A particle accelerator that has just been switched on can reveal rare forms of matter

Just a few hundred yards from where we sit is a large metal chamber devoid of air and draped with the wires necessary to operate the instruments within. A jet of particles silently passes through the interior of the chamber at about half the speed of light until it crashes into a solid piece of material, resulting in a burst of rare isotopes.

This all takes place at the Facility for Rare Isotope Beams, or FRIB, which is operated by Michigan State University for the US Department of Energy Office of Science. As of May 2022, national and international teams of scientists gathered at Michigan State University and began conducting science experiments at FRIB with the goal of creating, isolating and studying new isotopes. The experiments promised to provide new insights into the fundamental nature of the universe.

By accelerating heavy ions – electrically charged atoms of elements – FRIB will enable scientists like us to create and study thousands of isotopes never seen before.

We are two professors of nuclear chemistry and nuclear physics who study rare isotopes. Isotopes are, in a sense, different flavors of an element that have the same number of protons in their nucleus but different numbers of neutrons.

The accelerator at FRIB started operating at low power, but when it finishes ramping up to full power, it will be the most powerful heavy ion accelerator on Earth. By accelerating heavy ions – electrically charged atoms of elements – FRIB will enable scientists like us to create and study thousands of isotopes never seen before. A community of about 1,600 nuclear scientists from around the world has been waiting for a decade to begin science enabled by the new particle accelerator.

The first experiments at FRIB were completed in the summer of 2022. While the facility is currently running at only a fraction of its full capacity, multiple science collaborations working at FRIB have already produced and detected about 100 rare isotopes. These early results are helping researchers learn more about some of the rarest physics in the universe.

Rare isotopes are radioactive and decay over time as they emit radiation — visible here as the streaks emanating from the tiny piece of uranium in the center.

What is a Rare Isotope?

It takes an incredible amount of energy to produce most of the isotopes. In nature, heavy rare isotopes are produced during the catastrophic death of massive stars, called supernovae, or during the merger of two neutron stars.

To the naked eye, two isotopes of each element look and behave the same way — all isotopes of the element mercury would look just like the liquid metal used in ancient thermometers. However, because the nuclei of isotopes of the same element have different numbers of neutrons, they differ in how long they live, what kind of radioactivity they emit, and in many other ways.

FRIB can accelerate any naturally occurring isotope – whether as light as oxygen or as heavy as uranium – to about half the speed of light.

For example, some isotopes are stable and do not decay or emit radiation, so they are common in the universe. Other isotopes of the same element can be radioactive, so they inevitably decay when they turn into other elements. Because radioactive isotopes disappear over time, they are relatively rarer.

However, not all decay happens at the same rate. Some radioactive elements – such as potassium-40 – emit particles through decay at such a slow rate that a small amount of the isotope can last for billions of years. Other more radioactive isotopes like magnesium-38 exist for only a fraction of a second before decomposing into other elements. Short-lived isotopes, by definition, do not survive long and are rare in the universe. So if you want to study them, you have to make them yourself.

Making isotopes in a lab

Although only about 250 isotopes occur naturally on Earth, theoretical models predict that about 7,000 isotopes should exist in nature. Scientists have used particle accelerators to produce about 3,000 of these rare isotopes.

The FRIB accelerator is 1,600 feet long and made of three segments folded roughly into the shape of a paper clip. Within these segments are numerous, extremely cold vacuum chambers that alternately pull and push the ions using powerful electromagnetic pulses. FRIB can accelerate any naturally occurring isotope – whether as light as oxygen or as heavy as uranium – to about half the speed of light.

To make radioactive isotopes, all you need to do is smash this ion beam into a solid target, such as a piece of beryllium metal or a spinning disk of carbon.

The impact of the ion beam on the fragmentation target breaks apart the core of the stable isotope, simultaneously producing many hundreds of rare isotopes. To isolate the interesting or new isotopes from the rest, there is a separator between the target and the sensors. Particles with the correct momentum and electric charge are passed through the separator while the rest are absorbed. Only a subset of the desired isotopes will reach the many instruments built to observe the nature of the particles.

The probability of a specific isotope being created during a single collision can be very small. The odds of creating some of the rarer exotic isotopes can be on the order of 1 in a quadrillion – about the same odds as winning back-to-back Mega Millions jackpots. But the powerful ion beams used by FRIB contain so many ions and cause so many collisions in a single experiment that the team can reasonably expect to find even the rarest isotopes. According to calculations, FRIB’s accelerator should be able to produce about 80% of all theoretical isotopes.

The first two FRIB science experiments

A multi-agency team led by researchers from Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory (ORNL), University of Tennessee, Knoxville (UTK), Mississippi State University, and Florida State University, along with researchers from MSU, began conducting from the first experiment at FRIB on May 9, 2022. The group aimed a beam of calcium-48 – a calcium core with 48 neutrons instead of the usual 20 – into a beryllium target with a power of 1 kW. Even at a quarter percent of the facility’s maximum power of 400 kW, about 40 different isotopes passed through the separator to the instruments.

The FDSi device recorded the time each ion arrived, what isotope it was and when it fell away. Using this information, the collaboration deduced the half-lives of the isotopes; the team has already reported on five previously unknown half-lives.

The second FRIB experiment began on June 15, 2022, led by a collaboration of researchers from Lawrence Livermore National Laboratory, ORNL, UTK and MSU. The facility accelerated a jet of selenium-82 and used it to produce rare isotopes of the elements scandium, calcium and potassium. These isotopes are commonly found in neutron stars and the aim of the experiment was to better understand what kind of radioactivity these isotopes emit as they decay. Understanding this process could shed light on how neutron stars lose energy.

The first two FRIB experiments were just the tip of the iceberg of this new facility’s capabilities. In the coming years, FRIB will investigate four major questions in nuclear physics: first, what are the properties of atomic nuclei with a large difference between the number of protons and neutrons? Second, how are elements formed in the cosmos? Third, do physicists understand the fundamental symmetries of the universe, such as why there is more matter than antimatter in the universe? Finally, how can the information from rare isotopes be applied to medicine, industry and national security?


Sean Liddick, associate professor of chemistry, Michigan State University and Artemis Spyrou, professor of nuclear physics, Michigan State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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