Raph Hix Elected APS Fellow

October 19, 2023

Star-Stuff, Indeed

We are made of star-stuff, Carl Sagan said in the Cosmos TV series.

William Raphael (Raph) Hix knew that quote. As a high school kid in Maryland he’d taken advanced physics and chemistry. He’d watched Cosmos and heard Sagan talk about stars and elements. But something changed when he encountered this concept in one of his college astronomy textbooks. It took on a gravitas that has captivated him ever since, leading him to climb inside stars (theoretically) to see how the stuff that makes us—carbon, iron, etc.—came to be.

For his “contributions to understanding explosive thermonuclear burning and nucleosynthesis, particularly in contexts like supernovae,” Hix, a UT-Oak Ridge National Laboratory joint faculty professor, has been elected a Fellow of the American Physical Society. This honor is bestowed on only one half of one percent of the Society’s membership each year. Hix is one of 153 Fellows in the 2023 cohort and the second UT physicist elected in the past two years.

Adrian Del Maestro, UT Physics Professor and Department Head, had high praise for the department’s newest APS Fellow.

“Dr. Hix is exemplary of the unique and visionary researchers that bridge the University of Tennessee and Oak Ridge National Lab as joint faculty,” he said. “He is a driving force behind our astrophysics program and is a sought-after mentor and teacher, involving both graduate and undergraduate students in his cutting-edge research on stellar evolution.”

Late Bloomers

Hix is interested in how the chemical elements are made, or nucleosynthesis. The Big Bang gave us hydrogen, helium, and lithium. Since then, nuclear reactions accompanying the life and death of stars have created most of the other elements. As it turns out, stars are late bloomers.

“Most of the elements get made at the end of a star’s life,” he said.

Stars run on the fusion of hydrogen into helium for most of their lives. Hix explained that as a star begins to run out of fuel, temperatures go up and conditions become more extreme. That’s when the heavier elements, like carbon and iron, are made. Once the fuel is exhausted, an ordinary star (like our Sun) violently expels its outer layers, including elements made late in its life, and becomes a white dwarf. For more massive stars, like Betelgeuse, Rigel and Antares, the exhaustion of fuel leads to a supernova—sending those recently made elements into the cosmos—while the stellar core collapses, leaving a neutron star or a black hole.

With colleagues at ORNL and UT, Hix develops sophisticated models to understand how all this works. He leads ORNL’s Theoretical and Computational Physics group, utilizing some of the national lab’s powerful tools, like Summit and Frontier.

“We use the biggest supercomputer we can to model as much physics as possible within the intricate workings inside a star that we (then) blow up,” he said.

The results become part of a chain of handing off data—ultimately going to scientists who use telescopes to see if the model holds up to observation. Hix explained this is how they prove their models are accurate.

“It’s a way to climb inside a star and see the parts that are ordinarily hidden from view,” he said.

When Everything Was Cool and New

Hix finished undergraduate studies at the University of Maryland at College Park, where he re-discovered Sagan’s quote and graduated with bachelor’s degrees in physics and astronomy as well as math. He earned AM and PhD degrees in astronomy at Harvard University. Following a postdoctoral appointment at the University of Texas, he came to UT in Knoxville. He began as a postdoc, became a research professor, and then in 2004 moved (without moving) to ORNL. In 2010, he rejoined the UT faculty with a joint faculty appointment.

In his case, he explained, being joint faculty means that he’s an ORNL astrophysicist and the university subcontracts half of his time to teach courses (like Honors Introductory Astronomy) and supervise students. Hix said he really enjoys working with undergraduates. He loves seeing how excited they are when they come to the national lab and have an office for the summer. He likes being reminded, he said, “of that time in my career when everything was cool and new and interesting.”

About APS Fellows

The APS Fellowship Program was created to recognize members who may have made advances in physics through original research and publication, or made significant innovative contributions in the application of physics to science and technology. They may also have made significant contributions to the teaching of physics or service and participation in the activities of the Society.

Raph Hix is the 10th APS Fellow on UT’s current faculty.

Shape-Shifting Nuclei

August 22, 2023

What determines the shape of a nucleus? UT’s physicists played a key role in recently-reported findings that shed new light on that mystery. Their dedicated work to develop and deploy a sophisticated yet nimble detection system was central to an Oak Ridge National Laboratory-led study of how nuclear shapes evolve. The unexpected results could point to a deeper understanding of how nuclei stay together and how elements form.

