The long-awaited accelerator is now ready to investigate the origins of elements

One of the greatest hopes of nuclear physicists is about to come true. After decades of anticipation, a US$942 million accelerator in Michigan will open on 2 May. Its studies will map previously uncharted sections of the unusual nuclei landscape and give information on how stars and supernova explosions generate the majority of the elements in the Universe.

“This initiative has enabled the whole community of nuclear physicists to realize a long-held desire,” says Ani Aprahamian, an experimental nuclear physicist at the University of Notre Dame in Indiana. Kate Jones, a physics student at the University of Tennessee in Knoxville, concurs. “This is the facility that we have been waiting for,” she adds.

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The Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU) in East Lansing had a $730 million budget, with the majority of funding coming from the US Department of Energy and the state of Michigan contributing $94.5 million. Additional $212 million was given by MSU in a variety of ways, including the land. It takes the place of an older National Science Foundation accelerator at the same location, dubbed the National Superconducting Cyclotron Laboratory (NSCL). FRIB construction began in 2014 and was finished late last year, “five months ahead of schedule and under budget,” according to nuclear physicist Bradley Sherrill, FRIB’s scientific director.
Nuclear scientists have been clamoring for decades for a facility of this size — one capable of producing rare isotopes orders of magnitude quicker than the NSCL and comparable accelerators globally. The initial suggestions for such a machine date all the way back to the late 1980s, and agreement was established in the 1990s. “The community was convinced that we needed this technology,” says Witold Nazarewicz, a theoretical nuclear physicist and principal scientist at FRIB.

Internal mechanisms

All FRIB tests will begin at the basement of the facility. Ionized atoms of a particular element, often uranium, will be propelled into a 450-metre-long accelerator that bends like a paper clip to fit within the 150-metre-long hall. At the pipe’s terminus, the ion beam will collide with a graphite wheel that will spin continually to prevent overheating any one location. Although the majority of the nuclei will pass through graphite, a small percentage will collide with its carbon nuclei. This results in the disintegration of uranium nuclei into smaller combinations of protons and neutrons, each of which has a nucleus of a distinct element and isotope.
This beam of various nuclei will subsequently be directed upward to a ground-level ‘fragment separator.’ The separator is composed of a set of magnets that deflect each nucleus in a direction determined by its mass and charge. By fine-tuning this technique, the FRIB operators will be able to generate a fully isotope-free beam for each experiment.

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After that, the selected isotope may be sent via a labyrinth of beam pipes to one of the several trial rooms. Although production rates for the most rare isotopes may be as low as one nucleus per week, Sherrill believes the lab will be able to transport and analyse practically every single one.
A distinguishing aspect of FRIB is the presence of a second accelerator capable of smashing rare isotopes against a fixed target, simulating the high-energy collisions that occur within stars or supernovae.
FRIB will initially operate at a modest beam intensity, but its accelerator will progressively ramp up to create ions at a pace orders of magnitude greater than that of NSCL. Additionally, each uranium ion will travel quicker to the graphite target, carrying 200 mega-electronvolts of energy, compared to the 140 MeV carried by NSCL ions. FRIB’s increased energy is excellent for synthesizing a large variety of various isotopes, including hundreds that have never been synthesized previously, according to Sherrill.

The frontiers of knowledge

Physicists are anticipating the launch of FRIB, since their understanding of the isotope landscape is still incomplete. In theory, the forces that keep atomic nuclei together are the product of the strong force — one of nature’s four basic forces and the same force that holds three quarks together to form a neutron or a proton. However, nuclei are complicated things with many moving elements, and their structures and behaviors cannot be predicted precisely from basic principles, according to Nazarewicz.
As a result, researchers have devised a number of simplified models that accurately predict some properties of a particular range of nuclei but fail or provide only rough estimations beyond that range. This holds true even for fundamental problems, like as the rate at which an isotope decays — its half-life — or whether it can exist at all, Nazarewicz explains. “If you ask me how many isotopes of tin or lead exist, I will give you an answer with a big error bar,” he explains. FRIB will be able to create hundreds of hitherto undiscovered isotopes (see ‘Unexplored nuclei’) and will use their characteristics to test a variety of nuclear hypotheses.
Jones and others will be particularly interested in isotopes with’magic’ numbers of protons and neutrons — such as 2, 8, 20, 28 or 50 — because they generate entire energy levels (known as shells). Magic isotopes are important because they enable the most precise checks of theoretical predictions. Jones and her colleagues have spent years studying tin isotopes with increasingly fewer neutrons, creeping closer to tin-100, which has both magic quantities of neutrons and protons.
Additionally, theoretical uncertainties imply that researchers do not yet have a clear explanation for how the periodic table’s components arose. The Big Bang primarily created hydrogen and helium; the other chemical elements in the periodic table, up to iron and nickel, were synthesized mostly by nuclear fusion inside stars. However, heavier elements cannot be formed by fusion. They were created by other sources, most often radioactive decay. This occurs when a nucleus accumulates enough neutrons to become unstable, and one or more of its neutrons converts to a proton, resulting in the formation of new element with a higher atomic number.
This may occur as a result of neutron bombardment of nuclei during short yet catastrophic events like as supernovae or the merging of two neutron stars. The most investigated incident of this sort occurred in 2017, and it was consistent with theories in which colliding orbs generate materials heavier than iron. However, astrophysicists were unable to determine which particular atoms were produced or in what amounts, according to Hendrik Schatz, an MSU nuclear astrophysicist. FRIB’s primary strength, he argues, will be its exploration of the neutron-rich isotopes produced during these events.
The linear accelerator at the FRIB is composed of 46 cryomodules that accelerate ion beams at temperatures just above absolute zero.
The facility will contribute to the basic issue of “how many neutrons may be added to a nucleus and how does this affect the nucleus’s interactions?” According to Anu Kankainen, an experimental physicist from Finland’s University of Jyväskylä.
FRIB will complement existing state-of-the-art accelerators used to investigate radioactive isotopes, according to Klaus Blaum, a scientist at Germany’s Max Planck Institute for Nuclear Physics. Japan and Russia have optimized their facilities to create the heaviest elements conceivable, those at the end of the periodic table.
The €3.1 billion Facility for Antiproton and Ion Research (FAIR), an atom smasher now under construction in Darmstadt, Germany, is slated to be finished in 2027 (although Russia’s withdrawal from the project during the invasion of Ukraine may cause delays). FAIR will generate both antimatter and matter and will be capable of storing nuclei for extended periods of time. “A single computer cannot handle everything,” adds Blaum, who has served on advisory panels for both FRIB and FAIR.