When it starts in early 2022, the particle accelerator for the rare isotope beam (shown in the picture) will accelerate the ion beam to about half the speed of light.

The Borromean ring is carved on the 15th-century coat of arms of an Italian family and is decorated with an old Japanese shrine and has a powerful symbolic meaning. Take one ring out of the three interconnected circles, and the other two fall apart. The structure is only established when all three are intertwined. These rings represent the concept of unity, the Christian Trinity, and even some strange nuclei.

A rare species or isotope of lithium has a nucleus composed of three connected parts. The nucleus of lithium 11 is divided into main clusters of protons and neutrons, flanked by two neutrons, which form a halo around the core. Remove any one and the trio will disband, just like the Borromean ring.

Not only that, the nucleus of lithium 11 is huge. Although there are nearly 200 fewer protons and neutrons, its halo is very wide, the same size as the lead nucleus. The discovery of the lithium 11 expansion halo in the mid-1980s shocked scientists (SN: 8/20/88, p. 124), and its Borromean nature was also shocked. "No predictions were made for this," said Philomena Nunes, a nuclear theorist at Michigan State University in East Lansing. "This is one of the discoveries, like'What? What's going on?'"

Lithium 11 is just one example of what happens when the nucleus becomes strange. Nunes said such nuclei “have exciting properties.” They may be twisted into unusual shapes, such as pear shapes (SN: 6/15/13, p. 14). Or they can be wrapped in neutron skins—just like the peels on inedible stone fruits (SN: 6/5/21, page 5).

A new tool will soon help scientists pick these peculiar fruits from the atomic vine. Researchers are waiting in line at Michigan State University to use particle accelerators to study some of the rarest atomic nuclei. When it opens in early 2022, the Rare Isotope Beam Facility or FRIB (pronounced "eff-rib") will strip electrons from atoms to create ions, accelerate them to high speeds, and then crash them towards targets to create what scientists want The special nucleus to be studied.

FRIB's experiments will explore the limits of nuclei, examine how many neutrons can be inserted into a given nucleus, and study what happens when the nucleus is far away from the stable configurations found in everyday matter. Using FRIB data, the goal of scientists is to piece together a theory to explain the properties of all nuclei, even odd nuclei. Another central goal: to determine the origin story of the chemical elements born in the extreme environment of space.

If scientists are lucky, new and exciting nuclear mysteries will emerge, perhaps even stranger than lithium 11. FRIB’s scientific director and nuclear physicist Brad Sherrill said: “We will take a new look at an unexplored field.” “We think we know what we will find, but things are not. It's very likely as we expected."

The nucleus of lithium 11 has a center filled with protons and neutrons, surrounded by two neutrons, forming a broad halo. If one of these three components is removed, the nucleus cannot remain bound, which is the so-called Borromean nucleus.

The variety of nuclei is dazzling. Scientists have discovered 118 chemical elements, which are distinguished by the number of protons in the nucleus (SN: 1/19/19, page 18). Each of these elements has multiple isotopes, and different versions of the element are formed by changing the number of neutrons in the nucleus. Scientists predicted the existence of about 8,000 known isotopes of elements, but only about 3,300 appeared in the detector. Researchers expect that FRIB will have a considerable effect on the missing isotopes. It can identify 80% of the possible isotopes of all elements in uranium, including many isotopes that have never been seen before.

The most familiar nuclei are the nuclei of about 250 stable isotopes: they do not decay into other types of atoms. The grades of stable isotopes include nitrogen 14 and oxygen 16 in the air we breathe and carbon 12 found in all known organisms. The number after the element name indicates the total number of protons and neutrons in the nucleus.

A stable nucleus happens to have the right combination of protons and neutrons. Too many or too few neutrons will cause the nucleus to decay, sometimes slowly over billions of years, sometimes only within a fraction of a second (SN: 3/2/19, p. 32). In order to understand what happens inside these unstable nuclei, scientists have studied them before they decay. Generally speaking, as the proton-neutron balance becomes more and more unbalanced, and the nucleus becomes farther and farther away from stability, its properties become more and more strange.

This bizarre specimen tests the limitations of scientists' nuclear theory. Although a given theory may correctly explain near-stable nuclei, it may not explain more unusual nuclei. But physicists want a theory that can explain the most unusual to the most mediocre.

"We want to understand how the nucleus is built and how it works," said Witold Nazarewicz, FRIB chief scientist and theoretical nuclear physicist.

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Accelerating the ion beam in FRIB is like grazing.

In the beginning, “it was just a bunch of cats,” said Thomas Glasmacher, director of the FRIB laboratory. Cats meander in one way or another, but if you can push an unruly group of people in a certain direction—perhaps you open a can of cat food—the cats will start moving together, although They have a natural tendency to wander around. "Soon, it became a group of cats," he said.

In the case of FRIB, cats are ions—atoms with some or all of their electrons stripped away. Instead of cat food, electromagnetic force makes them move together.

