Researchers including those from Argonne have completed major construction of the Gamma-Ray Energy Tracking Array (GRETA), a precision detector that will expand our understanding of the structure and properties of atomic nuclei.
Understanding how atomic nuclei behave has led to many advances, including MRIs that diagnose disease and nuclear energy that powers homes. But our picture of the nucleus, the heart of the atomic world, is still incomplete. Researchers plan to improve our understanding with an advanced new instrument: GRETA, the Gamma-Ray Energy Tracking Array.
The project team, led by the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab), has now completed construction of GRETA’s key components: multiple germanium detector modules, the electronics system, the instrument’s mechanical frame and infrastructure, and the computing systems. The team includes scientists and engineers from Berkeley Lab’s nuclear science, engineering and computing divisions; Michigan State University (MSU); and DOE’s Argonne National Laboratory and Oak Ridge National Laboratory.
“Our goal was to make the best high-resolution, high-efficiency gamma-ray detector we possibly could, so we can answer big questions about the nature of matter and fundamental forces,” said Paul Fallon of Berkeley Lab, GRETA’s project director. ?“With every advance in this technology, we can improve our resolving power and can see weaker and weaker structures. GRETA will be 10 to 100 times more sensitive than previous nuclear science experiments.”
To get a glimpse inside the nucleus, researchers will smash a particle beam into a target placed at the center of GRETA, briefly creating energetic, rare nuclei. GRETA’s sensitive germanium detectors measure the 3D paths and energies of emitted gamma rays, particles of light made as excited nuclei return to a more stable state. That data can reveal many interesting insights, among them, the shape of the given nucleus and the de-excitation path to become stable.
This research is exciting for nuclear physicists and has implications for astrophysics and understanding of nuclear synthesis.”
Dariusz Seweryniak, Argonne physicist
GRETA was assembled at Berkeley Lab and is now ready to ship to the Facility for Rare Isotope Beams (FRIB), a DOE Office of Science user facility at MSU. There, researchers will install and commission the device, adding additional detector modules as they become available. FRIB will be able to create and study more than 1,000 new isotopes, including rare and short-lived isotopes. Scientists can also test the limits of how many protons and neutrons a nucleus can hold, exploring the point beyond which neutrons or protons can no longer bind within the nucleus and instead ?“drip” away.
Built for flexibility, as well as sensitivity, GRETA will move to different stations at FRIB, and later Argonne, to use particle beams of different types and energies. Other experiments, including those that will be conducted at the Argonne Tandem Linac Accelerator System (ATLAS), another DOE Office of Science user facility, will study pear-shaped nuclei. Studying these nuclei offers a way to search for subtle violations of fundamental symmetries in nature and explore why our universe is made mostly of matter (instead of antimatter). Researchers at FRIB and ATLAS will also use GRETA to shed light on the processes within stars that forge elements heavier than iron.
“Gamma-ray spectroscopy is among our most powerful tools to learn about the fundamental nature of the atomic nucleus,” said Heather Crawford, a scientist at Berkeley Lab and deputy project director for GRETA. ?“The excited states and gamma rays are a fingerprint for each isotope. GRETA is the world’s most powerful microscope to examine these fingerprints and answer questions about the nucleus and the forces that govern it.”
Coming Soon: Sphere of Germanium Crystals Will Increase Capabilities
GRETA is an expansion of an earlier project, GRETINA, that used 12 germanium detector modules to capture gamma rays. GRETINA is currently installed at Argonne’s ATLAS user facility for its fourth campaign. Once this campaign concludes, GRETINA will return to FRIB so its detector modules can be incorporated into GRETA.
GRETA will bring the total to 30 modules, completing a full sphere around the target and vastly increasing the instrument’s tracking capabilities. By catching more gamma rays, researchers will get a more accurate picture of what’s happening in the nucleus — and get it more quickly.
Each detector module is made of four tapered hexagonal crystals of ultra-pure germanium, each roughly the size of a 10-ounce coffee cup. The germanium crystals are such specialized and difficult pieces to make that only about four detector modules can be produced every year. Once tightly packed together and cooled to cryogenic temperatures (around negative 300 degrees Fahrenheit), the crystals are exceptionally good at measuring the energy and position of gamma rays, enabling researchers to reconstruct their interactions in the crystal.
