Exploring the Frontier of the Periodic Table

Chemistry’s Christopher Cahill has focused his research on some of the least-studied—and most dangerous—elements on the Periodic Table. It’s taken him from nuclear science labs to the heart of government policy-making.

Christopher Cahill (left) in his Science and Engineering Hall lab with PhD chemistry student researcher August Ridenour
Professor of Chemistry and International Affairs Christopher Cahill (left) in his Science and Engineering Hall lab with PhD chemistry student researcher August Ridenour. (Photo: Logan Werlinger)
March 14, 2018

Professor of Chemistry and International Affairs Christopher Cahill has spent most of his academic and research career at the bottom of the Periodic Table. There—far below such abundant elements as hydrogen, oxygen and carbon—resides a little-studied chemical family called the actinides. Some of their names are familiar (uranium, plutonium); others sound vaguely artificial (neptunium, americium). As the key components in nuclear weapons and nuclear energy, these last-row elements are highly radioactive, unstable to work with and have caused headaches for everybody from environmentalists to nuclear scientists to global policymakers. Even Cahill calls them “pretty nasty”—although he’s spent years taking their atomic structures apart.

“They are the frontier of the Periodic Table,” Cahill said. “These mysterious and toxic elements may not be on everybody’s radar, but they are incredibly relevant. They have, in some cases, made a mess of the environment and they may make a mess in the future. Or they may help mitigate climate change. Either way, there’s still a lot of chemistry to learn about them.”

Since 2005, Cahill has received nearly $1.2 million in research funding support from the U.S. Department of Energy, including a $373,199 grant awarded last month. His research team—which currently consists of two graduate students, one undergraduate and a visiting Fulbright scholar from Argentina—are looking at the fundamental chemical behavior of transuranics, the elements to the right of uranium on the Periodic Table. By testing them in environmentally relevant conditions, such as dissolving them in water, Cahill observes how they react under real-world stress. “There are grand challenges in this part of the Periodic Table, including nuclear waste disposal, environmental clean up and security issues,” he explained. “But if you don’t nail the fundamental science of how these elements behave, then everything else is a wash.”

Too Hot to Handle

With the exception of uranium—the last naturally occurring element on the Periodic Table—all of the bottom-row transuranics are man-made, created in reactors or specialized labs. But while many have existed for more than 70 years, they’ve remained under-studied precisely because they are so toxic and hard to obtain.

“The materials themselves can be quite dangerous with regard to radioactivity and that can be a limiting factor to studying or handling them,” explained August Ridenour, a PhD chemistry student researcher in Cahill’s lab. Ridenour and the others on Cahill’s team work with depleted uranium in his lab, but they’ve also traveled to the Pacific Northwest National Laboratory in Washington state to study radioactive isotopes in what is a highly secure facility. Their current work involves x-ray crystallographysynthesizing solid-state materials (crystals) and using X-ray diffraction to investigate their atomic structure. The process, which was the subject of Cahill’s recently-published paper for The Journal of the American Chemical Society, enables scientists to investigate the interaction of radioactive elements with organic material in potentially hazardous scenarios, like disposing nuclear waste or dismantling weapons.

“The research we are conducting is very exploratory,” Ridenour said. “But that fundamental knowledge is needed to explain the properties of materials that are relevant to real-world problems, such as the very complex chemistry occurring around nuclear fuel and nuclear waste.”

Science and Policy

During 2015-16, Cahill spent most of his time away from campus as an American Institute of Physics Fellow within the State Department’s Office of Weapons of Mass Destruction Terrorism. The experience, which primarily involved analyzing the global nuclear landscape, brought a new dimension to his teaching and research this year. “You would be hard pressed to find a discipline that is more driven by policy than the nuclear arena,” he said. “Nuclear science and policy are intimately enmeshed. One cannot be an expert in either without cognizance of the other.”

Former and current Cahill students weren't surprised to see the chemistry professor straddle the science arena and the policy world. “It is very clear that [Cahill] is passionate about this subject,” said Robert Surbella, PhD ’18, a former graduate research assistant in Cahill’s lab and currently the Linus Pauling Distinguished Post-Doctoral Fellow at the Pacific Northwest National Laboratory. “He is always trying to better himself as a scientist but also as an effective communicator so he can serve as a bridge between science and policy.” Indeed, Cahill encourages his students to adopt the role of “hybrid scientist”—prompting them to seek cross-disciplinary collaborations that broaden their knowledge both inside and outside the lab.

“I want my students to have an appreciation of the impact of their science on the public,” he said. “Perhaps more so than those who delve in other parts of the Periodic Table, nuclear scientists have to go an additional step and think about the importance and the impact of their work on the world at large."