GRETA: Gamma-Ray Energy Tracking Array

The study of atomic nuclei is central to our understanding of the world around us.  Comprising 99.9% of the visible matter in the universe nuclei are, in multiple aspects, central to fundamental questions in physics, such as our understanding of the origin of the elements and how complex many-body quantum systems organize.  Their properties depend sensitively on the number of protons (Z) and neutrons (N), and much of what we know about them comes from the measurement and characterization of their excitation modes, energy levels, and decays. As a consequence understanding nuclear properties, their role within the cosmos, and more broadly their application for society requires measurements on elements and isotopes far from β stability.


The Facility for Rare Isotope Beams (FRIB) [1] being constructed at Michigan State University and soon to come online will produce thousands of new short-lived radioactive (rare) isotopes and will greatly expand our reach to evermore exotic nuclei heavier in mass and closer to the limits of existence. GRETA  will be a key instrument at FRIB, capable of reconstructing the energy and three-dimensional position of γ-ray interactions. Its design provides the unprecedented combination of full solid-angle coverage and high efficiency, excellent energy and position resolution, and good background rejection (Peak-to-Total (P/T)) needed to carry out a large fraction of the nuclear structure and nuclear astrophysics science programs at FRIB. GRETA will be movable between the various beam-lines at FRIB and will also be used at other facilities such as the Argonne Tandem Linac Accelerator System (ATLAS) stable-beam facility at Argonne National Laboratory.


The technology and the scientific impact of a γ-ray tracking array has already been demonstrated. Between 2003 and 2011, the US low-energy nuclear physics community constructed GRETINA (Gamma-Ray Energy In-beam Nuclear Array) [2,3], a 1π tracking detector optimized for fast-beam nuclear physics experiments and employing the same segmented detector and signal-decomposition technology as GRETA. 

GRETA expands the existing GRETINA array to subtend the full 4π coverage of γ-ray tracking detectors.  The project scope covers the procurement of 18 Quad Detector Modules, as well as the design, assembly, integration, and testing of the Electronics, Computing, and Mechanical Systems to support all 30 Quad Modules. The 12 GRETINA Quad Detector Modules are integrated for a total of 30 Quad Detector Modules that cover 4π.

[1] Facility for Rare Isotope Beams at Michigan State University.
[2] S. Paschalis, I. Y. Lee, A. O. Macchiavelli et al., Nucl. Instrum. Methods in Phys. Res. A 709 (2013).
[3] P. Fallon, A. Gade and I.-Y. Lee, Annual Review of Nuclear and Particle Science 66 (2016)(1) 321.

The Gamma-Ray Energy Tracking Array, GRETA (shown above with the two hemispheres in the open position), is the realization of a 4π γ-ray tracking detector, capable of reconstructing the energy and three-dimensional position of γ-ray interactions within a compact sphere (shell) of High-Purity Germanium (HPGe) crystals. It consists of 120 highly segmented large-volume coaxial HPGe crystals, with four crystals combined to form a total of 30 Quad Detector Modules, and designed to cover a maximum solid angle in a close-packed spherical geometry. Each crystal is electrically segmented into 36 individual elements and a core contact, and read out over custom digital electronics. The detector signals are analyzed to reconstruct γ-ray energies and interaction points in a dedicated network-based Computing System.