Phase II Detectors
The Germanium detectors used for Phase II of the GERDA experiment will be segmented. The segmentation scheme was optimized to distinguish between signal and photon induced background events. Signal events have two electrons in the final state and the range of an electron of the relevant energy in germanium is about 1 mm. Photons most likely loose their energy in multiple Compton scatterings and the mean free path between interactions is of the order of centimeters. The cylindrical crystals ar 7 cm high and have a diameter of 7.5 cm. They are segmented three-fold in height and 6-fold cylindricly. A first prototype detector was built and intensiveley tested in a special test setup:
The Siegfried test setup with vacuum cryostat and copper electronics housing: (.jpg)
The results of the tests as well as extensive Monte Carlo studies confirm the power of segmentation. Some of the most dangerous backgrounds expected in the GERDA setup can be reduced by up to two orders of magnitude.
 I.Abt et al, Characterization of the first true coaxial 18-fold segmented n-type prototype detector for the GERDA project
Nucl.Instrum.Meth. A 577 (2007) 574 [nucl-ex/0701004]
 I. Abt et al,Identification of photons in double beta-decay experiments using segmented germanium detectors - studies with a GERDA Phase II prototype detector
 I. Abt et al,Pulse shapes from electron and photon induced events in segmented high-purity germanium detectors
 I. Abt et al,Test of pulse shape analysis using single Compton scattering events
A first trial exposing the detector to a neutron source also yielded interesting information about neutron ionteractions in germanium.
 I. Abt et al, coming soon
In the actual experiment the detectors will be assembled in strings hanging from the lock system. Each detector is suspended in a copper holder. Only 34 g copper are used to hold about 2 kg of germanium.
An 18-fold segmented detector in its copper holder:(.jpg)
Also seen is a prototype Kapton read-out cable.
37.5 kg of Germanium enriched in the isotope 76Ge were procured in the form of germanium-oxide. The abundance of 76Ge is above 80%. The material is currently in underground storage.
The reduction and purification of materials as well as crystal growing procedures are currently under investigation.