Artist’s conception of calcium ions traveling down the accelerator at a high velocity toward the rotating californium target. Credit: Sabrina Fletcher and Thomas Tegge/LLNL
Based on the briefest of glimpses, scientists are mapping out a shadowy “island of stability,” a harbor of hope within a vast and deadly “sea of instability.” These modern-day Magellans are nuclear physicists, and by colliding atoms at high speeds, they are creating the heaviest elements ever seen with the hope that they will find this island: the few precious isotopes that do not decay instantaneously.
Now they’ve discovered the largest element yet, number 118: In a paper published recently in the journal Physical Review C, a collaboration of Russian and American researchers presents evidence for the as-yet-unnamed element. They say they observed 118 three times over a three-year search.
Atoms have dense nuclei that are composed of neutrons and positively-charged protons. The higher an element is on the periodic table, the more protons and neutrons—and therefore the more mass—a nucleus in one of its atoms has.
Creating superheavy nuclei is a delicate balancing act. Within the nucleus, two forces fight for control. The protons’ electric charges make them repel each other fiercely. But the strong nuclear force, which binds protons and neutrons alike, pulls them together. For each element, there are only a few combinations of protons and neutrons—each of which is called an isotope—that will stick together long enough to count as an actual atom.
“At some point, the forces that are holding the nucleus together are not going to let another proton be shoved in there,” said Nancy Stoyer, co-author of the study and a scientist at the Lawrence Livermore National Laboratory. “You put [another proton] in there and it’s going to immediately break apart. At some point, we’re going to reach the end of the elements. Where that is, we don’t know.”
Because of the high number of protons and neutrons they contain, isotopes of the heaviest elements are very unstable. But sitting at the edge of the known atoms, element 118 is believed to dwell on the outskirts of a small “island of stability,” a group of isotopes that are unusually stable for their bloated size. While the atoms of 118 produced lasted less than one millisecond, scientists have hypothesized that elements actually on the island could live for seconds, minutes, or even longer.
Finding heavier elements allows scientists to test theories about nuclear forces under extreme conditions, Stoyer said.
“As we’re exploring this extreme heavy stuff, at this upper end, we’re getting more information about what is holding these protons and neutrons together in the nucleus,” she said. “There are many theories that explain these forces using various mathematical functions. A good theory works well in the regions they are formulated to explain. A great theory works well in all regions, even at the extremes.”
The biggest elements—including number 118—were all discovered in experiments at the Joint Institute for Nuclear Research in Dubna, Russia. Researchers there smash a beam of calcium atoms (element 20) into a target made of highly radioactive californium (element 98). Most of the calcium ions fly right through the californium, missing or only grazing the californium nuclei. But a lucky few calcium ions hit smack on, fusing the two nuclei to form a new element.
In 2002, the group found what appeared to be one atom of element 118, but didn’t want to claim a discovery yet: In 1999, a team at the University of California, Berkeley reported the discovery of element 118, but the group wasn’t able to replicated the results. When the scientists returned to their raw data, they found no proof for the element’s creation. It was alleged that one team member had fabricated the results, and the rest of the group retracted the report in 2001.
But in 2005, the Russian-American collaboration racked up two more sightings of the elusive element. Then, the researchers felt confident they could announce the element’s discovery and give an approximate measure of its lifetime: about one-thousandth of a second.
Researchers can’t actually see element 118 directly, but they can see its so-called decay chains. The nuclei break down in a series of steps into smaller, more stable atoms. The researchers track what shoots out of the nuclei to reconstruct what element was present at the beginning of the chain.
“These Dubna people, during the last seven years, have measured about 80 decay chains in this new region of superheavy elements,” said Sigurd Hoffman of the Society for Heavy Ion Research in Darmstadt, Germany. These findings all agree with each other, and with existing theories, he said. “From this point of view, this data looks very convincing.”
Some are hesitant to say the discovery is for real, though.
“I don’t think we are ready to celebrate,” said Witold Nazarewicz, a physicist at the University of Tennessee, who was not part of the research. In studies of superheavy nuclei, there have been several claims of discoveries that didn’t hold up, he said. “We always take first results as exciting, but we celebrate only when the results are confirmed by independent methods.”
