It's taken years, but physicists have finally filled in a persistent gap in the periodic table. Eight years after the creation of element 118, the heaviest known atom, researchers have made a few atoms of its slightly lighter neighbor, element 117, by shooting an intense beam of calcium ions into a target of berkelium. Besides sketching in the blank space in the table, the discovery bolsters the notion of an "island of stability," a group of superheavy nuclei still tantalizingly out of reach that theorists predict may be as stable as more familiar elements.
The 92 elements naturally found on Earth have one thing in common: they have been stable enough to hang around over the 4.5 billion years of our planet's existence. Those beyond the 92nd element, uranium, have shorter half-lives and have been manufactured in nuclear reactors or by particle accelerators. As more were discovered, the trend seemed be toward shorter and shorter half-lives as mass increased. But in the 1960s, nuclear physicists discovered that certain key numbers of protons and neutrons conferred extra stability on a nucleus. If there were such “magic numbers” larger than those seen in existing elements, perhaps some superheavy elements with quantities of protons or neutrons close to those numbers would last much longer, producing a so-called island of stability. If such stable superheavies could be found and made in quantity, they could have exotic and useful chemical properties.
Hopes of reaching the island of stability have increased as experimenters fabricated nuclei with numbers of protons and neutrons ever closer to the predicted magic numbers. The Joint Institute for Nuclear Research (JINR) in Dubna, Russia, has been the most successful element hunter in recent years, having cooked up elements 113 through 116 and 118. But making element 117 presented a particular challenge. Using a projectile of calcium, which has 20 protons, Dubna physicists needed a target with 97 protons and plenty of neutrons—an isotope of the devilishly hard-to-synthesize berkelium. Nuclear chemists at the world's most intense neutron source, the High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee, spent 250 days scraping together a mere 22.2 milligrams of the stuff—about the size of a fingernail paring—and another 90 days purifying it to one part in 10 million. Then the clock began to tick—berkelium's half-life is 320 days.
The berkelium's next stop was Russia’s Research Institute of Atomic Reactors in Dimitrovgrad, where scientists crafted the berkelium into a thin target and then on to JINR, where physicists pummeled the target with calcium-48 ions day and night for 5 months. A gas-filled separation chamber diverted atoms blasted off the wheel into an array of detectors. From thousands of potentially interesting events, the physicists pinned down just six atoms of element 117, they will report Friday in Physical Review Letters. For the time being, it will be called "ununseptium," the Latin word for 117.
In order to identify the atoms, the detectors recorded decay chains, the sequence of particles spat out by the radioactive nuclei as scientists seek a more stable configuration. Which particles, their timings, and energies reveal what the starting atom was. An isotope of 117 with 177 neutrons, for example, emits a series of alpha particles to metamorphose first into daughter nucleus 115 and on to great-great-great-great granddaughter dubnium (105). The elements in this chain—115, 113, 111, and so on—had all been made before except not with as many neutrons. The extra neutrons conferred by the berkelium brought them closer to a neutron magic number and so made them more long-lived than normal. This props up the notion that elements become more stable as they approach the island of stability's magic number of neutrons, says team member Krzysztof Rykaczewski, a nuclear physicist at Oak Ridge.
"Experimentally, it's an enormous tour de force," says nuclear physicist Konrad Gelbke, director of the National Superconducting Cyclotron Laboratory at Michigan State University in East Lansing. What's more, "they're developing a picture that's starting to make a lot of sense." Like sailing expeditions of old, the findings are solidifying the existence of an island of stability, made possible through the detailed interactions of neutrons and protons inside the nucleus, he says. "This is decades of very careful and painstaking work that is slowly coming to fruition."