Rare isotopes for choice

&ball; Physics 15, 69

The Rare Ion Beam Facility opens its doors to experiments that will study the formation of heavy elements in the Universe and provide critical tests of nuclear theories.

Tomasz Zajda/stock.adobe.com

The Rare Ion Beam (FRIB) facility will produce the widest range of rare isotopes of any existing facility.

The nuclear physics community welcomes the launch of a long-awaited radioactive isotope beam production facility, with a cohort of users preparing for the first experiments. The Facility for Rare Ion Beams (FRIB) at Michigan State University opens its doors to experimenters this week. FRIB is expected to provide the widest range of rare isotopes of any existing facility, including many isotopes never synthesized before. The facility will also allow researchers to control isotope beam energies to match those relevant to nuclear processes in stars and supernovae.

Rare isotopes get their name from their rarity – these unstable nuclei decay radioactively and therefore cannot be found naturally on Earth. But making significant amounts of these elements in a lab will allow scientists to tackle important open problems in physics. Current nuclear theories, for example, cannot describe many nuclei, and rare isotopes provide edge cases on which to test why these theories fail. Rare isotopes are also relevant to the cosmic nucleosynthesis of heavy elements, a process for which there is as yet no satisfactory explanation. On the applied side, radioactive isotopes can be useful for medical imaging, cancer treatment, and other industrial applications.

In light of these motivations, physicists have been calling for a rare ion facility for decades. In 2008, the U.S. Department of Energy and Michigan State University provided funding for the FRIB project, and construction began in 2014. Despite the challenges of the COVID pandemic, construction was completed in early this year, a few months ahead of schedule, according to FRIB science. director Bradley Sherrill.

FRIB relies on a powerful linear accelerator, which accelerates ions to about half the speed of light, then smashes them into a target of heavier nuclei. There, the ions react with other nuclei, producing a whole host of different isotopes. Using a ‘fragment separator’, researchers can filter out the desired isotopes and route them to ‘stop stations’, where the fast beams are slowed down in a gas. This reduced speed allows isotopes to be probed with high-precision techniques, such as laser spectroscopy. The stopped isotopes can then be re-accelerated and delivered to other experimental chambers at the desired energies.

Two key aspects make FRIB a unique facility, Sherrill says. The first is the superconducting linear accelerator, which can achieve the highest beam power of any heavy ion accelerator facility. The efficiency of the machine is enhanced by the ability to accelerate up to five ionization states of a given element, such as uranium. (Other accelerators are typically designed to accelerate only ions with a specific charge.) The second aspect is the reacceleration stage, where the speed of the isotopes produced can be fine tuned. “Isotopes can be created at half or two-thirds the speed of light, but we actually want to slow them down to energies relevant to astrophysics,” he says.


Table of nuclides showing stable isotopes (black), previously detected isotopes (green), theoretically predicted isotopes (yellow), and potentially FRIB-producible isotopes (blue).

FRIB’s beam power “will give us the greatest range in the rarest isotopes,” Sherrill says. FRIB is expected to generate hundreds of isotopes never before synthesized, some of which are particularly prized by scientists. “The process that developed the FRIB science dossier identified certain key regions of isotopes, and we directed our efforts to provide access to those particular isotopes,” Sherill explains. These regions include isotopes at the limits of nuclear stability, such as those near the neutron “drip line”, and isotopes relevant to the formation of heavy elements in the cosmos.

The neutron drip line marks the boundary in the nuclide table between bound and unbound isotopes of a given element. If more neutrons are added to a nucleus at the drip line, they leak or drip out of the nucleus. Today, this boundary is only known for the ten lightest elements down to neon, but FRIB’s ability to deliver neutron-rich isotopes should allow researchers to extend the drip line to dozens of heavier elements. “It will take us to where we have asked questions and gotten no answers for decades,” says nuclear physicist Ani Aprahamian, who chairs the FRIB program advisory board. His hope is that understanding these extreme cases will allow researchers to develop a unified nuclear theory that applies to nuclei of all stripes and colors.

And if the isotopes at the limit of stability are not found on Earth, they are involved in the production of heavy elements in stars and supernovae. Many of these elements are believed to derive from the so-called r-process, which involves fast neutron capture reactions. The details of the r-processes are poorly understood, in part because of the challenges of performing experiments under the same conditions found in the cosmos. FRIB will provide unprecedented capabilities to mimic astrophysical scenarios by harnessing most isotopes that can be present in a stellar environment and accelerating them to relevant energies, says nuclear astrophysicist Kelly Chipps of Oak Ridge National Laboratory in the Tennessee. She says a unique feature of FRIB is that it can produce different isotopic isomers – versions of the same isotopes in different energy states. Most models for the r-process assume that isomers have no effect, but “there’s no good reason to believe that’s the case,” Kelly says. The ability to finely control these and other details gives FRIB the “potential to have a huge impact on our understanding of nuclear astrophysics,” she says.

Sherrill says there are important synergies between the research that FRIB will enable and recent developments in gravitational wave detection and observational astronomy. The detection by LIGO and Virgo in 2017 of a neutron star merger, accompanied by emission of radiation across the electromagnetic spectrum, provided evidence that heavy elements can be created in such mergers (see Point of view: neutron star merger seen and heard). FRIB will provide a wealth of data that could help researchers analyze fusion spectra to extract details of the processes that forge these elements. By combining multi-messenger astronomy with the constraints of nuclear physics, researchers might be able to understand “the bottom line of the reality of what’s going on inside a neutron star merger,” says Chipps. .

–Matteo Rini

Matteo Rini is the editor-in-chief of Physics.


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