GRETA’s backbone is a complex, meter-wide aluminum frame that supports the germanium detectors and electronics. The sphere is built in two halves that separate, opening space for researchers to change out targets at the center of the instrument. To make sure the sphere comes back together seamlessly, the base plate and rails are aligned within one millionth of an inch. Each half can also rotate, allowing researchers to safely install the detector modules before moving them into their final orientation for operations. The team put the support assembly through its paces with dummy weights last year before beginning integration of the other components.
A Faster Gamma-Ray Detector
A key part of the project was to design compact and efficient new electronics and a dedicated computing system for GRETA. The new electronic system can perform with up to 50,000 signals per second in each crystal. A dedicated computing cluster will process up to 480,000 gamma-ray interactions every second in real time. Tests this spring showed GRETA surpassing its design goal, processing as many as 511,000 gamma-ray interactions per second.
Argonne has been part of the broader GRETA collaboration, contributing its expertise in gamma-ray detection, according to Dariusz Seweryniak, an experimental physicist and interim leader of the Low Energy Nuclear Physics group within Argonne’s Physics division.
“Our engineers designed the trigger system for GRETA’s data acquisition system, which is crucial as it’s the heart of the system,” Seweryniak said.
Additionally, Argonne is developing and deploying the gamma-ray tracking algorithms, which are key to maximizing GRETA’s performance. Argonne Physicist Torben Lauritsen has led this effort. Recently, Lauritsen has been using artificial intelligence (AI) to enhance the software’s performance to further improve the sensitivity of the detector. This work is being done in collaboration with Argonne Distinguished Fellow Sven Leyffer, senior computational mathematician and deputy division director of Mathematics and Computer Science at Argonne, and Thomas Lynn, a former postdoctoral researcher at Argonne.
The electronics for GRETA were also a significant focus of Argonne’s contribution. ?“Our team, including John Anderson and Michael Oberling, designed and built the trigger system for GRETA, which is essential for identifying gamma-ray events,” said Michael Carpenter, an experimental physicist at Argonne who serves as a deputy manager for electronics on GRETA. Anderson, now retired, was a manager for the GRETA and GRETINA trigger systems; a position now held by Oberling — an electronics engineer in the Physics division.
The primary role of a trigger system is to identify and select events of interest from a large number of signals generated during an experiment. In the context of GRETA, the trigger system is essential for processing the vast amounts of data generated by the detector and ensuring that only relevant gamma-ray interactions are recorded for further analysis. GRETA’s trigger system was tested at Argonne before being integrated with the rest of the components at Berkeley.
GRETA is also a potential first use case for an accelerated data pipeline called DELERIA, a new software platform for streaming enormous amounts of data at high speeds. Researchers will be able to transfer data through DOE’s high-speed network, ESNet, to be crunched at supercomputing facilities off-site and returned almost immediately. That rapid feedback will help researchers optimize their experiments as they take place even without a dedicated computing cluster.
Once GRETA incorporates GRETINA’s detector modules, it will replace GRETINA as the flagship gamma-ray detector at FRIB. Installation is expected in the fall and first experiments should begin in 2026.
GRETA is expected to move between facilities, including Argonne’s ATLAS and MSU’s FRIB, to maximize its use. ?“Both facilities offer complementary beams for different experiments, and the research community supports this approach,” Seweryniak said. It is estimated that Argonne will host GRETA in 2028 or 2029.
“This research is exciting for nuclear physicists and has implications for astrophysics and understanding of nuclear synthesis,” Seweryniak said.
In addition to Anderson, Carpenter, Lauritsen, Leyffer, Lynn, Oberling and Seweryniak, Argonne team members involved with GRETA include Edward Boron, a senior technician with the Experimental Operations and Facilities division of the Nuclear Technologies and National Security directorate, and Marco Siciliano, a physicist who serves as the lead for the GRETINA detector while it’s at ATLAS for experiments.
This project received funding from DOE’s Office of Science, Office of Nuclear Physics.