Powerful linear accelerator begins smashing atoms - how it could reveal rare forms of matter

Powerful linear accelerator begins smashing atoms – how it could reveal rare forms of matter

A new particle accelerator at Michigan State University aims to detect thousands of never-before-seen isotopes. Photo Credit: Rare Isotope Rays Facility, CC BY-ND

Just a few hundred yards from where we were seated is a large metal chamber with no air, covered with wires needed to control the instruments inside. A beam of particles silently traverses the interior of the chamber at about half the speed of light until it hits a solid piece of material, resulting in an explosion of rare isotopes.

All of this takes place at the Facility for Rare Isotope Beams (FRIB) operated by Michigan State University for the US Department of Energy Office of Science. Beginning in May 2022, national and international teams of scientists came together at Michigan State University and began conducting scientific experiments at the FRIB with the goal of generating, isolating, and studying new isotopes. The experiments promised new insights into the fundamental nature of the universe.

We are two professors of nuclear chemistry and nuclear physics working on rare isotopes. Isotopes are sort of different flavors of an element with the same number of protons in their nucleus but different numbers of neutrons.

The accelerator at FRIB started out at low power, but when it reaches 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 never-before-seen isotopes. A community of around 1,600 nuclear scientists from around the world has been waiting for a decade to start the science made possible by the new particle accelerator.

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

What is a rare isotope?

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

Rare isotopes are radioactive and decay over time as they emit radiation – seen here as the streaks coming from the small nugget of uranium in the center.

To the naked eye, two isotopes of any element look and behave the same—any isotope of the element mercury would look the same as the liquid metal used in ancient thermometers. However, since the nuclei of isotopes of the same element have a different number of neutrons, they differ in their lifespan, in what kind of radioactivity they emit and in many other things.

For example, some isotopes are stable and do not decay or emit radiation, so they are widespread in the universe. Other isotopes of the same element can be radioactive, so they will inevitably decay as they transform into other elements. Since radioactive isotopes disappear over time, they are relatively rare.

However, not all decay happens at the same rate. Some radioactive elements – like potassium-40 – emit particles when they decay at such a slow rate that a small amount of the isotope can last for billions of years. Other, more radioactive isotopes such as magnesium-38 only exist for a split second before decaying 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.

Creating isotopes in the laboratory

While 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 around 3,000 of these rare isotopes.

The FRIB accelerator is 1,600 feet long and consists of three segments folded roughly in the shape of a paper clip. Within these segments are numerous, extremely cold vacuum chambers that alternately pull and push the ions with strong 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 create radioactive isotopes, all you have to do is slam that ion beam onto a solid target, like a piece of beryllium metal or a spinning disc of carbon.

The impact of the ion beam on the fragmentation target breaks up the nucleus of the stable isotope while simultaneously creating many hundreds of rare isotopes. To isolate the interesting or new isotopes from the rest, a separator sits between the target and the sensors. Particles with the right momentum and electrical charge are passed through the separator while the rest are absorbed. Only a subset of the desired isotopes reach the many instruments that have been built to observe the nature of the particles.

The probability of producing a given isotope in a single collision can be very small. The odds of creating some of the rarer exotic isotopes can be in the order of 1 in a quadrillion – roughly the same odds as winning consecutive Mega Millions jackpots. But the powerful ion beams used by FRIB contain so many ions and produce so many collisions in a single experiment that the team can reasonably expect to find even the rarest isotopes. According to calculations, the FRIB accelerator should be able to produce about 80% of all theorized isotopes.

The first two scientific FRIB experiments

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

The FDSi device recorded the time each ion arrived, what isotope it was and when it decayed. Using this information, the collaboration derived the half-lives of the isotopes; The team has already reported 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 beam of selenium-82, producing rare isotopes of the elements scandium, calcium and potassium. These isotopes are common in neutron stars, and the goal of the experiment was to better understand what type of radioactivity these isotopes emit during 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 what this new facility could do. In the coming years, the FRIB will explore 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 rare isotope information be applied in medicine, industry and national security?

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