Over the past 35 years, Mills built and refined photon sources to advance research in chemistry, physics, biology and beyond

OCTOBER 17, 2024

Mills served as the deputy director of Argonne’s Advanced Photon Source, which he helped to design and build.

X-ray light sources provide ultra-bright, high-energy X-ray beams that allow scientists to explore the innermost structure and function of materials. Synchrotron light sources are critical instruments for research exploring combustion engines, microcircuits, pharmaceuticals, nanotechnologies and more. One of the most renowned of these facilities is the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at the DOE’s Argonne National Laboratory. APS is used by more than 5,500 scientists in a typical year.

Dennis Mills joined Argonne in 1988 and served as deputy director of the APS from 2002 until his retirement in late 2023. He played a key role in planning, building and upgrading the APS, establishing it as a world-class scientific facility. He also brought together a team of experts in X-ray research, which has since flourished into the X-Ray Sciences Division at Argonne.

“I’ve really enjoyed seeing the field grow and flourish because people truly understand that photon sources are a great tool.” — Dennis Mills, Argonne National Laboratory

Part of Mills’ success can be attributed to the knowledge he gained during his first forays into synchrotron light sources. During his PhD work at Cornell University in the 1970s, he built a beamline at the Cornell 12 GeV synchrotron, a high-energy physics facility, to use in his thesis project. After graduating in 1979, Mills stayed at Cornell to work at the Cornell High Energy Synchrotron Source that had just started operating with the new Cornell Electron Storage Ring. Over the next decade, Mills developed expertise on how to build and operate the new generation of X-ray beamlines. He also published seminal papers on synchrotron X-ray optics and related techniques, cementing himself as a leading researcher in the field.

In 1987, Mills earned a prestigious Guggenheim Fellowship to work both at the Synchrotron Radiation Source at the Daresbury Laboratory in the U.K. and at Argonne. ​“I spent six months at Argonne and got to know the people there who were developing the technical designs for a new high-energy storage ring. The following year, they offered me a position to work on the development of X-ray optics and beamlines,” he said.

By that time, Argonne had already made plans to build the APS with a larger and more powerful electron storage ring at its center. During the early construction phases of the APS, Mills contributed to important technical decisions about beamlines, office and lab infrastructure, and how to design optical components. However, building the new X-ray light source came with a suite of challenges that required in-depth knowledge and innovative problem solving.

“Previous machines were smaller and lower energy,” Mills said. ​“The X-ray beams coming out of the newer machines were so intense that they could literally melt the optical components we were supposed to use. One of the first jobs that I was assigned to when I started at Argonne was to figure out how to solve this high heat load problem.”

Over the course of several years, Mills and his collaborators at Argonne and other light sources conducted numerous experiments to test different methods for cooling components of the X-ray beamlines. They ultimately solved the heat-load problem by cooling the silicon monochromators in the APS using liquid nitrogen. This solution, which earned Mills and his team the esteemed APSArthur H. Compton Award in 1998, is now implemented at the APS and photon sources around the world.

During his 35-year career at Argonne, Mills climbed the ranks, moving from a position as group leader to division director to deputy associate laboratory director for science and deputy director of the APS. In his role as deputy director, Mills managed the APS User Office that coordinates visits for the thousands of scientists that come to the APS each year.

Mills was responsible for overseeing the allocation of user beam time for the facility’s 60 plus beamlines and finding ways to improve the user experience. He also provided important insights to the APS management team about which new beamlines to build and which research areas to invest in based on the needs of the scientists within the light source community.

Along with his research and managerial accomplishments, Mills said one of the proudest aspects of his career at Argonne was providing educational opportunities to early-career researchers. ​“Twenty-five years ago, we realized that it was really important to train young scientists to help them understand what they could do with these new machines,” said Mills. With funding from DOE, Mills and his team launched the National School on Neutron and X-ray Scattering, a two-week summer school to teach graduate students about the use of neutrons and X-rays for their research.

“Each year they would come spend a week at the APS listening to lectures and doing X-ray experiments before heading to a neutron source for a similar experience,” Mills said.

In 2010, Mills was elected to be a fellow of the American Physical Society for his work developing synchrotron X-ray optics, building the APS beamlines and creating training programs for early-career scientists. He also served as editor of the Journal of Synchrotron Radiationfor nearly a decade and gave annual lectures to the APS staff about various topics within the field of X-ray science.

Mills retired in November 2023, but his career at Argonne isn’t quite over yet. He continues to support APS staff and scientists through his new role as a contractor. But Mills said he looks forward to pursuing his hobbies outside of work and the more flexible schedule afforded by retirement. ​“I’d like to learn how to play golf better and spend more time at home and traveling,” he said.

When reflecting on his career at Argonne, Mills highlights the unique experiences that working at the APS provided him.

“I got to meet people that I might have never met otherwise, and I was afforded a lot of opportunities to travel to visit other light sources around the world,” he said. ​“I’ve really enjoyed seeing the field grow and flourish because people truly understand that photon sources are a great tool not just to advance the fields of chemistry and physics, but are also now supporting cutting edge research in life sciences, engineering and cultural heritage studies.”