Design & Technology
GRETA is based on the same detector design as the detectors in the GRETINA array. These are large (90mm long, 40mm maximum radius) high-purity germanium (HPGe) coaxial detectors, with a central contact for the full volume energy signal in the central bore, and 36-fold segmented outer electrical contact. Each crystal is individually encapsulated, and four crystals are housed in a single cryostat to make up each GRETINA quad module.
The central contact of each crystal is instrumented with a cold FET, while the 36 segment FETs are warm, housed outside of the cryostat in an accessible compartment for ease of repair. All signals are processed through preamps also housed directly on the quad module, before going to output through the Radiall connectors on the outside of the detector module.
Figure 1: Photo of a GRETINA quad module being prepared for CMM scanning at LBNL. The detector module houses 4 large-volume 36-fold segmented HPGe crystals, which have a geometry optimized for close-packing into a sphere.
Figure 2: Front face of a GRETINA quad module, and an engineering drawing showing the position of the crystals inside the detector housing.
Figure 3: Segmentation scheme for the GRETINA/GRETA HPGe crystals. Each detector is electrically segmented on the outer contact into 6 longitudinal slices and 6 wedges.