A long-awaited accelerator is ready to explore the origins of items 2022-04-29 11:19:10


An aerial view of the Rare Isotope Ray Facility on the Michigan State University campus.

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

One of the most important wishes of nuclear physicists is about to come true. After waiting for decades, the $942 million Michigan accelerator is officially opened on May 2. His experiments will map unexplored regions of the landscape of exotic atomic nuclei and shed light on how star explosions and supernovae form most of the elements in the universe.

“This project was the fulfillment of a dream for the whole community in nuclear physics,” says Annie Abrahamian, 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 Rare Isotope Packaged Facility (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. 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 location. Construction of the FRIB began in 2014 and was completed late last year, “five months ago and within budget,” says nuclear physicist Bradley Sherrill, FRIB’s director of science.

For decades, nuclear physicists have been pushing for a facility of its own — 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 80s, and consensus was reached in the 90s. “Society was adamant that we needed to get an instrument like this,” says Witold Nazarewicz, a theoretical nuclear physicist and FRIB chief scientist.

internal business

All FRIB experiences will begin in the facility’s basement. Atoms of a particular element, usually uranium, would be ionized and sent to a 450-meter accelerator that would bend like a paperclip to fit inside the 150-meter chamber. At the end of the tube, the ion beam will strike a constantly spinning graphite wheel to avoid overheating any particular spot. Most nuclei will pass through graphite, but some will collide with its own carbon nucleus. This causes uranium nuclei to disintegrate into smaller groups of protons and neutrons, each with a nucleus of a different element and isotope.

This bundle of miscellaneous cores will then be routed to a “fragment terminator” at ground level. The separator consists of a series of magnets that deflect each nucleus towards the right, each at an angle dependent on its mass and charge. By fine-tuning this process, FRIB operators will be able to produce a package consisting entirely of one peer for each given experiment.

The desired isotope can then be directed through a maze of beam tubes to one of the many experimental halls. With the rarest of isotopes, production rates can be as low as one nucleus per week, Cheryl says, but the lab will be able to connect and study nearly every single isotope.

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

FRIB will start operating at a relatively low beam intensity, but its accelerator will gradually ramp up to produce ions at a rate of orders of magnitude higher than NSCL. Each uranium ion will also travel faster to the graphite target, carrying an energy of 200 MeV, compared to the 140 MeV carried by ions in the NSCL. Sherrill says the higher FRIB energy is in the ideal range for producing a large number of different isotopes, including hundreds that have not been synthesized before.

edge of knowledge

Physicists are excited about FRIB’s coming online, because their knowledge of isotope landscapes 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 bodies with many moving parts, and their structures and properties are impossible to predict completely from first principles, says Nazarevich.

So the researchers devised a variety of simplified models that predict some features of a particular range of cores, but may fail or give only rough estimates outside of that range. This applies even to basic questions, such as how quickly the isotope decays — its half-life — or whether it can form at all, Nazarewicz says. “If you ask me how many isotopes of tin are there, or what is the precursor, the answer will be given with a large error bar,” he says. FRIB will be able to synthesize hundreds of previously unobserved isotopes (see ‘Unexplored nuclei’) and, by measuring their properties, will begin testing many nuclear models.

Unexplored nuclei.  Diagram showing measured and observed isotopes versus those likely produced by FRIB.

Source: Neufcourt, L. et al. Phys. Reverend J 101044307 (2020)

Jones and others will be particularly keen to study isotopes that contain “magic” numbers of protons and neutrons – such as 2, 8, 20, 28 or 50 – that make the structure of the nucleus particularly stable because they form entire energy levels (known as shells). Magical isotopes are of particular interest because they provide the cleanest tests of theoretical models. For many years, Jones and her team have studied isotopes of tin with progressively fewer neutrons, heading toward tin 100, which contains magic numbers of 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 are formed. The Big Bang produced mainly hydrogen and helium only. The other chemical elements in the table up to iron and nickel were formed mostly through nuclear fusion within stars. But heavier elements cannot be formed by fusion. It was falsified by other means – usually through beta radioactive decay. This happens when a nucleus gains so many neutrons that it becomes unstable, and one or more of its neutrons turns into a proton, forming an element with a higher atomic number.

This can happen when cores are bombarded with neutrons in short but catastrophic events, such as a supernova or the merger of two neutron stars. The most studied event of this type, observed in 2017, was consistent with models in which colliding celestial bodies produce elements heavier than iron. Hendrik Schatz, a nuclear astrophysicist at Michigan State University, says astrophysicists haven’t been able to observe what specific elements were made up, or in what quantities. One of FRIB’s major strengths will be exploring the neutron-rich isotopes that are synthesized during these events, he says.

A RF Superconducting Linear Accelerator located in the Linac Tunnel at the Rare Isotope Beam Facility.

FRIB’s linear accelerator consists of 46 immobilizer units, which accelerate ion beams during operation at temperatures a few degrees above absolute zero.Credit: Rare Isotope Ray Facility

This facility will help answer the basic question of “How many neutrons can one add to the nucleus, and how do they change the interactions within the nucleus?” says Anu Kankainen, an experimental physicist at Jyväskylä University in Finland.

FRIB will be a complement to the latest accelerators that study nuclear isotopes, says Klaus Blum, a physicist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Facilities in Japan and Russia have been optimized to produce the heaviest elements possible, those at the end of the periodic table.

The €3.1 billion Facility for Proton and Ion Research (FAIR), an atom-smasher under construction in Darmstadt, Germany, is scheduled for completion in 2027 (although a freeze on Russia’s participation in the wake of the invasion of Ukraine may lead to some delay). FAIR will produce antimatter as well as matter, and will be able to store cores for longer periods of time. “You can’t do everything with one machine,” says Bloom, who has been on the advisory committees to both FRIB and FAIR.