Shape Shifters

Nuclei typically appear as spherical or deformed (football-like). Some can shift their shape depending on their energy level— deformed at higher energy (excited state) and spherical at low energy (ground state). The reverse (deformed at low energy; spherical at high energy) has been harder to pin down, especially in regions of the nuclear landscape where little experimental data is available.

In this work, scientists found that a sodium-32 nucleus has an exceptionally long-lived excited state, also known as an isomer. This nucleus sits at the heart of the “island of inversion,” where previous experiments have documented spherical-to-deformed shape reversal. Isomers can help probe nuclear structure, and the one observed in sodium-32 is a rare microsecond isomer in this particular area of the nuclide chart. It can provide a window into the underlying conditions where the spherical-to-deformed transition begins.

The analysis is based on data collected from the very first experiment at the Facility for Rare Isotope Beams (FRIB)—specifically the FRIB Decay Station Initiator (FDSi). This is Professor Robert Grzywacz’s home office, so to speak, as he and his group have invested years in this sensitive, modular detector system, starting with the plans on paper and now actually “catching” the fragments of a rare isotopes created by FRIB’s powerful linear accelerator and measuring their decay.

“We poured an enormous amount of work into this experiment,” he said. “It had to succeed.”

Getting an Early Start

While the observation of the sodium-32 isomer is new, the premise is not. As a graduate student Grzywacz was a lead author on papers outlining a novel method suited to scanning large swaths of the nuclear chart in search of new isomers. His work eventually led him to Tennessee, where in 1998 he became a postdoctoral fellow at UT and in 2003 joined the faculty.

Since then he’s built a talented nuclear physics team of fellow faculty, students, and staff. These latest findings are outlined in Physical Review Letters and UT Physics co-authors include Grzywacz as well as Miguel Madurga (assistant professor); Zhengyu Xu and Kevin Siegl (postdoctoral research associates), Noritaka Kitamura (postdoctoral research associate, now assistant professor at University of Tokyo); Joseph Heideman, Shree Neupane, and Maninder Singh (PhD alumni); Ian Cox and James Christie (graduate students); Harrison Huegen and Amanda Nowicki (undergraduates); and Jason Chan (Electronics Shop Supervisor).

Learn more about the results at Oak Ridge National Laboratory’s website.

With thanks to Dawn Levy of Oak Ridge National Laboratory.

Where Instability is a Good Thing

November 14, 2022

FRIB just published the first paper from its first experiment. UT’s physicists built the tools that made it possible.

The email came with a simple subject line: “Now with ribbon.”

Professor Robert Grzywacz was at Michigan State University sending photos as dignitaries cut a giant green ribbon to open the Facility for Rare Isotope Beams (FRIB). For him and his colleagues it was more than christening a powerful research facility. It was a new chapter in a story of scientific creativity, and community, and turning drawbacks into opportunities.

Things Fall Apart

Grzywacz, like Professor Kate Jones and Assistant Professor Miguel Madurga, dwells in the land of low-energy nuclear physics. They’re interested in how a nucleus is structured, how stable it is, and the way its components interact. Rare isotopes are an ideal vehicle to find out.

FRIB creates these exotic nuclei by stripping electrons from stable atoms and guiding the resulting ions into a linear accelerator where they travel faster than half the speed of light. When the beam hits a target, nuclei lose protons or neutrons, creating unstable, short-lived rare isotopes. FRIB can stop and re-accelerate the beam, filtering out isotopes so that researchers get the ones they want.

Studying isotopes as they fall apart gives scientists a window into a nucleus’s innermost mysteries, knowledge that moves science forward and also meets practical needs.

“There are applications from medical imaging and therapy to energy,” Jones explained.

In June FRIB completed its first experiment. Scientists slammed a stable beam of calcium-48 into a beryllium target. The fragments from that collision gave them a bunch of exotic isotopes to study. Observing their decay, which takes less than a second, led to the first reported measurements of half-lives for five exotic isotopes of phosphorous, silicon, aluminum, and magnesium. The findings were published in Physical Review Letters as the first paper from FRIB’s first experiment.

“This result builds on a lot of our past work,” Grzywacz said.

Making those measurements required sophisticated instrumentation called the FRIB Decay Station initiator (FDSi), a system he knows well.

Finding a Silver Lining

The FDSi didn’t come out of nowhere. It’s descended from generations of detectors designed before FRIB was born. Grzywacz has been part of this effort since 1998, when he started proposing experiments at FRIB’s predecessor, the National Semiconductor Cyclotron Laboratory (NSCL).

