GRETINA is a new, advanced of gamma-ray detector system optimized to study the structure and properties of subatomic nuclei. Built from large crystals of hyper-pure germanium, it is the first system to use gamma-ray energy tracking whereby the path of gamma-rays emitted from nuclear reactions are tracked through the crystals in order to get unprecedented energy resolution. Such tracking has the potential for high-sensitivity gamma-ray source imaging, which has many applications in medical imaging (PET scanners) and homeland security (cargo inspections).
GRETINA, the first stage of the (future) Gamma-Ray Energy Tracking Array (GRETA), consists of 28 coaxial germanium crystals, each of which is segmented into 36 electrically isolated elements. Signal decomposition algorithms translate electrical signals on these elements from passing gamma rays into a set of positions and energies. The tracking enables the Compton scattering energy loss of the gamma rays to be precisely determined as they traverse the crystals. The GRETINA detector elements fit together in a close-packed spherical geometry that covers one quarter of a sphere.
ORNL led the effort on the GRETINA signal decomposition algorithm and Liquid Nitrogen system, and strongly contributed to the analysis programs. The signal decomposition work, for example, enabled the system to determine final position resolutions of gamma rays to between 1 and 2 mm (standard deviation) with all tested detectors, while simultaneously meeting the CPU timing requirements. See the image at right of projections of the interaction sites inside a Ge crystal of a pencil-like beam of gamma rays from a collimated 137Cs source. During tests in 2011, an array of twenty crystals yielded a peak-to-total of 62%, with a relative efficiency of 75%, following signal decomposition and tracking. This performance exceeded the GRETINA requirements. Construction, programming, and installation of the GRETINA liquid nitrogen filling system has also been completed. The new system is highly reliable and incorporates numerous fail-safe systems, reporting modes, and user-friendly features. Detailed logs and diagnostic information are generated and made available online, and also sent to an email distribution list.
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For More Information
The following links will let you learn more about this topic: GRETINA home page [LBNL]
David Radford, radforddc at ornl.gov