An unexpected radiolysis result paves way for isotope harvesting at FRIB

Researchers update their understanding of resulting reactions when heavy-ion beams hit a flowing-water target, a key variable to the eventual collection of radionuclides for use in medicine, materials science, and other applied contexts

When it comes to isotope harvesting at the Facility for Rare Isotope Beams (FRIB), the question of context is everything. What will happen to the chemistry of water flowing around a spinning drum of titanium alloy being hit with a heavy-ion beam? The beam-on-water reactions should create a wealth of radionuclides potentially useful in medicine, astrophysics, materials science, and stockpile stewardship science.

Isotope harvesting enables FRIB to extend its impact and is a new area of opportunity for researchers. During routine operation for its nuclear physics mission—without interfering with FRIB’s primary users—extra, unused isotopes can be “harvested” using additional tools and infrastructure.

Included in the surrounding water are compounds such as molecules of hydrogen peroxide (H2O2), molecular hydrogen (H2), and molecular oxygen (O2). All are produced by radiolysis, the breaking of bonds in water molecules due to the beam’s energy. The concentrations of these oxidizing and reducing agents will affect efforts to gather other sought-after radioisotopes slated to be produced as useful byproducts of experiments at FRIB. Having an idea about their concentration is the first step in developing methods for how to deal with them.

At Michigan State University (MSU), Katharina Domnanich, assistant professor of chemistry at FRIB and in the MSU Department of Chemistry, and Gregory Severin, associate professor of chemistry at FRIB and in the MSU Department of Chemistry, combined experimental results and an updated simulation to better understand what to expect in FRIB’s closed water circulation system. It turns out there could be fewer of these oxidizing and reducing agents than previously thought. Their paper (“A Model for Radiolysis in a Flowing-Water Target during High-Intensity Proton Irradiation”) was published in the American Chemical Society’s ACS Omega magazine in July 2022.

In 2019, Domnanich and colleagues loaded a car with equipment and traveled to the University of Wisconsin-Madison (UW-Madison) Cyclotron Laboratory. The purpose was to mimic part of the isotope-harvesting setup at FRIB, albeit at a much more modest scale. In a one-day experiment, a beam blocker 3D-printed from a titanium alloy (Ti64) alloy was blasted with a high-intensity proton beam while traversed by the water flow in a 40-liter water matrix.

With Domnanich that day was E. Paige Abel, then an FRIB graduate student. Abel later wrote up the results of the target durability. Domnanich intended to contribute a small section to the paper on what was happening with radiolysis in the experiment. However, the data was different than expected from earlier calculations and experiments. So, she reported her observations as a coauthor on Abel’s 2020 paper, “Durability test of a flowing-water target for isotope harvesting.” Later, Domnanich returned to the dataset in earnest, working from her home office. She collaborated with Severin to update a simulation that would fit the data. The result was a model that explained the Madison observations. That bodes well for FRIB’s isotope harvesting, expected to be operating by 2024.

The puzzle was that the quantities of H2O2, H2, and O2 in the water matrix were all considerably lower than expected. Concentrations of these and other related molecules were produced in the relatively small zone where the beam hit the target. Earlier work suggested that those concentrations would increase over time in the overall volume of circulating water. As an analogy, consider how a toddler's chaos usually extends from the child’s bedroom to an entire home. When it comes to radiolysis, it appears that instead the many interactions occurring in this zone (at UW-Madison, in just a 0.14 milliliter volume of water immediately around the target) quickly reach a steady state of rapid creation and decay of these molecules. This limits the concentration in the overall system. That is, it’s more like a toddler so occupied by chaos in the immediate environment of the bedroom that the child never makes it to the rest of the house.

More work needs to be done. However, the fact that FRIB isotope harvesting may have to contend with lower concentrations of oxidizing molecules is potentially good news.

“I use a contact lens cleaning solution each night that’s just 3-percent hydrogen peroxide. It bubbles wildly before eventually decomposing,” said Domnanich. “Imagine this in higher concentrations. It corrodes metals and other equipment, and quickly reacts with other isotope species we are trying to harvest.”

Isotope harvesting at FRIB will involve filtering out oxidative molecules from the water cooling system. Domnanich’s updated model may help estimate just how much filtering may be needed.

“It would be nice to know what we can expect beforehand,” said Domnanich.

Geoff Koch is a freelance science writer. 

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), supporting the mission of the DOE-SC Office of Nuclear Physics. Hosting what is designed to be the most powerful heavy-ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions and applications for society, including in medicine, homeland security and industry.

The U.S. Department of Energy Isotope R&D and Production Program (DOE Isotope Program) supports isotope harvesting at FRIB. MSU operates FRIB as a user facility for the Office of Nuclear Physics in the U.S. Department of Energy Office of Science, supporting the mission of the DOE-SC Office of Nuclear Physics.

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