Students Study Exotic Nucleus at Department of Energy-funded Accelerator Facility

The Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU) is a scientific user facility funded by the U.S. Department of Energy Office of Science where “researchers from all career stages and backgrounds come together to … study the properties and fundamental interactions of rare isotopes and nuclear astrophysics,” according to the university’s website.

“It’s the premiere facility for doing rare isotope studies, and by ‘rare isotope,’ I mean nuclei that are short-lived. You can’t dig them up. You can’t find them in nature because they decay into other things. They’re radioactive,” said Riley. “They have to be created on site and studied very quickly before they turn into something else. That’s the exciting thing about this lab.”

Riley’s research team consists of computer science major Jay Kosa ’25; physics majors Ian Conroy ’24 and Mill Heinze ’24; and physics and math double major Alyssa Himmelreich ’26. Once in March and again in July, members of the group had “beam time to study the very exotic nucleus Si-42, an isotope of silicon with twice as many neutrons as protons,” said Riley. Unfortunately, the initial attempt at the experiment in March was not able to be fully run.

“Even though we didn’t get the results we wanted, it was still a great experience to see the lab and understand how running an experiment works,” said Himmelreich of the first trip to MSU. “The coolest part was being able to see the target, the detectors, and all the other machinery used up close. You don’t realize how big everything is and how much is going on there until you see it in person. There is so much that goes into each experiment and being able to go to the lab and see this firsthand was a memorable experience.” She will return to MSU this spring to collect data she will analyze in summer 2024.

The second attempt in July, however, was fruitful, and for five days, the accelerator ran around the clock, with Riley, Kosa, Conroy, and Heinze, along with a crew of collaborators from MSU and FSU, taking day, night, and graveyard shifts to collect as much data as possible.

It was the second summer trip for Conroy and Heinze, who also spent five weeks at Florida State University (FSU) conducting an experiment in the accelerator lab there. “We took stable nuclei and shot them at targets of other stable nuclei. You can do some cool stuff at FSU, but you’re limited to nuclei that you can find in the Earth’s crust.” The FSU experiment is the focus on Heinze’s honors project, while the preliminary analysis of the FRIB data from MSU is the focus of Conroy’s honors thesis.

“At FRIB you can make stuff that is very exotic, and you can extend the boundaries of what we know about how nuclei behave,” said Riley. Using a beam moving at 40% the speed of light, the group put Silicon 42 nuclei in an excited state and waited for the presence of gamma rays. Their goal was to collect the full energy of these gamma rays 50 times, and they actually observed about 70 of these events.

“Already we can tell that this nucleus is behaving in an interesting way. Nuclei with 28 neutrons are supposed to be a closed shell, but over the past two decades, researchers have collected evidence that Silicon 42 acts differently than all the other N = 28 nuclei. We proposed this experiment because we wanted to see if there is evidence that this closed shell may have vanished. The fact that we got over 50 counts is a sign that the shell is gone, but extraordinary claims require extraordinary evidence, so we need to be very sure before we say that.”

Riley has been trying to do this experiment on Silicon 42 since 2010. It was approved to run in 2016 at MSU when, prior to FRIB, the university housed the National Superconducting Cyclotron Laboratory, but a vacuum leak in one of the cyclotrons prevented the experiment from fully running. “I think of this nucleus like my white whale,” said Riley. “This is the first time that we actually saw a spectrum from Silicon 42.” The group will continue digesting the data and working toward at least one publication.

“The MSU lab was mind-blowing, both in scale and complexity. Everything was so new and advanced, it felt like the set of a movie,” said Kosa. “The chamber which actually generated our data was three stories tall cocooned inside a larger facility, and represented only the beginning of what MSU has in store for the future. Three power grids work together to power the system, and they’re not even at full throttle yet. If I had to pick the coolest part, it was the marvel of witnessing the bleeding edge of scientific research in an already massive facility that is still in its infancy. What a gift that was, and we made memories I’ll cherish my whole life.”

“The really cool thing is that undergraduate students got to go and see how this kind of science is done,” said Riley. “This is the big leagues, and we were invited in. The National Science Foundation funds my research group in support of this kind of undergraduate research because they know that’s how you prepare the next generation of scientists to work at labs like this. It’s very much a scientific workforce pipeline.”