Anti-atoms are rare, but researchers working with them are swimming in data compared with those chasing superheavy atoms. In an experiment that required prodigious patience, researchers at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, spent almost five months last year firing titanium-50 ions — each with 22 protons and 28 neutrons — into a berkelium-249 target at the rate of about 5 trillion particles per second. The hope was that, just once or twice, two atoms would fuse to make an element with 119 protons, more than any created before.
Smashing beams of heavy atoms together has served physicists well over the past 70 years, allowing them to create increasingly heavy agglomerations of protons and neutrons, and to expand the periodic table far beyond the heaviest naturally occurring elements. The confirmed record-holder is element 116, livermorium, with 116 protons and, depending on the isotope, between 174 and 177 neutrons.
There have been claims to elements 117 and 118 too, but these have not been officially confirmed. And so far, “none of the current experiments have reported finding 119 or 120”, says Christoph Düllmann, spokesman for the GSI-led collaboration — although he adds that his own team's analysis of last year's work is not quite complete.
There is a strong sense that the quest is coming to a dead end. The chance that nuclei will fuse decreases as they get heavier, because the protons and neutrons resist sticking together. Most researchers agree that beyond 120, the chance of getting two nuclei to fuse directly is vanishingly small. “So this leaves us with the question,” says Düllmann, “what do we do next?”
To answer that requires an understanding of what motivates the superheavy search. Curiosity and national pride undoubtedly have a role, with politicians and scientists both looking to stamp their country's name into a new box on the periodic table. But each superheavy element is extremely short-lived, splintering in milliseconds.
Theorists have posited that some superheavy combinations of protons and neutrons will turn out to be stable for seconds, minutes or even days. This fabled 'island of stability' is thought to exist at between 114 and 126 protons, and around 184 neutrons. It is now clear that any attempt to make new superheavy elements by smashing a light particle into a heavier one will not reach the island: the isotopes spat out have too few neutrons. So researchers are changing tactics by trying to make heavier isotopes of elements that have already been created.
That is what scientists will attempt next year at the Joint Institute for Nuclear Research in Dubna, Russia. They plan to make neutron-rich isotopes of element 118 by firing beams of calcium-48 into radioactive californium-251.
The Russian team and others also want go back to the elements already made and create hundreds or thousands of atoms, rather than the handful necessary to claim a discovery. “We should set ourselves the goal of making not one or two atoms, but macroscopic quantities that we can use to study chemistry and nuclear structure in much greater detail,” says Rolf-Dietmar Herzberg, a physicist at the University of Liverpool, UK. That might allow theorists to make more accurate predictions about where the island of stability lies.
But the temptation to expand the periodic table is strong. Researchers will probably turn away from head-on collisions and instead try knocking two heavy nuclei together in a glancing blow, which may stand a better chance of successfully fusing them to create new elements.
Physicists have a history of surprising themselves in their quest to create ever heavier atoms. In the early 1990s, no one thought that they could get past element 112 and then a tweak to the fusion process made it possible, says GSI team member Michael Block. “The next element is always the hardest.”