The GERDA Array and Detector Holders
The GERDA detector array has a hexagonal layout with a maximum of three tiers, i.e. 25 positions. A string with a maximum load of 5 detrectors could be placed at any one of them. The distance between two positions is in general 9 cm. Two outer positions, tier three, have a wider spacing to accomodate Phase I detector strings. The nominal Phase II array occupies the 7 inner position, each string with a load of three detectors.
Figure 1: Layout of a Phase II array with 7 strings of 5 detectors each: (.jpg)
The GERDA string can combine up to 5 detector holders into a unit called string which can be introduced into the lock system. Most of the strings will have, however, not more than three detectors.
An individual Phase II detector holder consists of 34 g of copper and 7 g of teflon supporting about 2 kg of germanium. Thin Kapton cables are used to read out the signals. Another Kapton cable is used to supply the detector with high voltage. Figure 1 shows a complete design with a detector loaded. Figure 2 reveals the delicate structure of the support which was optimized to minimize material and thus background.
Figure 1: Detector holder plus crystal and cable: (.jpg)
Figure 2: Detector holder and cables: (.jpg)