and neutrons it can hold. For protons, thecapacity of the first shell is 2, followed inturn by shells containing 6, 12,8, 22, 32 and44. Neutrons are thought to exist within anidentical framework. Crucially, the modelshows that when a nucleus has a fullcomplement of protons or neutrons in itsouter shell, it is highly stable, and even morestable if it has a full complement of both typesof particle. For example, an oxygen nucleusis extremely stable because it contains 8protons and 8 neutrons, which means itsouter shell is full, containing 6 protons and6 neutrons. As a result of all this, the numbersof protons and neutrons that lead to full shellshave been dubbed the "magic numbers". Theyare 2, 8, 20, 28, 50, 82 and 126 (due to amodification in the theory to account for theeffects of huge nuclei, the proton numbers of114and 120 are also considered magic).

This model successfully explains whysome heavy-ish elements are unstable,such as the highly radioactive polonium(atomic number 84), while their neighbourson the periodic table, such as lead (82), arenot. It's because lead is magic. In fact, strictlyspeaking, it's doubly so: its most stable isotope, .lead-zox, has a magic number of both protons(82) and neutrons (126).Change the numberof neutrons and you get a different-isotopeof lead, which is not doubly magic any moreand therefore less stable.

Another prediction of the shell modelis the existence oflong-lived superheavyelements - the island of stability - amidsta sea of unstable elements (see diagram,page 34). These elements should have magicnumbers of protons and neutrons that lendstability to an otherwise shaky ensemble.According to the theory, the next doubly magicelement will have 114protons and 184 neutrons.

Hot on the trail is Oganessian's team.Along with colleagues at the LawrenceLivermore National Laboratory (LLNL)in ~

The quest for the biggest atoms in the universe isteetering on the brink of the possible, says Tim Dean

Higher, higher!•

IN APRIL,the Israeli scientist AmnonMarinov announced that he hadglimpsed a mysterious island. It was

something that he and many rival explorershad long been seeking - but few of those rivalswere prepared to accept that he had reallyfound what he said he'd found. Yet ifhe isright, physicists may have to reconsider whatthey know about the outer limits of matter.

What Marinov of the Hebrew Universityof Ierusalem and his colleagues claim to havediscovered is a monstrous atom with by farthe heaviest nucleus ever seen, packing awhopping 122protons and 170 neutrons.Crucially, the team had not synthesised it inthe lab, but found it in nature, in a sample ofpurified thorium. The discovery implied thatthe element had a half-life of no less than 100million years (www.arxiv.org!0804.3869).

Had his team finally stumbled upon thefabled "island of stability" - a region populatedby superheavy atoms that is widely believedto exist beyond the outer reaches oftheperiodic table? POSSibly,but a key problemfor Marinov's atomic giant is that roo-million-year half-life. "It couldn't be shorter or wewouldn't see it," he explains. This is incrediblylong for such a massive element, and goesagainst the theoretical grain. No wonderhis claim has been viewed with scepticism.

The island of stability is no flight offancy,though. It is predicted by well-establishedtheory, the same theory that casts doubton Marinov's claim. By smashing atomicnuclei together, some ofthe rival teams havesynthesised elements just a handful of protonsshort of Marinov's purported record-breaker.Step by step, they just might reach their goal."People have always been interested in thisproblem, especially the limits of the materialworld's existence," says Yuri Oganessian ofthe Joint Institute of Nuclear Research (JINR)in Dubna, Russia.

Leaving aside Marinov's possible find, the

321 NewScientist 126 July 2008

heaviest known naturally occurring elementis uranium, with an atomic number of 92 - thenumber of protons in its nucleus. In 1941,Nobelprizewinner Glenn Seaborg and his team at theUniversity of California, Berkeley, synthesisedthe first element heavier than uranium bybombarding it with neutrons. The new elementhad an atomic number of 94 and was namedplutonium. Subsequently, Seaborg madeSignificant extensions to the periodic table,predicting several more elements heavierthan uranium. His team went on to synthesisea total of 10 so-called transuranium elements,culminating in 1974 with what was eventuallynamed seaborgium, atomic number 106.

Superheavy monstersSince then, others have taken on the challengeof making "superheavy" elements (anythingwith an atomic number of 104 or morequalifies as superheavy). This is more thanjust an exercise in machismo: pushing theboundaries of matter puts theoretical modelsof atomic structure to the test.

When Seaborg started his work ontransuranium elements, the accepted theoryof the nucleus was the "liquid drop model",developed by Niels Bohr and John Wheeler.This envisaged the nucleus as a drop ofcharged liquid whose surface tensioncounteracted the mutual repulsion of theprotons within, keeping the nucleus stable.In the 1960s, however, researchers at JINRshowed that the lifetimes of some isotopesof the transuranium elements were ordersof magnitude less than that predicted bythe model, so it fell by the wayside.

