GRETA Comes Online for a New Era of Nuclear Science

The Gamma-Ray Energy Tracking Array (GRETA), the world's most advanced gamma-ray spectrometer for nuclear science, has taken first beam and successfully completed a commissioning and characterization run at the Facility for Rare Isotope Beams ( FRIB ) at Michigan State University. The milestone confirms the detector is performing as anticipated, giving researchers a way to observe atomic nuclei in greater detail than ever before.

GRETA uses high-purity germanium detectors to reveal the structure and behavior of nuclei with unprecedented sensitivity and precision. The detector was first assembled with a subset of detector modules at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), which leads the project, before being shipped to FRIB in fall of 2025.

The commissioning and characterization marks the culmination of years of design, construction, testing, and installation by a collaboration of experts from Berkeley Lab's nuclear science, engineering, and computing divisions; FRIB at Michigan State University; and Argonne and Oak Ridge national laboratories.

"It's very exciting to see GRETA now fully installed with a complete complement of detectors at FRIB," said Heather Crawford, deputy project director for GRETA and head of the low-energy nuclear physics program at Berkeley Lab. "GRETA has more high-purity germanium than we've ever had around a target. It's an exceptional array and capability for our field. It has been a long time coming, and the result of years of effort from a very big team."

At FRIB, researchers can produce short-lived rare isotopes for study by directing high-energy heavy-ion beams at a target. As the resulting excited atoms return to more stable states, they emit gamma rays that GRETA tracks to reveal the inner workings of the nucleus.

During commissioning, scientists and engineers integrated GRETA at FRIB's ReAccelerator beamline, calibrated the full array's systems, and collected the experimental data to verify its performance. In the characterization run, researchers collided two lighter nuclei to form excited nuclei that then decayed multiple times, emitting cascades of gamma rays.

"By exploring the sensitivity of GRETA to the weakest gamma-ray transitions, we can benchmark the performance against previous gamma-ray detector arrays," said Paul Fallon, GRETA project director and director of Berkeley Lab's Nuclear Science Division. "There's still a lot of analysis work to be done, but the data are fantastic."

The collaboration is now preparing for GRETA's upcoming science experiments at FRIB, which is a DOE Office of Science user facility that supports researchers from around the world. GRETA will enable researchers to investigate how nuclei are assembled, how many protons or neutrons they can hold, how they behave under extreme conditions, and how the elements heavier than iron are forged in stars. They can also search for subtle asymmetries in nature that could help explain why our universe is made of matter rather than antimatter.

The measurements that scientists make with GRETA will improve the theoretical models of the atomic nucleus that underpin ongoing research in nuclear physics, astrophysics, energy, medicine, and national security.

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