GRETA, sensitive and flexible, heads to Michigan State to study the nucleus
Researchers announced earlier this month that they have completed major construction of the Gamma-Ray Energy Tracking Array (GRETA), a precision tool for gamma ray spectroscopy that, according to Paul Fallon, a researcher at University of California–Berkeley and GRETA’s project director, will be 10 to 100 times more sensitive than previous nuclear science experiments. Fallon was quoted in an August 8 article published by Lawrence Berkeley National Laboratory (Berkeley Lab)—where GRETA’s project leaders are based and GRETA was assembled.
A team including scientists and engineers from Berkeley Lab’s nuclear science, engineering, and computing divisions; Michigan State University; and Argonne and Oak Ridge National Laboratories has now completed construction of GRETA’s key components: electronics, computing, and mechanical systems as well as the majority of the sensitive, high-purity germanium detector modules.
Paul Fallon, project director, and Heather Crawford, deputy project director, stand at the center of GRETA’s two halves. (Credit: Marilyn Sargent/Berkeley Lab)
Next stop is FRIB: GRETA is now ready to ship as planned to the Facility for Rare Isotope Beams (FRIB) at MSU, where FRIB’s rare isotopes will be directed to GRETA’s center. GRETA was built for flexibility as well as sensitivity, according to Berkeley Lab, and will be able to move from station to station at FRIB for exposure to particle beams of different types and energies. GRETA will later hit the road and visit the Argonne Tandem Linac Accelerator System (ATLAS). Argonne expects to host GRETA in 2028 or 2029 and for GRETA to move between facilities to maximize its use.
At FRIB, a Department of Energy Office of Science user facility, researchers expect to receive GRETA from Berkeley Lab this summer, install it in the fall, and begin the first experiments in 2026.
Structure and power: GRETA is an expansion of an earlier project, GRETINA, which used 12 germanium detector modules to capture gamma rays. GRETA will incorporate detector modules from GRETINA, which is currently operating at Argonne, and bring the total to 30 modules, completing a full sphere around the target to increase the instrument’s tracking capabilities. GRETA’s spherical aluminum frame was built in two halves that separate to allow targets to be replaced.
Each detector module is made of four tapered hexagonal crystals that are difficult to make—only about four detector modules can be produced every year. When those detector modules are cooled to cryogenic temperatures, the crystals will measure the 3D paths and energies of emitted gamma rays—particles of light made as an excited atom returns to a more stable state—enabling researchers to reconstruct their interactions in the crystal.
Scientists will be able to test the limits of how many protons and neutrons a nucleus can hold, exploring “the drip lines,” the point beyond which neutrons or protons can no longer bind within the nucleus and instead “drip” away, according to Berkeley Lab. Other experiments will study pear-shaped nuclei, a way to search for subtle violations of fundamental symmetries in nature and explore why our universe is made mostly of matter and not antimatter.
Argonne collaboration: Argonne has been part of the broader GRETA collaboration, and on August 20 published its own news article, adapted from that issued by Berkeley Lab and detailing the work of Argonne researchers on GRETA’s trigger system, gamma ray–tracking algorithms, and electronics.
“Our engineers designed the trigger system for GRETA’s data acquisition system, which is crucial as it’s the heart of the system,” said Dariusz Seweryniak, an experimental physicist and interim leader of the Low Energy Nuclear Physics group within Argonne’s Physics division. GRETA’s trigger system was tested at Argonne before being integrated with the rest of the components at Berkeley.
The primary role of the trigger system is to identify and select events of interest from a large number of signals generated during an experiment. For GRETA, the trigger system allows data from only the most relevant gamma-ray interactions to be recorded for further analysis.