The facility was nearly 20 years old and “the nuclear physics community was in the process of discussing what and where the new generation facility (would) be,” he said.

At the time, he and Jones were deeply integrated at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL). Grzywacz’s group built and contributed to new instruments to study radioactive, neutron-rich isotopes. They were the first to use digital signal processing broadly for nuclear spectroscopy experiments for low-energy nuclear physics. They partnered with industry to develop a digital data acquisition system (which eventually made its way to FRIB).

Among their key accomplishments was building a suite of instruments to detect and measure decay patterns. Krzysztof Rykaczewski led an ORNL-centered program with buy-in from other universities (Georgia Tech, Louisiana State, Mississippi State, and Vanderbilt) through the University Radioactive Ion Beam Consortium (UNIRIB) and the Joint Institute of Nuclear Physics and Applications (JINPA).

“The decay program at HRIBF was strong and unique,” Grzywacz said.

In 2008, the U.S. Department of Energy decided the future was with FRIB and in 2011 HRIBF was closed. Despite their disappointment, Grzywacz and his colleagues found a silver lining.

“The scientific strength of the research program at HRIBF and available instrumentation encouraged us to engage in the construction of one of the key FRIB projects: the FRIB Decay Station,” he said.

Use What You Have

The FRIB Decay Station (FDS) focuses on four strategic areas: nuclear structure, nuclear astrophysics, tests of fundamental symmetries, and applications of isotopes for society.

In 2016 some 60 scientists gathered at JINPA and established the FDS collaboration, with Grzywacz as spokesperson. The FDS would be a new instrument: an efficient, modular system of multiple detectors under one infrastructure that would provide parallel and simultaneous measurements. It came with a hefty price tag, so in the interim researchers formed the FDS Initiator (FDSi) group.

“This project aimed to realize the FDS vision but using existing instrumentation,” Grzywacz said. “The FDSi was to provide a unified framework for the variety of instrumentation provided by the community. The essential elements of the FDS were to be realized by existing detectors.”

In August 2021 FDSi went into production with a talented, collaborative cast.

ORNL staff led by James Allmond designed the elements and led construction. Machinists from UT Physics made precision parts for the frame. UT’s low-energy nuclear physics group built new detectors, making sure they could be integrated in FRIB’s digital data acquisition framework. Grzywacz came up with the idea of using two focal planes for easy switching between two sets of apparatus, as well as the designs for the implantation detectors. These are necessary to stop the nuclei and measure the charged particle decays.

While this was happening the FRIB Program Advisory Committee announced the first approved experiments. Of 34 accepted proposals, eight included the FDSi.

By February 2022 FDSi installation began at Michigan State. In June the system was ready for the calcium fragments that came its way, as were Grzywacz and Madurga; graduate students Ian Cox and James Christie; and postdocs Noritaka Kitamura, Kevin Siegl, and Zhengyu Xu, all of whom worked on FRIB’s inaugural experiment.

Twelve Neutrons from Stability

After years of work to get FRIB up and running, scientists can now reap the benefits. The FDSi is one of multiple instruments along the beam, giving them lots of options to explore nuclei.

Jones will use a high-resolution spectrograph and gamma-ray energy tracking array in an experiment on the structure of light tin isotopes, slated to run this December.

“Tin is magic,” she explained. “It has 50 protons, which forms a closed nuclear shell.”

The isotopes Tin-100 (50 neutrons) and Tin-132 (82 neutrons) are “doubly-magic.”

“Doubly-magic nuclei are cornerstones of the nuclear shell model,” Jones said. “Knowing the single-particle states outside of these nuclei allows theorists to calculate the properties of more complex nuclei.”

Tin-112 is the element’s last stable isotope. This makes Tin-100, at 12 neutrons from stability, particularly difficult to reach.

“The further from stability, the harder isotopes are to produce,” Jones said. “Hence, FRIB.”

By knocking out a neutron from Tin-104, her group will get a look at the states in Tin-103. They expect the ground and first excited states to have the same nature as Tin-105 and Tin-107. Ultimately, as they get closer to Tin-101 (which can be reached when FRIB reaches full power) they anticipate those states will be flipped.

This kind of research—learning about the structure of key exotic nuclei—is in line with FRIB’s aspiration to be the world’s leading laboratory in rare isotope science.

