A team of scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory have detected six isotopes, reportedly never previously discovered, of superheavy elements 104 through 114. The findings will be published Oct. 29 in Physical Review Letters.
Starting with the creation of a new isotope of the yet-to-be-named element 114, the researchers observed successive emissions of alpha particles that yielded new isotopes of copernicium (element 112), darmstadtium (element 110), hassium (element 108), seaborgium (element 106) and rutherfordium (element 104). Rutherfordium ended the chain when it decayed by spontaneous fission.
Information gained from the new isotopes will contribute to a better understanding of the theory of nuclear shell structure, which underlies predictions of an "Island of Stability," a group of long-lasting isotopes thought to exist amidst a sea of much shorter-lived, intrinsically unstable isotopes of the superheavy elements, according to the researchers.
The new, neutron-deficient, superheavy element isotope was produced in 48Ca irradiations of 242Pu targets at a center-of-target beam energy of 256 MeV. The alpha decay of the new isotope was followed by the sequential alpha decay of four daughter nuclides, 281Cn, 277Ds, 273Hs and 269Sg. 265Rf was observed to decay by spontaneous fission.
"We were encouraged to try creating new superheavy isotopes by accelerating 48Ca projectiles with Berkeley Lab's 88-inch cyclotron and bombarding 242Pu targets inside the Berkeley gas-filled separator here," said Heino Nitsche, PhD, head of the Heavy Element Nuclear and Radiochemistry Group in Berkeley Lab's Nuclear Science Division (NSD) and professor of chemistry at the University of California at Berkeley.
The sum of protons and neutrons of 48Ca and 242Pu is 114 protons and 176 neutrons. To make the desired "neutron poor" nucleus, one having only 171 neutrons, first required a beam of 48Ca projectiles whose energy was carefully adjusted to excite the resulting compound nucleus enough for five neutrons to "evaporate."
"The process of identifying what you've made comes down to tracking the time between decays and decay energies," said Ellison. As a check against possible mistakes, the data from the experiment were independently analyzed using separate programs devised by Paul Ellison of NSD, a graduate student in the University of California, Berkeley department of chemistry; Ken Gregorich, a senior staff scientist in NSD and team member, Jacklyn Gates of NSD.
In this way, after more than three weeks of running the beam, the researchers observed one chain of decays from the desired neutron-light 114 nucleus. The first two new isotopes, the yet-to-be-named, and copernicium 281 produced by its alpha decay, lived less than a fifth of a second before emitting alpha particles. The third new isotope, darmstadtium 277, lived a mere eight-thousandths of second. Hassium 273 lasted a third of a second. Seaborgium 269 made it to three minutes and five seconds but managed to emit an alpha particle. Finally, after another two and a half minutes, rutherfordium 265 decayed by spontaneous fission.
"Our new isotopes are on the western shore of the Island of Stability"—the shore that's less stable, not more, said Gregorich. Yet the discovery of six new isotopes, reaching an unbroken chain of decays from element 114 down to rutherfordium, is a major step toward better understanding the theory underlying exploration of the region of enhanced stability that is thought to lie in the vicinity of element 114—and possibly beyond.