GRETA—the Gamma-Ray Energy Tracking Array—uses a shell of high-purity germanium detectors to reconstruct gamma rays emitted by unstable atomic nuclei. After completing commissioning at the Facility for Rare Isotope Beams in Michigan, the $58 million instrument has begun its first scientific experiments.
The opening experiment is led by nuclear physicists from the University of Surrey. It asks whether a short-lived germanium nucleus has the elongated shape predicted by older models or a flatter, disc-like shape suggested by newer evidence.
What is GRETA?
GRETA is a gamma-ray spectrometer built for nuclear science. Lawrence Berkeley National Laboratory leads the project, with contributions from Argonne National Laboratory, Oak Ridge National Laboratory and the Facility for Rare Isotope Beams, or FRIB, at Michigan State University.
The instrument surrounds an experimental target with highly segmented germanium crystals. Its job is not to make a conventional picture. It records the energy, time and position of interactions produced as gamma rays move through the detector material.
GRETA completed its commissioning and characterisation run in July 2026. That work tested calibration, data handling and performance before the first science campaign began.
How can gamma rays reveal the shape of a nucleus?
At FRIB, beams of atomic nuclei can be accelerated into targets to create rare and unstable isotopes. The newly formed nuclei are often left in excited energy states. As they move toward lower-energy arrangements, they release gamma rays.
The energies and patterns of those gamma rays reflect the steps between quantum states. By measuring them precisely, physicists can infer how protons and neutrons are arranged and how the nucleus rotates or changes shape.
A gamma ray often interacts more than once inside germanium, scattering and depositing part of its energy at several locations. GRETA’s segmented crystals and software connect those interactions into a likely path. Tracking the path improves the estimate of the ray’s original energy and direction, especially when the emitting nucleus is moving quickly.
What is the first experiment testing?
The Surrey-led team is studying a radioactive isotope of germanium. Established calculations predicted a prolate, rugby-ball-like nucleus. More recent evidence points toward an oblate, flattened shape—more like a small disc.
That difference matters because nuclear shapes emerge from the collective behaviour of protons and neutrons. If an unusual isotope does not match the model, physicists may need to improve the rules used to predict neighbouring nuclei.
The experiment has been completed, but analysis is still under way. GRETA’s first use should therefore be understood as the start of a measurement campaign, not an announced answer to the shape question.
Why is GRETA more sensitive than earlier instruments?
Gamma rays can be difficult to catch, and rare isotopes may exist for only fractions of a second or be produced at low rates. More efficient detection means researchers can extract useful measurements from limited beam time and from nuclei that were previously too scarce to study.
For the Surrey experiment, the university says GRETA is around five times more efficient than the detector originally planned. The instrument combines a large amount of high-purity germanium with fine segmentation, real-time processing and software that reconstructs individual gamma-ray tracks.
During earlier development tests, the associated electronics and computing system were designed to process hundreds of thousands of interactions per second. Fast analysis also helps teams monitor an experiment while valuable beam time is still running.
What could GRETA help scientists discover?
- How many protons or neutrons a nucleus can hold before becoming unstable.
- How nuclear shapes and quantum states change in extreme isotopes.
- How elements heavier than iron are formed in explosive stellar events.
- Whether subtle asymmetries in nuclear behaviour can inform the matter-antimatter puzzle.
- How to improve nuclear models used across physics, medicine, energy and other research.
Does GRETA detect dangerous radiation outside the laboratory?
That is not its purpose. GRETA is a specialised research instrument installed at an accelerator facility. It measures radiation produced in controlled nuclear-physics experiments rather than monitoring public environments or serving as a medical scanner.
Why does the Surrey role matter?
Access to rare-isotope facilities is awarded through competitive scientific proposals. The Surrey team secured the first experiment after international review and is working with FRIB and Berkeley Lab collaborators. The work also connects with the university’s FAUST detector project, which is intended to operate alongside GRETA in future experiments.
Sources
- University of Surrey: first science experiment using GRETA, 5 August 2026.
- Berkeley Lab: GRETA comes online for a new era of nuclear science, 28 July 2026.
The first experiment’s scientific conclusions were not yet available when this article was prepared.