The Nuclear Family

A premier research facility needs a community with a shared purpose. Grzywacz and Jones’s nuclear connections span decades and continents. Grzywacz still works with colleagues from his alma mater, the University of Warsaw. Jones can trace her nuclear family’s roots to her undergraduate days at the University of Surrey and follow them through postdoc appointments across Germany and France, where she and Grzywacz met. FRIB is home to scientists they’ve met along the way as well as former students and postdocs they’ve mentored. And, of course, there’s Witek Nazarewicz, a longtime professor of physics at UT before he signed on as FRIB’s chief scientist.

“There is a scientific community around FRIB that we are integrated in,” Jones said. “There are people who are critical links in making FRIB happen, such as those who develop the rare ion beams, who we met back in Europe or here in East Tennessee.”

This community is essential for FRIB, and nuclear science, to thrive.

“The future of low-energy nuclear physics in the U.S. depends to a large extent on the success of FRIB,” Jones said. “Without building new facilities it would be difficult for the U.S. to stay ahead in our field.”

She and Grzywacz met some of their colleagues long before they celebrated the ribbon-cutting at FRIB’s opening. The importance of those early (and continuing) ties can’t be overstated.

“There are decades of knowledge that are passed between researchers, not only in papers, but working together in labs, at workshops and conferences, and teaching in classrooms and professors’ offices,” Jones said. “If you break the chain, it’s hard to rebuild.”

Getting Under the (Neutron) Skin

October 20, 2022

Every atomic nucleus, miniscule or massive, is governed by a basic law of nature: the strong force holding it together. The more we know about the complex workings of nuclei the better we understand different kinds of matter, including elements produced in cosmic collisions (like the gold in our jewelry boxes). In Nature Physics, UT’s physicists and their colleagues at Chalmers University of Technology present a groundbreaking model to calculate the properties of a lead nucleus with a method that can be used across the nuclear landscape—all the way to neutron stars.

The Story of Stars in a Single Nucleus

Scientists use computational models to get the most complete picture of a nucleus. Heavier nuclei typically don’t fit well in those models. They may be unstable and fall apart quickly. They might have so many protons and neutrons that it’s difficult to keep track of all the interactions between them. Nuclei with lots of neutrons present a particular challenge. The new approach turns that obstacle into an asset.

When neutrons outnumber protons, equal numbers of each gather in the nucleus’s center while the surplus neutrons collect on its surface, forming a skin. Lead (Pb-208) is a good example. With 126 neutrons and 82 protons, it’s a heavy (yet stable) nucleus covered in a neutron skin. This outer layer is sensitive to the strong force, which holds its protons and neutrons together.

Professor Thomas Papenbrock described how he and his colleagues connected this single, simple nucleus to massive neutron stars, a teaspoon of which would weigh 4 billion tons on Earth.

Combining advanced methods, statistical tools, and computational modeling, they were able to calculate the neutron skin in lead. This informs scientists about the “nuclear equation of state,” which describes matter under various conditions like pressure, volume, or temperature. That equation in turn links neutron skin thickness to the structure and size of neutron stars. Further, the model can be adapted across the nuclear landscape to predict properties of other heavy nuclei that have eluded models in the past.

The work also shows that a heavy nucleus can be computed with tools theorists routinely use: Hamiltonians (functions that describe a dynamic system) rooted in the theory describing the strong force (quantum chromodynamics, or QCD). These tools have well-understood parameters that help quantify uncertainties, making predictions more precise.

The Physics in Your Jewelry Box

Papenbrock outlined multiple reasons why understanding nuclear structure and properties is important. First, there’s understanding the complexity arising from the strong force, one of nature’s four basic interactions (along with the weak, electromagnetic, and gravitational forces).

“Second,” he said, “is understanding the production of elements in the early universe and in stellar processes such as neutron-star mergers and supernovae explosions.”

(Helium and hydrogen were born in the early seconds of our universe, while gold and platinum trace their beginnings to merging neutron stars.)

“This require us to understand very neutron-rich nuclei that cannot all be produced and studied in laboratories,” Papenbrock said. “So, we have to compute them.”

Understanding the nucleus not only helps us understand what happened eons ago or what goes on in far-away objects. It also helps scientists shape future discoveries.

“Searches for physics beyond the Standard Model of elementary particles almost all involve hypothetical reactions of such particles with or inside atomic nuclei,” he explained.

James Bond and Nuclear Physics

Papenbrock pursues nuclear studies through a joint faculty appointment with Oak Ridge National Laboratory. He worked with ORNL’s Zhonghao Sun (former UT postdoc) and Gaute Hagen (an adjunct professor with UT Physics) to calculate lead’s neutron skin, a feat made possible by ORNL’s Summit supercomputer.

