Miniball: a $\gamma$-ray spectrometer with position-sensitive Ge detectors for nuclear structure studies at REX-ISOLDE

Miniball is a dedicated Ge detector array which has been developed for the investigation of rare $\gamma$ decays at the new radioactive beam facility REX-ISOLDE at CERN. The array is optimised for high full-energy peak efficiency and for high granularity needed to perform Doppler corrections of $\gamma$-rays emitted by fast moving nuclei. Miniball will finally consist of 40 six-fold segmented, encapsulated detectors which are clustered in eight cryostats with three detectors each and four cryostats with four detectors, respectively. It is shown that from an analysis of the pulse shapes and of the amplitudes of the mirror charges in the adjacent segments the effective granularity of Miniball can be enhanced from 240 to approximately= 4000. The properties of Miniball are compiled on the basis of experimental data. Examples of the first data measured with Miniball are presented. (11 refs).