The journey starts from one of FRIB's two ion sources, where the elements are evaporated and ionized. After some initial acceleration to move the ions, the beam enters the linear accelerator, which is what makes the particles really cruise. The linear accelerator looks like a shrinking freight train-a row of 46 boxes of pistachio ice cream colors, each box is about 2.5 meters high and varies in length. But the beam from the accelerator is much faster than a train full of cargo—about half the speed of light.

In a green box called a cryogenic module, the superconducting cavity is cooled to a few Kelvin, a little bit higher than absolute zero. At these temperatures, the cavity can use rapidly oscillating electromagnetic fields to accelerate ions. A chain of pistachio modules wraps around the facility in the shape of a paper clip, which is a necessary twist so that the approximately 450-meter-long accelerator fits into its 150-meter-long tunnel.

When the beam is fully accelerated, it will hit the graphite target. This heavy blow separates the protons and neutrons from the nucleus of the incoming ion, forming new and rarer isotopes. Then, the specific object that the scientist wants to study is separated from the ruffian by a magnet, which is redirected according to the mass and charge of the particle. The particles of interest are then sent to the experimental area, where scientists can use various detectors to study how the particles decay, measure their properties, or determine how they react.

The FRIB accelerator is bent into a paperclip shape to fit the 450-meter-long equipment in the tunnel. 46 cryogenic modules (green boxes) contain superconducting cavities that accelerate particles. Once the ions are accelerated, they hit the target to produce new isotopes. Going further down, the magnet separates the specific isotope that the scientist wants to study.

The energy of the FRIB beam is carefully selected to produce rare isotopes. When the nucleus collides with the target, the excess energy will explode the nucleus. Therefore, the design goal of FRIB is to reach less than one percent of the energy of the Large Hadron Collider at the European Nuclear Research Center near Geneva, which is the most powerful accelerator in the world.

On the contrary, the potential of the new accelerator depends on its powerful strength: in essence, there are many, many particles in its beam. For example, FRIB will be able to hit 50 trillion uranium ions into its target every second. Therefore, it will produce a more intense flow of rare isotopes than its predecessor.

For isotopes that are relatively easy to produce, FRIB will produce about 1 trillion per second; a lot of learning. This opens up prospects for examining isotopes that are more difficult to manufacture. These isotopes may appear in FRIB once a week, but this is still much more frequent than in weaker beams. It's like a low water pressure in a bathroom: "If it's just a trickle, you can't take a shower," said Nunes, one of the leaders of the coalition of theoretical physicists who support FRIB research. Now, "FRIB will be equipped with fire hoses."

This fire hose can also come in handy to precisely locate the critical boundary called the neutron drip line.

Try to stuff too many neutrons in an atomic nucleus, and it will decay almost immediately by spitting out a neutron. Imagine a greedy chipmunk whose cheeks are full of nuts. When it tries to push another one in, another nut pops out immediately. The threshold at which the nucleus decays in this way marks the ultimate limit to which the nucleus is bound. On a chart of known elements and their isotopes, the boundary draws a line, the neutron drop line. So far, scientists know at most the location of this key dividing line of the 10th element neon in the periodic table.

Heather Crawford, a nuclear physicist at Lawrence Berkeley National Laboratory in California, said: “FRIB will be the only method heavier and far enough to define the drip line.” FRIB is expected to identify the 30th element zinc, and beyond Neutron drip line.

Scientists have discovered a large number of isotopes (green) of chemical elements. FRIB is expected to find new (turquoise) in the full range of predicted isotopes (gold). The neutron drip line, the bottom edge of the colored region, marks the limit of the nucleus, but scientists don't know exactly where it is.

Near that drip line, the number of neutrons greatly exceeds that of protons, which is where the nucleus becomes particularly strange. Lithium-11 has a wide halo, next to the drip line. Crawford focuses on magnesium isotopes near the drip line. The most common stable isotope of magnesium has 12 protons and 12 neutrons. Crawford's main target is magnesium 40, which has 12 protons and twice as many neutrons in its nucleus-28.

"This is the limit of existence," Crawford said. There, theories predicting the properties of atomic nuclei are no longer reliable. Theoretical physicists cannot always determine the size and shape of a given nucleus in this field, or even whether it qualifies as a bound nucleus. When predicting how much energy is required to strike an atomic nucleus into its various charged states, a given theory may not meet the requirements. The distance between these energy levels is like a fingerprint of the nucleus, highly sensitive to details of the shape and other characteristics of the nucleus.

Sure enough, the performance of Magnesium 40 was unexpected, as Crawford and his colleagues reported in the 2019 Physical Review Letters. Although the theory predicts that its energy level will match that of magnesium isotopes with slightly fewer neutrons, the energy level of magnesium 40 is significantly lower than its neighbors.

In August, Crawford learned that she would be one of the first scientists to use FRIB. The two experiments proposed by her and her colleagues were selected as the first round of about 30 experiments, which will be conducted in the first two years of FRIB. She will carefully study Magnesium 40, which, like Lithium 11, also has a Borromean core. Crawford's goal now is to determine whether the isotope she chose also has a halo nucleus. This is one possible explanation for the strangeness of magnesium 40. Although nuclei with halo have been known for decades, theories still cannot reliably predict which nuclei will have halo. Understanding magnesium 40 can help scientists consolidate their interpretation of nucleus neutron decoration.