“Computing has been a game changer over the last two-to-three decades,” Papenbrock said. “Moore’s law states that computing power doubles roughly every 16 months. Our phones can do things today that seemed science fiction (or James Bond) just a few decades ago.

“This has forced nuclear physicists to collaborate with computer scientists to really use all these technologies,” he continued. “We had to make so many changes to our codes over the years to adapt to this evolution of computing.”

Using collaborative strengths and computing power for nuclear physics is familiar territory for Papenbrock and Hagen. They’re part of the NUCLEI (the NUclear Computational Low Energy Initiative) collaboration that recently won $13 million in funding through SciDAC, the U.S. Department of Energy’s Scientific Discovery Through Advanced Computing program (part of the Office of Science). Papenbrock became the group’s principal investigator this year.

Comprising five national laboratories and seven universities, NUCLEI aims to advance the computations of atomic nuclei and processes. The Nature Physics paper was a result of the collaboration’s research, earning a News and Views article in the journal for its significance. This work, the review explains, “has allowed practitioners to reach for the stars.”

Andrew W. Steiner Elected APS Fellow

October 19, 2022

When a giant star dies, Andrew W. Steiner gets a little more lab space.

An associate professor with a joint appointment at Oak Ridge National Laboratory, Steiner studies the neutron stars left after massive stars collapse. His outstanding research has earned him election as a Fellow of the American Physical Society (APS), making him the 10th APS Fellow on the current physics faculty.

APS Fellowship recognizes members who have made advances in physics through original research and publication or significant innovative contributions in applying physics to science and technology. They may also have made significant contributions to physics teaching or service. Steiner was cited “for pioneering a data-driven approach to constraining neutron star properties and the dense matter equation of state that combines advanced statistical methods, state-of-the-art nuclear theory, experimental constraints on bulk nuclear properties, and astrophysical data.”

A Stellar Laboratory

Dense, with a mass much larger than that of our sun, an average neutron star could fit in the space between the Tennessee Theatre and McGhee Tyson Airport. Yet these stellar bodies are actually good models for atoms. Steiner uses neutron star observations as a kind of laboratory to learn more about how neutrons and protons interact, especially in terms of the strong force (one of nature’s four basic forces) that holds them together. He was a bit surprised the citation mentioned “advanced statistical methods,” because he’s never actually had any courses in statistics.

“It is, in part, a testament to how much our field has changed over the past decade or so,” he said. “Statistical methods, computational tools, and machine learning continue to transform our ability to use data to elucidate our knowledge of the strong nuclear force.”

Steiner’s work has ties to important progress in nuclear astrophysics. The team that first detected gravitational waves produced by colliding neutron stars cited his group’s predictions about tidal deformability (what he calls “squishiness”) in these massive objects. He’s also director of the Nuclear Physics from Multi-Messenger Mergers (NP3M) research hub. The National Science Foundation (NSF) awarded UT $3.25 million to launch this national effort, bringing together scientists with expertise in nuclear physics and astrophysical simulations to study the properties of matter not available on Earth. Among the hub’s goals is training a diverse new generation of scientists.

“Professor Steiner works at the forefront of theoretical nuclear astrophysics where he uses cutting edge computational tools to probe the properties of hot and dense strongly interacting matter,” said Physics Professor and Department Head Adrian Del Maestro. “His dedication to building open source tools ensures that his research has a broad impact on the field as recognized by his peers through election as a Fellow of the APS. We are proud to add him to the list of 10 APS Fellows forging new knowledge and teaching and mentoring students in the Department of Physics and Astronomy at the University of Tennessee.”

Steiner earned a bachelor’s degree in physics at Carnegie Mellon University, followed by master’s and doctoral degrees in physics at the State University of New York at Stony Brook. He joined the UT Physics faculty in 2014. In 2016 he won an NSF Faculty Early Career Development Program (CAREER) Award, following that up in 2019 with a Senior Career Award for Research and Creative Achievement from the College of Arts and Sciences.

Steiner credits a long list of supporters for his career success: his wife, his daughter, his parents, and colleagues from UT Physics and around the country (including Madappa Prakash, his graduate advisor).

Each year, no more than one half of one percent of the society’s membership (excluding student members) is recognized by their peers for election to the status of Fellow. Steiner was recommended for Fellowship by his colleagues in the APS Division of Nuclear Physics. He will be acknowledged at the Division’s annual meeting later this month.