BGO shields for the MINIBALL spectrometer

An enhanced detection sensitivity of the high-resolution MINIBALL spectrometer is required for future experiments at the HIE-ISOLDE accelerator at CERN. These measurements will be based on direct reactions and fusion-evaporation reactions, populating excited nuclei at high excitation energies. Moreover, high angular-momentum transfer will cause higher γ-ray multiplicities. The existing MINIBALL spectrometer with its closely packed eight triple cluster detectors was designed for highest solid-angle coverage, causing best γ-ray efficiency for low-multiplicity events. Therefore the triple-cluster detectors will be surrounded with additional BGO Compton suppression detectors in order to cope with the scattering between detectors from high energetic γ-rays and double hits from higher γ-ray multiplicities. After extended Monte-Carlo studies with a newly developed MINIBALL simulation code based on GEANT4 a final Compton suppression detector geometry for the MINIBALL spectrometer was determined. This additional BGO detectors in combination with the MINIBALL array will improve the peak-to-total ratio up to 50% and cause just a slight reduction from 8.0% to 7.2% in efficiency. In collaboration with the IPN Orsay the first BGO prototype is under construction. The status of the project and first measurements with the newly developed BGO detector are presented.