Scientists have created new extraheavy versions of three silvery metals in an advance that could lead to better understanding of how some elements are forged in stars.
The new heavyweights are isotopes of the metals thulium, ytterbium and lutetium. Isotopes are versions of atoms with irregular numbers of neutrons in the nuclei. In this case, researchers packed 113 and 114 neutrons in with thulium’s 69 protons, 116 and 117 neutrons next to ytterbium’s 70 protons and 119 neutrons alongside lutetium’s 71.
None of these five isotopes has ever been created before—at least, not on Earth. Although each decays away almost immediately and has no known practical use, they’re all exciting for researchers seeking to know more about the deep origins of gold and other familiar heavy elements.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
“It’s been an ongoing challenge to try to understand where heavier elements like gold and lead and uranium are made in the cosmos,” says Brad Sherrill, a physicist at Michigan State University (MSU) and a co-author of the new study, which was published on February 15 in the journal Physical Review Letters. These and other heavy metals must form from the decay of even heavier unstable elements born in environments positively bristling with neutrons, such as within and around merging neutron stars. But the granular details of this process are almost impossible to unravel without knowing more about the nuclei of heavy, neutron-rich atoms. Just how massive can they be? Exactly how long do they take to decay? The most reliable way to find answers to such questions is to actually have suitably neutron-packed isotopes on hand for direct study, but these have proved prohibitively difficult to make and measure—that is, until now. The five ultraheavy isotopes all emerged from early operations of the U.S. Department of Energy’s new Facility for Rare Isotope Beams (FRIB) at MSU, where Sherrill leads advanced isotope-separation efforts.
“Not only have they made new isotopes in this very heavy region but they’ve been able to identify them conclusively,” says Rebecca Surman, an astrophysicist at the University of Notre Dame, who was not involved in the research.
The isotopes of thulium, ytterbium and lutetium that the researchers created aren’t even the ones they’re really after. FRIB’s real goal is to go heavier.
Heather Crawford, a nuclear chemist at the Lawrence Berkeley National Laboratory, who was not involved in the study, says the facility’s early results are exciting. “Even with the facility just barely coming online, there were already new isotopes,” she says. “We knew that would be the case eventually, but the speed with which it came was just really nice to see.”
A rich assortment of astrophysical processes can create elements and isotopes; an element’s mass is one of the most important arbiters of how it’s made. Elements with a greater mass than iron are often created in what’s called the r-process, which is short for “rapid neutron capture.” In the r-process, unstable nuclei capture free neutrons from the environment quickly before the nuclei start to fizzle apart via radioactive decay. This requires extreme astrophysical settings, such as the environs of exploding stars.
“It’s like a big cooking pot, and you try to close the lid, raise the pressure and make those nuclei,” says Robert Grzywacz, an experimental nuclear physicist at the University of Tennessee, Knoxville, “except that the cooking pot is also exploding.”
Unraveling what must have happened to yield the soup of stable atoms at the end of all these unstable atomic transformations is a difficult challenge. Astrophysicists try to model the process in supercomputer simulations, but they lack basic information about how heavy nuclei behave, Surman says.
That’s where FRIB comes in. The new facility allows researchers to bombard a stationary target with intense, heavy beams of atoms of metals such as lead or uranium. In the new study, the beam was composed of platinum and aimed at a carbon target. The collisions knocked protons and neutrons off the original platinum nuclei, and by sheer chance a fraction of them resulted in nuclei losing more protons than neutrons, yielding very neutron-rich isotopes of lighter metals.
“We call this fragmentation,” Sherrill says. “We’re breaking the initial nucleus into pieces, and occasionally one of the pieces is interesting”—very occasionally. For every 100,000,000,000,000 collisions, the scientists only found two atoms of what they were looking for, Sherrill says.
A sensitive process of differentiating the resulting particles—which at this point are traveling at half the speed of light—by mass and electric charge ensues. The researchers haven’t done all of the detailed measurements on the new isotopes yet, but each probably has a half-life in the range of one second, Sherrill says.
To make the extraheavy thulium, lutetium and ytterbium, the researchers used only one two-hundred-seventieth of the beam’s ultimate planned intensity, Sherrill says. They also plan to boost the sensitivity of their detector by a factor of 10. “There are big dramatic future increases that will be on the way,” he says.
With those increases in power, the study’s team, led by MSU physicist Oleg Tarasov, hopes to make even heavier isotopes. An Earth-based creation of certain ultraheavy isotopes routinely born inside dying stars is probably beyond FRIB’s reach, Grzywacz says, but the further that researchers can push, the more they can learn and extrapolate about those isotopes they can’t make.
“There is an interplay of different phenomena which can change as you change the neutron number,” he says. “Some of them become more important than others and will change, for example, nuclear lifetime in a dramatic way. Some of those things are hard to predict.”
FRIB’s future findings should help researchers clarify the cosmic circumstances in which particular metals are made. With a good enough knowledge of the nucleus, Sherrill says, it would be possible to predict precisely which atoms would coalesce from, say, a neutron star merger. That kind of deep understanding could also have practical applications, such as the ability to process or dispose of nuclear waste while minimizing the emergence of deadly radioactive byproducts.
“This result is exciting,” he says, “because it’s a step toward getting to these heavy isotopes that have been really difficult or, up to now, impossible for us to produce.”
Editor’s Note (3/4/24): This story was edited after posting to clarify the description of the planned intensity of Facility for Rare Isotope Beams’ (FRIB’s) main beam, Brad Sherrill’s role at FRIB and the fact that it is a U.S. Department of Energy facility.