The nucleus of unstable magnesium 40 contains more neutrons (blue) than the more common stable magnesium 24, even though both have the same number of protons (red). Scientists want to know whether magnesium 40 has a typical nucleus or a nucleus with a large neutron halo.

Physicists hope to explore like mechanics under the hood to understand the cosmic nuclear reactions that make the universe work. "Nuclear physics is like the engine of a sports car. What happens in the engine determines the performance of the car," said Ani Aprahamian, a nuclear physicist at the University of Notre Dame in Indiana.

The universe powered by this engine may be a fierce place for atomic nuclei, with violent star explosions and extreme conditions from time to time, including matter crushed by gravity and squeezed into ultra-compact space. These environments have produced miracles of nuclear physics that are different from those commonly found on earth. FRIB will give scientists a glimpse of some of these processes.

For example, physicists believe that certain neutron-rich environments are cauldrons, and many chemical elements in the universe are cooked in the cauldron. This cosmic connection allowed nuclear physicist Jolie Cizewski to realize his childhood dream.

She said that when Cizewski was a little girl, she discovered an astronomical error. "I decided to become an astronomer so that I can go into space." It seems that she turned left from her childhood obsession. She has never been in orbit, nor has she become an astronomer.

But the echo of that childhood dream now anchors her research. She will soon use FRIB to reveal the secrets of the universe instead of observing the stars with a telescope.

Cizewski of Rutgers University in New Brunswick, New Jersey, is working to reveal the details of the cosmic nuclear reactions that cause the nuclei around us. "I tried to understand how these elements are synthesized, especially those elements heavier than iron," she said.

Many elements around us and in our bodies are formed inside stars. As the age of large stars grows, they gradually fuse larger nuclei in their cores, resulting in more distant elements in the periodic table-oxygen, carbon, neon, etc. But the process stopped on the iron. The rest of the elements must be born in another way.

A process called the rapid neutron capture process or r process is responsible for many other elements found in nature. In the r process, the nucleus quickly absorbs neutrons and expands to a large mass. The neutron nodes are interspersed with radioactive decay forming new elements. Observations of the merger of two neutron stars in 2017 indicate that this type of collision is where the r process takes place (SN: 11/11/17, page 6). But scientists suspect that it may also occur in other cosmic locations (SN: 6/8/19, p. 10).

Cizewski and his colleagues are studying the abbreviation of the r-process that may flourish in supernovae, and it may not have enough charm to complete the complete r-process. The team has focused on germanium 80, which plays a pivotal role in the weak r process. Physicists want to know how likely this nucleus is to capture another neutron and become germanium 81. In FRIB, Cizewski will impact a beam of germanium 80 into deuterium, which has a proton and a neutron in its nucleus. Knowing how often germanium 80 captures neutrons will help scientists determine the neutron uptake chain of the weak r process, wherever it may appear.

Like interconnected Borromean rings, different aspects of nuclear physics are closely intertwined, from the mysteries of the universe to the inner workings of the nucleus. The strange nuclei made by FRIB also allow physicists to dig the cornerstones of physics by testing some basic laws of nature. The facility also has a practical side. For example, scientists can collect some of the isotopes produced by FRIB for use in medical procedures.

The physicist is ready for surprises. "Every time we build such a facility, there will be new discoveries and scientific breakthroughs," Nazarevich said. Just like the Borromean nucleus that discovered lithium 11 in the 1980s, scientists may discover something completely unexpected.

In addition to studying extreme nuclei and exploring the nuclear physics of stars, scientists also hope to use FRIB to make progress in two other key areas.

Scientists plan to collect the isotopes produced in FRIB for social applications. For example, in medicine, certain isotopes (such as terbium-149) can be used for radiotherapy or medical imaging.

When this isotope of the rare earth metal terbium decays, it releases alpha particles (helium nuclei) that can kill cancer cells. With a half-life of 4.1 hours, it’s at its best: it’s fast enough to make an impact — it won’t take hundreds of years to decay — but not so fast that it disappears in a few seconds before it can be effective .

Scientists plan to examine certain physical rules, such as the idea that matter and antimatter behave as mirror images. Certain hypothetical physical effects may cause particles to ignore this rule, which helps explain why there is more matter than antimatter in the universe.

The effects that may make matter and antimatter behave differently may also lead to separation of charges in atoms, with slightly more positive charges on one side of the atom and slightly more negative charges on the other side. In most atoms, this separation may be too small to be measured. But in radium 225 with a pear-shaped nucleus, this effect will be stronger, because the asymmetry of the nucleus will increase the asymmetry of the atomic charge.

Have questions or comments about this article? Send an email to feedback@sciencenews.org

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Physics writer Emily Conover has a PhD. PhD in Physics from the University of Chicago. She has twice won the News Briefing Award from the DC Science Writers Association.

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