Heavy elements approach fabled ?island of stability?.
Tantalising evidence of two new chemical elements has been produced by a team of Russian and American scientists. Their observations indicate that we may be getting close to the fabled ?island of stability? in the periodic table, where heavy elements should be more stable than their neighbours. If confirmed, the discovery will bring the tally of known elements to 116.
?It?s one of the most fundamental questions ? how many elements are there?? says Paddy Regan, a nuclear physicist at the University of Surrey. ?There must be an upper limit, and this work suggests that we should be able to find that within the next decade.?
Uranium, the heaviest element found in nature, has an atomic number of 92, meaning it has 92 protons in its nucleus. Atoms bigger than this are more likely to break apart spontaneously in radioactive decay, because the strong nuclear force that holds protons and neutrons together gets weaker as more particles jostle for space at the core of the atom. Also, protons have a positive charge and the more there are the greater the strain on the nucleus due to the repulsion between them. Eventually the nucleus shatters, spraying out smaller, more stable atoms.
But physicists have predicted ?islands of stability? at atomic numbers 114, 120 and/or 126, where the protons and neutrons might be able to jostle themselves into a shape that minimises contact between the protons. That would allow the nucleus to hang together for much longer than its neighbours in the periodic table. Creating such elements may give scientists access to unusual and exciting chemistry.
“It's one of the most fundamental questions - how many elements are there? Paddy Regan, , University of Surrey”
The only way to make these heavy elements is to smash smaller atoms together at huge energies. The team of scientists from Lawrence Livermore National Laboratory in California and the Joint Institute for Nuclear Research in Dubna, Russia, fired a beam of heavy calcium atoms at a target made from americium, the radioactive metal found inside smoke detectors.
The result of the collision was just four atoms of element 115, which lived for about 90 milliseconds before decaying into a second new element, 113. Interestingly, the atoms of 113 survived for up to 1.2 seconds, ?long enough to allow you to do some interesting chemistry?, according to Francis Livens, a nuclear chemist at the University of Manchester.
The new elements have provisionally been named ununtrium (113) and ununpentium (115). They will be given proper names if and when the discovery is confirmed.
The Dubna group has an extensive track record in this kind of alchemy. ?Dubnium? was named to commemorate the group's creation of element 105, and it has also recorded evidence for elements 114 and 116.
Nevertheless, Regan remains cautious. ?For this to be real, it has to be reproducible, so I?m keeping an open mind on this,? he says. ?Basically, if you want proof, you need a smoking gun. In this case, you need to see the alpha particles and X-rays that come from radioactive decay ? and you have to see them at precisely the right energy that is caused by that particular decay.?
Retraction and accusation
Many others in the field are equally tentative. Embarrassment over the discovery of element 118, announced with great fanfare and then retracted amid accusations of scientific fraud, has left the nuclear physics community feeling bruised.
?We haven?t so much got egg on our face over 118, more like a full omelette,? says Regan.
He adds that many in the field think there is an inherent problem with the technique used in these experiments. Since the americium target used is itself radioactive, it will always contain traces of other decay products that interfere with the reaction.
However, the US Department of Energy recently promised $850 million towards a new rare isotope accelerator. This will allow physicists to use as the target a beam of radioactive americium atoms that is absolutely pure, unlike the stationary target used in this latest research.
University of Surrey