in their heels until the International Union of Pure and Applied Chemistry (IUPAC), the authority on chemical nomenclature, stepped in to resolve the mess in the 1990s. Finally the Russian priority was acknowledged in the name, which after all was a small riposte to the earlier American triumphalism of americium (element 95), berkelium (element 97) and californium (98).

These ‘superheavy’ elements, with atomic numbers reaching into double figures, are generally made now not by piecemeal addition to uranium but by trying to merge together two smaller but substantial nuclei. One – typically zinc or nickel – is converted into electrically charged ions by having electrons removed, and then accelerated in an electric field to immense energy before crashing into a target made of an element like lead. This is the method used by the laboratory in Darmstadt that, since the 1980s, has outpaced both the Americans and the Russians in synthesizing new elements. Called the Institute for Heavy Ion Research (GSI), it has claimed priority for all the elements from 107 to 112, and their names reflect this: element 108 is hassium, after the state of Hesse, and element 110 is darmstadtium. But this crowing is a little less strident now: many of the recent elements have instead been named after scientists who pioneered elemental and nuclear studies: bohrium, mendelevium (after the periodic table’s discoverer Dmitri Mendeleyev), meitnerium (after Lise Meitner), rutherfordium (Ernest Rutherford). In 2010 IUPAC approved the GSI team’s proposal for element 112, copernicium, even though Copernicus is not known ever to have set foot in an (al)chemical lab.

If, then, we already have elements 114 and 116, why the fuss over 113? Although the elements get harder to make as they get bigger, the progression isn’t necessarily smooth: some combinations of protons and neutrons are (a little) easier to assemble than others. Efforts to make 113 have been underway at least since 2003, when a group at the Nishina Center for Accelerator-based Science in Saitama, near Tokyo, began firing zinc ions at bismuth. The Japanese centre, run by the governmental research organization RIKEN, was a relative newcomer to element-making, but it claimed success just a year later. It’s precisely because they are unstable that these new elements can be detected with such sensitivity: the radioactive decay of a single atom sends out particles – generally an alpha particle – that can be spotted by detectors. Each atom initiates a whole chain of decays into successive elements, and the energies and the release times of the radioactive particles are characteristic ‘fingerprints’ that allow the decay chain – and the elements within it – to be identified.

At least, that’s the theory. In practice the decay events must be spotted amidst a welter of nuclear break-ups from other radioactive elements made by the ion collisions. And with so many possible isotopes of these superheavy elements, the decay properties of which are often poorly known, there’s lots of scope for phantoms and false trails – not to mention outright fraud (Bulgarian nuclear scientist Victor Ninov, who worked at Berkeley and GSI, was found guilty of fabricating evidence for the claimed discovery of element 118 at Berkeley in 2001). When you consider the figures, some scepticism is understandable: the Japanese team estimated that only 3-6 out of every 100 quintillion (10**20) zinc ions would produce an atom of 113.

Last year, IUPAC representatives decided the Japanese results weren’t conclusive. But neither were they persuaded by subsequent claims of scientists at Dubna and Berkeley, who have begun collaborating after decades of bitter rivalry. However, on 26 September the RIKEN team released new data that make a stronger case. The team leader Kosuke Morita attests that he is “really confident” they have element 113 pinned. Again they’ve only a single decay chain to adduce – starting from a single atom of 113 – but some experts now find the case convincing. If so, it looks like the name game will get solipsistic again: rikenium and japonium are in the offing.

Given how hard it is to make this stuff, why bother? Plutonium isn’t the only artificial element to find a use: for example, minute amounts of americium are used in some smoke detectors. Yet as the superheavies get ever heavier and less stable, typically decaying in a fraction of a second, it’s harder to imagine how they could be of technological value. But according to calculations, some isotopes of element 114 and nearby elements should be especially stable, with half-lives of perhaps several days, years, even millennia. If that’s true, these superheavies could be gradually accumulated atom by atom. But some other estimates say this ‘island of stability’ won’t appear until element 126; others suspect it may not really exist at all.

There are another, more fundamental motivations for making new elements. They test to destruction the current theories of nuclear physics: it’s still not fully understood what the properties of these massive nuclei are, although they are expected to do weird things, such as take on very deformed, non-spherical shapes.

Artificial elements also pose a challenge to the periodic table itself, chemistry’s organizing scheme. It’s periodic because, as Mendeleyev and others realised, similar chemical properties keep reappearing as the elements’ atomic numbers increase: the halogens chlorine (element 17), bromine (35) and iodine (53) all form the same kinds of chemical compounds, for example. That’s because atoms’ electrons – negatively charged particles that govern chemical behaviour – are arranged in successive shells, and the arrangements for elements in the same column of the periodic table are analogous: all the halogens are one electron short of a filled outermost shell.

But a very massive nucleus starts to undermine this tidy progression of electron shells. The electrons closest to the nucleus feel the very strong electric field of that mass of protons, which makes them very energetic – they circulate around the nucleus at speeds approaching the speed of light. Then they feel the effects of special relativity: as Einstein predicted, particles moving that fast gain mass. This alters the electrons’ energies, with knock-on effects in the outer shells, so that the outermost electrons that determine the atom’s chemical behaviour don’t observe the periodic sequence. The periodic table then loses its rhythm, as such elements deviate from the properties of those with which it shares a column – it might form a different number of chemical bonds, say. Some anomalous properties of natural heavy elements are caused by these “relativistic” effects. They alter the electron energies in gold so that it absorbs blue light, accounting for the yellow tint of the light it reflects. And they weaken the chemical bonds between mercury atoms, giving the metal its low melting point.

Relativistic deviancy is expected for at least some superheavies. To look for it, researchers have to accomplish extraordinarily adroit chemistry: to figure out from just a handful of atoms, each surviving for perhaps seconds to minutes, how the element reacts with others. This could, for example, mean examining whether a particular chemical compound is unusually volatile or insoluble. The teams at GSI, Dubna and Berkeley have perfected methods of highly sensitive, quick-fire chemical analysis to separate, purify and detect their precious few exotic atoms. That’s enabled them to establish that rutherfordium (element 104) and dubnium buck the trends of the periodic table, whereas seaborgium (106) does not.

As they enter the artificial depths of the periodic table, none of these researchers knows what they will find. The Dubna group claims to have been making element 115 since 2003, but IUPAC has not yet validated the discovery. They are on firmer grounds with 117 and 118, which are yet to be named, and both GSI and the RIKEN team are now hunting 119 and 120.

Is there any limit to it? Richard Feynman once made a back-of-the-envelope calculation showing that nuclei can no longer hold onto electrons beyond an atomic number of 137. More detailed studies, however, shows that to be untrue, and some nuclear scientists are confident there is no theoretical limit on nuclear size. Perhaps the question is whether we can think up enough names for them all.

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