Long-awaited accelerator ready to explore the origins of the elements

An aerial view of the Rare Isotope Beam Facility on the Michigan State University campus.Credit: Facility for Rare Isotope Beams

One of the main wishes of nuclear physicists is about to come true. After a decades-long wait, a $942 million accelerator will officially open in Michigan on May 2. Their experiments will map uncharted regions of the landscape of exotic atomic nuclei and shed light on how stars and supernova explosions create most of the elements in the Universe.

“This project has been the realization of a dream for the entire nuclear physics community,” says Ani Aprahamian, an experimental nuclear physicist at the University of Notre Dame in Indiana. Kate Jones, who studies nuclear physics at the University of Tennessee at Knoxville, agrees. “This is the long-awaited facility for us,” she says.

The Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU) in East Lansing had a budget of $730 million, most of it funded by the US Department of Energy, with a contribution of $ 94.5 million from the state of Michigan. MSU contributed an additional $212 million in various ways, including land. It replaces an earlier National Science Foundation accelerator, called the National Superconducting Cyclotron Laboratory (NSCL), at the same site. Construction of FRIB began in 2014 and was completed late last year, “five months early and on budget,” says nuclear physicist Bradley Sherrill, FRIB’s chief science officer.

For decades, nuclear physicists had been pushing for a facility of their power, one that could churn out rare isotopes orders of magnitude faster than is possible with the NSCL and similar accelerators around the world. The first proposals for such a machine emerged in the late 1980s, and a consensus was reached in the 1990s. “The community was adamant that we needed a tool like this,” says Witold Nazarewicz, a theoretical nuclear physicist and chief scientist of FRIB.

inner workings

All FRIB experiments will begin in the basement of the facility. Atoms of a specific element, usually uranium, will be ionized and sent into a 450-meter-long accelerator that doubles as a paper clip to fit inside the 150-meter-long room. At the end of the pipe, the ion beam will hit a continuously rotating graphite wheel to prevent any particular spot from overheating. Most of the nuclei will pass through the graphite, but a fraction will collide with its carbon nuclei. This causes uranium nuclei to break into smaller combinations of protons and neutrons, each of which is a nucleus of a different element and isotope.

This bundle of mixed cores will be directed towards a ‘fragment separator’ at ground level. The separator consists of a series of magnets that deflect each core to the right, each at an angle depending on its mass and charge. By fine-tuning this process, FRIB operators will be able to produce a beam consisting entirely of one isotope for each particular experiment.

The desired isotope can then be routed through a maze of beam tubes to one of many experimental rooms. For the rarer isotopes, production rates could be as low as one core per week, but the lab will be able to deliver and study almost all of them, says Sherrill.

A unique feature of FRIB is that it has a second accelerator that can take the rare isotopes and smash them against a stationary target, to mimic the high-energy collisions that occur inside stars or supernovae.

FRIB will start operating with a relatively low beam intensity, but its accelerator will gradually increase to produce ions several orders of magnitude faster than NSCL. Each uranium ion will also travel faster toward the graphite target, with an energy of 200 megaelectronvolts, compared to the 140 MeV carried by the ions in the NSCL. FRIB’s highest energy is in the ideal range for producing a large number of different isotopes, Sherrill says, including hundreds that have never been synthesized before.

the edge of knowledge

Physicists are excited about bringing FRIB online, because their understanding of the isotope landscape is still tentative. The forces that hold atomic nuclei together are, in principle, the result of the strong force, one of the four fundamental forces of nature, and the same force that binds three quarks together to form a neutron or a proton. But nuclei are complex objects with many moving parts, and it’s impossible to predict their structures and properties exactly from first principles, says Nazarewicz.

Therefore, researchers have come up with a variety of simplified models that predict some features of a certain range of cores, but may fail or give only rough estimates outside that range. This applies even to basic questions, such as how fast an isotope decays, its half-life, or whether it can form, says Nazarewicz. “If you ask me how many isotopes of tin or lead are there, the answer will be given with a big error bar,” he says. FRIB will be able to synthesize hundreds of previously unobserved isotopes (see ‘Unexplored Nuclei’), and by measuring their properties, it will begin to test many nuclear models.

UNEXPLORED CORE.  Graph showing measured and observed isotopes versus those that FRIB will potentially produce.

Source: Neufcourt, L. et al. physics Rev C 101044307 (2020)

Jones and others will be especially interested in studying isotopes that have “magic” numbers of protons and neutrons, such as 2, 8, 20, 28, or 50, which make the structure of the nucleus especially stable because they form entire energy levels ( known as shells). Magical isotopes are particularly important because they provide the cleanest tests for theoretical models. For many years, Jones and his group have studied tin isotopes with progressively fewer neutrons, approaching tin-100, which has magic numbers for both neutrons and protons.

Theoretical uncertainties also mean that researchers do not yet have a detailed explanation of how all the elements on the periodic table were formed. The Big Bang produced essentially just hydrogen and helium; the other chemical elements in the table up to iron and nickel were formed primarily through nuclear fusion within stars. But heavier elements cannot be formed by fusion. They were forged by other means, usually via radioactive β decay. This happens when a nucleus gains so many neutrons that it becomes unstable and one or more of its neutrons becomes a proton, creating an element with a higher atomic number.

This can happen when nuclei are bombarded with neutrons in brief but catastrophic events, such as a supernova or the merger of two neutron stars. The best-studied event of that kind, observed in 2017, was consistent with models in which colliding orbs produce elements heavier than iron. But astrophysicists couldn’t look at which specific elements were made, or in what amounts, says Hendrik Schatz, a nuclear astrophysicist at MSU. One of FRIB’s main strengths will be exploring the neutron-rich isotopes that are produced during these events, he says.

The superconducting radio frequency linear accelerator housed in the linac tunnel at the Facility for Rare Isotope Beams.

FRIB’s linear accelerator is made up of 46 cryomodules, which accelerate ion beams while operating at temperatures a few degrees above absolute zero.Credit: Facility for Rare Isotope Beams

The facility will help answer the fundamental question of “how many neutrons can be added to a nucleus and how does it change the interactions within the nucleus?” says Anu Kankainen, an experimental physicist at the University of Jyväskylä in Finland.

FRIB will be complementary to other state-of-the-art accelerators studying nuclear isotopes, says Klaus Blaum, a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Facilities in Japan and Russia are optimized to produce the heaviest elements possible, the ones at the bottom of the periodic table.

The €3.1bn Center for Antiproton and Ion Research (FAIR), an atom smasher being built in Darmstadt, Germany, is scheduled to be completed by 2027 (although Russia’s freeze on participation after the invasion from Ukraine could bring some delays) . FAIR will produce both antimatter and matter, and will be able to store nuclei for longer periods of time. “You can’t do everything with one machine,” says Blaum, who has been on advisory committees for FRIB and FAIR.

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