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: Rare Isotope Beam Facility

One of nuclear physicists’ fondest wishes is about to come true. After a decades-long wait, a US$942 million accelerator in Michigan officially opens on May 2. His experiments will trace unexplored regions of the landscape of exotic atomic nuclei and shed light on how stars and supernova explosions create most of the elements of the Universe.

“This project was a dream come true 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 in 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 which was funded by the US Department of Energy, with a contribution of $94.5 million from the State of Michigan. The University of Michigan contributed an additional $212 million in various ways, including land. It replaces a former 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 ahead of schedule and on budget,” says nuclear physicist Bradley Sherrill, who is the scientific director of FRIB. FRIB.

For decades, nuclear physicists have been pushing for a facility of its might — one that could produce rare isotopes orders of magnitude faster than is possible with NSCL and similar accelerators around the world. The first proposals for such a machine came 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, theoretical nuclear physicist and FRIB chief scientist.

Internal function

All FRIB experiments will begin in the basement of the facility. Atoms of a specific element, usually uranium, will be ionized and sent through a 450 meter long accelerator which bends like a paperclip to fit inside the 150 meter long hall. At the end of the pipe, the ion beam will hit a graphite wheel that spins continuously to avoid overheating a particular spot. Most of the nuclei will pass through the graphite, but a fraction will collide with its carbon nuclei. This causes uranium nuclei to break down into smaller combinations of protons and neutrons, each of which is a nucleus of a different element and isotope.

This beam of matched cores will then be directed to a “fragment splitter” at ground level. The separator is made up of a series of magnets that deflect each core to the right, each at an angle that depends on its mass and charge. By refining 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 several experiment halls. For the rarest isotopes, production rates could be as low as one nucleus per week, but the lab will be able to deliver and study almost any of them, Sherrill says.

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 at a higher rate than NSCL. Each uranium ion will also travel faster towards the graphite target, carrying an energy of 200 mega-electronvolts, compared to the 140 MeV carried by the ions in the NSCL. FRIB’s higher 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 tip of knowledge

Physicists are excited about bringing FRIB online, as their knowledge 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 in 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 accurately predict their structures and properties from first principles, Nazarewicz says.

Researchers have therefore concocted a variety of simplified models that predict certain characteristics of a certain range of nuclei, but might fail or give only rough estimates outside this range. This even applies to basic questions, such as how quickly an isotope decays — its half-life — or whether it can form at all, Nazarewicz says. “If you ask me how many isotopes of tin exist, or of lead, the answer will come with a big error bar,” he says. FRIB will be able to synthesize hundreds of hitherto unobserved isotopes (see “Unexplored nuclei”), and by measuring their properties, it will begin to test many nuclear models.

UNEXPLORED NUTS.  Graph showing measured and observed isotopes versus those that will potentially be produced by FRIB.

Source: Neufcourt, L. et al. Phys. Rev. VS 101044307 (2020)

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

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

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

The superconducting radiofrequency linear accelerator housed in the linear accelerator tunnel of the Facility for Rare Isotope Beams.

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

The facility will help answer the fundamental question of “how many neutrons can be added to a nucleus, and how does this affect 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 that study nuclear isotopes, says Klaus Blaum, a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. The facilities in Japan and Russia are optimized to produce the heaviest possible elements, those at the end of the periodic table.

The €3.1 billion Facility for Research on Antiprotons and Ions (FAIR), an atom breaker under construction in Darmstadt, Germany, is expected to be completed in 2027 (although the freezing of the participation of Russia following the invasion of Ukraine could lead to delays). FAIR will produce both antimatter and matter and can store nuclei for longer. “You can’t do it all with one machine,” says Blaum, who has served on advisory boards for FRIB and FAIR.

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