Leveraging advanced accelerator techniques to probe nuclear structure

Each year, the Physical Sciences and Engineering (PSE) directorate at the U.S. Department of Energy’s (DOE) Argonne National Laboratory recognizes exceptional early-career researchers breaking into their fields with the PSE Early Investigator Named Awards. In 2025, the lab announced that six awardees would be receiving support in the form of funding and mentorship to conduct groundbreaking research aligned with Argonne’s strategic mission.

One member of the 2025 cohort is Bernhard Maass, an assistant physicist in the Low Energy Physics group in Argonne’s Physics division. Maass, whose work focuses on probing nuclear size, shape and structure, will be working under the guidance of Peter Mueller, principal physicist and interim group leader of the division’s fundamental symmetry group, on a proposal titled, ​“Quantum Control of Rare Isotopes.” This proposal aims to apply advanced laser spectroscopic methods, which are typically developed and deployed for applications in quantum optics, studying the nuclear structure and properties of radioactive ions.

“Our goal is to measure and understand fundamental properties of nuclei, which are at the center of every atom in the universe.” — Bernhard Maass, Argonne assistant physicist

Here, Maass discusses his research and other work he supports at Argonne.

Q: What role do you play at the lab?
A: I’m working in the Low Energy Physics group of the Physics division. Our goal is to measure and understand fundamental properties of nuclei, which are at the center of every atom in the universe. At ATLAS (Argonne Tandem Linac Accelerator System), a DOE Office of Science user facility at Argonne, the group maintains a number of instruments to investigate nuclear properties by exciting them or even breaking them apart. In addition to these fast and destructive techniques, there are slower high-precision measurements, which rely on stopped radioactive beams in ion traps. I’m supporting the strong stopped-beam program at ATLAS, which is renowned for measuring the mass of nuclei and other nuclear properties in the past decades. My specialty is a technique called laser spectroscopy. The size and the shape of nuclei leave tiny fingerprints on the atomic spectrum, and with very precise measurements, we can extract certain nuclear properties. I’m developing a laser spectroscopy program at ATLAS that uses the unique beams and existing infrastructure.

Q: What initiatives or projects are you most excited about being involved in at Argonne?
A: I’m very excited about being involved in the ongoing ATLAS program. We are in the process of commissioning two new and upgraded sources for stopped radioactive beams: nuCARIBU and the N=126 Factory. Both will offer unique beams and opportunities for laser spectroscopy experiments. It is a very nice and rewarding perspective to not just be a user of these sources but to also participate in the process of their development.

Q: Can you talk a bit about the research you’re conducting for your proposal for which you received the 2025 PSE Early Investigator Named Award?
A: My goal is to improve the reach of laser spectroscopy by developing new techniques that go beyond ​“classic” setups, to increase both the sensitivity and resolution. This is necessary because the isotopes of interest are further away from stability, shorter-lived and produced in increasingly smaller quantities. At the same time, theory has become more powerful and can make predictions — such as nuclear shape parameters — with much higher precision.

I plan to develop new high-precision ion traps that will allow us to cool radioactive ions to temperatures of a few millikelvin — a very low temperature, just above absolute zero — in very short time and to implement quantum controls to probe the atomic levels. In traps, ions can be probed more than once, increasing efficiency, and narrower atomic lines can be accessed, which allows higher resolution. I also intend to move away from linear scans of our laser frequency but instead have machine learning-based scanning patterns that maximize the information gain per time.

Q: What do you like most about your job?
A: I enjoy working in the Physics division — everyone is open to discussions and ideas, and the program spans a wide range of different experiments and theories. I had some of the most fruitful and stimulating discussions of my career somewhere in the hallways of the Argonne Physics building.

On a day-to-day basis, I like how diversified the job is. My tasks range from developing the very first idea of a measurement, designing and implementing the hardware, performing the experiment, and finally, often after years of work, publishing and presenting results at international physics conferences.

Q: How does your work support the lab’s mission?
A: I use the laboratory’s resources to push the limits of what is technically possible in the context of fundamental science. ATLAS is a world-leading accelerator facility, and by adding new capabilities, we’re expanding its global outreach and relevance.

Q: What do you enjoy doing outside of work?
A: I love the great outdoors — I’ve been to almost half of all U.S. national parks, many of which I discovered doing backpacking trips. My wife and I like discovering new things, and when we are not out in nature, we also spend our time visiting cultural events, musicals, festivals and cities.

Q: What other sorts of career or professional development opportunities has Argonne provided?
A: The first time I came to Argonne was in 2016 — I was a young graduate student from a German university, working on setting up a laser spectroscopy experiment in intermittent stays for the next three years. From the start, I was inspired by how work was done at Argonne and enjoyed working with the scientists at the lab. Some of them became my mentors and had a great and very positive impact on my career.

Q: What encouraged you to get involved in the scientific discipline you are in?
A: I can remember that, as a little boy in a rural town in Germany, I started building toy airplanes out of firewood and wooden slats, using every tool that I could find in my father’s garage. They never flew further than down the balcony, and this probably set the precedent for finding out what holds things together by crashing them. My fascination for engineering remained — only the toys, machines and instruments became more sophisticated, complicated and expensive.

I excelled in mathematics and physics in school, and I knew that I would not be happy settling on engineering without the aspect of finding out how everything works on a fundamental level. This brought me to major in physics.

Why laser spectroscopy? The university had two big physics departments: quantum optics and nuclear physics. Naturally, I wanted to do both, so for my Ph.D., I applied to the professor who was using one to do the other. Looking back, I think this was a very good choice, and I’m very thankful for the support and opportunities he provided for my career, which brought me here to Argonne.