The effort to construct GRETA (Gamma-Ray Energy Tracking Array), a cutting-edge spherical array of high-purity germanium crystals that will measure gamma-ray signals to reveal new details about the structure and inner workings of atomic nuclei, has received key approvals needed to proceed toward full build-out.
GRETA, which will also provide new insight about the nature of matter and how stars create elements, is expected to reach the first phase of completion in 2023, and to achieve final completion in 2025. It builds on the existing GRETINA (Gamma-Ray Energy Tracking In-beam Nuclear Array) instrument, completed in 2011, which features fewer gamma-ray-detecting crystals. Gamma rays are very energetic, penetrating forms of light that are emitted as unstable atomic nuclei decay into more stable nuclei.
The U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has had a leadership role in both GRETINA and GRETA, and Berkeley Lab nuclear physicists and engineers are working with teams at Argonne and Oak Ridge national laboratories, and Michigan State University, in the development of GRETA.
On Wednesday, Oct. 7, 2020, DOE officials approved key milestones for the GRETA project, including the scope of work and its schedule, and the final construction engineering plans that will guide the project through to completion. The formal approval steps are known as Critical Decision 2 and Critical Decision 3 (CD-2 and CD-3).
“The approvals were a major achievement for the project and the team. It marks the successful completion of the final design, and demonstrates we are ready to build the array,” said Paul Fallon, GRETA project director and a senior staff scientist in Berkeley Lab’s Nuclear Science Division. A key next step is to fabricate the complex, meter-wide aluminum sphere that will house the detectors.
New user facility will put GRETA to work
GRETINA, and later GRETA, will be installed at Michigan State University’s Facility for Rare Isotope Beams (FRIB), when that facility begins operations in 2022. On Sept. 29, FRIB was officially designated as the newest member of the DOE Office of Science’s user facilities. There are now 28 of these user facilities, which are accessible to scientists from across the country and around the world. Already, an estimated 1,400 scientific users are lined up to participate in nuclear physics experiments at FRIB once that facility starts up in 2022. Still under construction, FRIB is about 94% complete.
GRETINA is equipped with 12 detector modules and 48 detector crystals, and GRETA will add 18 more detector modules, for a total of 30 modules and 120 crystals. About 18-20 detector modules are expected to be installed in GRETA before the end of 2024, with the final modules installed in 2025.