A few hundred yards from where we are sitting is a large metal chamber devoid of air and covered with the cables needed to control the instruments within. A particle beam silently passes through the interior of the chamber at about half the speed of light until it strikes a piece of solid material, resulting in an explosion of rare isotopes.
This all takes place in the Facility for Rare Isotope Beams, or FRIB, which is operated by Michigan State University for the US Department of Energy Office of Science. Beginning in May 2022, national and international teams of scientists converged 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.
We are two professors of nuclear chemistry and nuclear physics who study rare isotopes. Isotopes are, in a sense, different flavors of an element with the same number of protons in their nucleus but a different number of neutrons.
FRIB’s accelerator has started operating at low power, but when it reaches full power it will be the most powerful heavy ion accelerator on Earth. By accelerating heavy ions, atoms of electrically charged elements, FRIB will allow scientists like us to create and study thousands of isotopes never seen before. A community of around 1,600 nuclear scientists around the world have been waiting a decade to start doing science with the new particle accelerator.
The first experiments at FRIB were completed in the summer of 2022. Although the facility is currently operating at only a fraction of its full power, several scientific collaborations working at FRIB have already produced and detected around 100 rare isotopes. These early results are helping researchers learn more about some of the rarest physics in the universe.
What is a rare isotope?
It takes incredibly high amounts of energy to produce most isotopes. In nature, rare heavy isotopes are produced during the cataclysmic death of massive stars called supernovae or during the merger of two neutron stars.
To the naked eye, two isotopes of any element look the same and behave the same – all isotopes of the element mercury would look exactly like the liquid metal used in old thermometers. However, since the nuclei of isotopes of the same element have different numbers of neutrons, they differ in their lifetimes, the type of radioactivity they emit, and in many other ways.
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 into other elements. As radioactive isotopes die out over time, they are relatively rarer.
However, not all cavities occur at the same rate. Some radioactive elements, such as potassium-40, emit particles by decay at such a low rate that a small amount of the isotope can last for billions of years. Other more radioactive isotopes like magnesium-38 only exist for a fraction of a second before they decay 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.
Create isotopes in the lab
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 is made up of three segments folded roughly into the shape of a trombone. Within these segments are numerous extremely cold vacuum chambers that alternately pull and push 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 create radioactive isotopes, you simply smash that ion beam into a solid target like a piece of beryllium metal or a rotating disc of carbon.
The impact of the ion beam on the fragmentation target breaks up the stable isotope nucleus and simultaneously produces several hundred rare isotopes. To isolate interesting or new isotopes from the rest, a separator sits between the target and the sensors. Particles with the right amount of momentum and the right electrical charge will pass through the separator while the others will be absorbed. Only a subset of the sought-after isotopes will reach the many instruments built to observe the nature of the particles.
The probability of creating a specific isotope during a single collision can be very low. The odds of creating some of the rarest exotic isotopes can be on the order of 1 in a quadrillion, roughly the same 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 around 80% of all theorized isotopes.
The first two scientific experiments of the FRIB
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, together with researchers from MSU, began conducting the first experiment. at FRIB on May 9, 2022. The group directed a beam of calcium-48 – a calcium nucleus with 48 neutrons instead of the usual 20 – towards a beryllium target at 1 kW of power. Even at a quarter of a percent of the facility’s 400 kW peak output, about 40 different isotopes passed through the instrument separator.
The FDSi device recorded the time each ion arrived, what isotope it was, and when it decayed. From this information, the collaboration deduced the half-lives of the isotopes; the team has already reported five previously unknown half-lives.
The second FRIB experiment began June 15, 2022, led by a collaboration of researchers from Lawrence Livermore National Laboratory, ORNL, UTK, and MSU. The facility accelerated a selenium-82 beam and used it to produce rare isotopes of the elements scandium, calcium and potassium. These isotopes are commonly found in neutron stars, and the purpose of the experiment was to better understand what kind of radioactivity these isotopes emit when 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. Over the next few 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 the 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 information on rare isotopes be applied to medicine, industry and national security?
Provided by The Conversation
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