MINIBALL - A Ge detector array for radioactive ion beam facilities

Dedicated Ge-detector arrays are being developed for the investigation of rare gamma decays with low gamma -ray multiplicity at the upcoming radioactive ion beam facilities. These arrays are optimized for the high full-energy peak efficiency and angular resolution of the gamma -ray detection needed for a proper Doppler correction of the gamma -rays emitted by fast recoiling nuclei. MINIBALL will consist of 40 six-fold segmented, encapsulated Ge detectors which are clustered in eight cryostats with three detectors each and four cryostats with Four detectors, respectively. The individual components the six-fold segmented Ge detector, the cryostats, the fast preamplifier, the digital pulse-processing electronics and the mechanical frame - and their properties are described. The results of test measurements with the first MINIBALL cluster detector using a Cs-137 source and the in-beam reaction D(Cl-37, n) Ar-38 are presented. It is shown that from pulse-shape analysis of the events within a detector segment the effective granularity of the MINIBALL array can be enhanced from 240 to similar to 4000. The specifications of MINIBALL are compiled on the basis of experimental data. First results with a 12-fold segmented, encapsulated detector are discussed with respect to the feasibility of future gamma -ray tracking arrays.