The liquid drop model has now beenreplaced by another model in which protonsand neutrons populate discrete energy levels,or "shells", within the nucleus, much aselectrons exist in shells around it. Eachshell has a maximum number of protons

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California, the team has pioneered thesynthesis of atoms heavier than seaborgium.So far, they have made elements with atomicnumbers 1l3, 1l4, 1l5, 116 and 118 - the heaviestmade to date. But because these elementshave only 173to 177neutrons, far fewer thanthe magic number 184, they are incredibly ••unstable with half-lives from ~::::~., ~.....fractions of a millisecond to a few .::::::~:........seconds. Still, the researchers are •••• ::mastering the techniquesthat could help them reachthe island of stability.

The procedure involvesbombarding certain heavyelements with a beam of lighter "naked"nuclei - atoms with their electrons strippedaway - in the hope that they might fuse intoa new element. The key is to use nuclei for thebeam that have just the right number ofprotons to form the superheavy element youwant. For example, element 118 was formedby fusing californium, which has 98 protons,and calcium, which has 20.

It's a fiddly and laborious procedure.The beam has to have just the right amountof energy to encourage fusion with the target.If the atoms have too little energy, electrostaticrepulsion between the two atomic nucleicauses them to ricochet apart. Use too muchenergy and the resulting combined nucleusis overexcited and unstable. "The probabilityof fusing a beamed particle with one of thetarget nuclei is extremely small," says LLNLteam member Dawn Shaughnessy.Experiments can last for months, andproduce as little as one superheavy elementper month. "But one atom is all that'sneeded to confirm the existence of a newelement," she says.

Despite the difficulties, the race is onto synthesise element 120 (which should beeasier than 119).Shaughnessy's challenge isto produce the exotic materials required forthe beam and the heavy target atom. Onereason the joint Russian-US team successfullysynthesised element u8 is their use ofthe veryrare and expensive isotope calcium-ax. Thisdoubly magic isotope has 20 protons and 28

neutrons, which also makes it unusuallyneutron-rich for such a light element. That'simportant, because when synthesising asuperheavy element you need all the neutronsyou can get. Something like element u8 needs

..........•..•...........•••••..................

"one atom is all thatis needed to confirma new element"

184 neutrons to reach its peak stability.Nevertheless, Shaughnessy reckons

calcium-ad may have outlived its usefulness."We have basically run out of reactions usingthe calcium-ax beam, so we are now trying todevelop new beams, such as iron," she says.

Combine a beam ofiron nuclei with aplutonium target, and element 120 couldbe within reach. However, this is not anyold plutonium. It has to be plutonium-zaa,an extremely rare and extraordinarily stableisotope. "We currently have what amountsto the world's supply of plutornum-zaa," saysShaughnessy. This highlights the challenge ofsynthesising superheavy elements: researchersare being forced to use exotic elements suchas curium, americium and californium, whichthemselves have to be made in the lab.

Meanwhile, theoreticians are investigatingjust how stable some of these superheavyelements are. Early this year, Partha

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Nuclear physicists are extending the periodic table way beyond the heaviest naturally occurring element, uranium (atomic number 92),edging ever closer to the fabled "island of stability" - a region of extraordinarily long·lived monster atoms (right)

ium 117 Ununseplium 118 UnunoctiumN/A 1006,JINR

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341 NewScientist 126 July 2008 www.newscientist.com

Atomic needle in a cosmic stackIf superheavy elements reallyexist with half-lives in the orderofthousands or millions of years,could they be lingering rightunder our noses here on Earth?

"ItGinnot be exduded, intheory, though it seems ratherunlikely, n says Yuri Oganessianofthe Joint Institute of NudearResean:h in Dubna, Russia. Histeam has been trying to findhassium, element 108, insamples of natural osmium.They haven't found any natural

hassium yet, and it may takeyears to do so.

Indeed, research since the1950s has failed to find any signsof superheavy elements fromeither cosmic or terrestrialsources. This might be becausethey simply cannot be createdthrough natural processes, saysDavid Hinde, a nuclear physicistat the Australian NationalUniversity in Canberra. Even thehigh pressure and temperaturein exploding stars - which Gin

Chowdhury and Chhanda Samanta of the SahaInstitute of Nuclear Physics in Kolkata, India,modelled the stability of 1700 isotopes ofsuper heavy elements, with atomic numbersfrom 100 to 130 (Physical Review C,vol Tl,p 044603). Their calculations confirm thata smattering of superheavy elements shouldbe stable enough to resist spontaneous decayfor long periods, some for up to hundreds ofthousands of years.

But not for millions of years, so it's littlewonder that Marinov's claim for element 122,

with a half-life of at least 100 million years,raised eyebrows, ifnot hackles. However,it was not just the claims of a half-life thatdrew fire; the experimental technique wasalso criticised. Marinov's team ran a purifiedsolution of natural thorium - atomic number90, atomic mass 232 - through a massspectrometer, which measures the mass ofindividual atoms. Besides thorium, the team

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form elements as heavy asuranium - may not be capableof creating a superheavyelement. The problem is workingout how to jam enough neutronsinto a heavy nucleus to get to themagic number of 184 withoutthe nucleus undergoing fissionfirst. "It's a bit of a stretch, n

Hinde admits.Perhaps the only place these

fleeting superheavy elementsexist in the entire universe isin nuclear physics labs on Earth.

saw a handful of atoms that had much greatermass, just over 292. Marinov's team ruled outcontamination and said that what they foundwas element 122 with 170 neutrons, or possiblyelement 124 with 168 neutrons.

A handful of counts in a mass spectrometeris not enough to prove the existence of sucha massive atom, argues David Hinde, a nuclearphysicist at the Australian National Universityin Canberra. Furthermore, he says: "Ourextensive current understanding of nuclearphysics cannot accommodate a lifetime of100 million years for any configuration ofelement 122. However, if compelling evidencewere found, of course we would have to lookfor holes in our knowledge. But the evidenceis not compelling."

Marinov's experiment aside, Chowdhury'sand Sarnanta's calculations show thatsuperheavy elements might be stable forhundreds ofthousands of years, which willencourage others to keep looking for element120 and above.

Quantum spannerIf that task was not hard enough already,quantum mechanics also throws a spannerin the works: even ifa superheavy nucleuscan resist spontaneous decay for hundredsof thousands of years, there is a second wayit could fall apart long before then.

It's easy to picture the protons andneutrons sitting firmly in their atomic shellsin precise locations, but quantum mechanicstells us things are not so clear-cut. In fact, theposition of anyone proton or neutron is betterrepresented by a cloud of probable locations.And sometimes, particularly with large atoms,this cloud will move beyond the "boundary"ofthe nucleus. When this happens, there'sa chance that a proton or neutron will escape-effectively "tunnelling" its way out.

This quantum tunnelling effect is

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responsible for the radioactive alpha-decayof heavy atoms, where they emit two protonsand two neutrons (a helium nucleus) and sliptwo slots down the periodic table in theprocess. So while a super heavy atom mightresist spontaneous decay for millennia,as Chowdhury and Samanta suggest, it maynevertheless undergo alpha-decay withinmicroseconds. This makes it extremelyunlikely that we'll find stable superheavyelements in nature (see "Atomic needle ina cosmic stack", left).

Nevertheless, hope remains that it willbe possible to synthesise one or more of them.One candidate that might last long enough forchemists to play with it is a potential isotopeof darmstadtium, element 110, with a magical184 neutrons. This is predicted to have a half-life of over 300 years. However, no one is closeto synthesising darmstadtium with so manyneutrons. The heaviest isotope created so faris still 13neutrons short and has a half-life ofjust 11seconds.

If a superheavy element does getsynthesised with a long enough half-lifeto study its chemistry, what could we expect?Definitely something unusual. The hugepositive charge of the extremely large nucleuswould accelerate the electrons around it tospeeds approaching the speed oflight,radically skewing the element's chemicalproperties. This effect explains why mercuryis liquid at room temperature: it weakens thebonds between atoms thus lowering themelting point. And the bigger the nucleus,the faster the electrons go and the strongerthe effect. Some predictions suggest thatelement 114would be a gas rather than a solid.

According to Shaughnessy, a shortageof neutron-rich nuclei beams and targetsis currently the biggest practical barrier tosynthesising long-lived superheavy elements."We may get to a point where we just don'thave the materials to get there, but all thetheorists tell us we would be able to seesomething that at least lasts longer thanwhat we have now."

If they do manage to solve thetechnological problems, can the researcherskeep creating heavier and heavier matter?Is there an end in sight to superheavyelements? "At some point, the repulsionbetween the protons in the nucleus will beso high that no number of neutrons will beable to make it exist, even for a fraction ofa second," says Shaughnessy.

Oganessian is more emphatic. "Of course,there is an end, as there is an end in everything,"he says. But it may yet take decades of diligenteffort to find the last island of stability in theturbulent sea of matter. •

Tim Dean is a writer based in Sydney, Australia

26 July20081 NewScientist135