RoSiB - a 4 PI silicon ball for charged-particle detection in EUROBALL

A 4 PI silicon ball for detection and identification of light charged particles in large multidetector -arrays as
EUROBALL is presented. The design is based on a N=42 ball with 12 pentagons and 30 hexagons as used in the
GASP array. The absorptive material for -rays is minimized to the detector thickness of 300 or 500µm and a
0.63 mm ceramic backing. The geometrical coverage is designed for about 90% of 4. A pulse shape
discrimination method with totally depleted detectors working in the reverse mount allows identifying protons
and -particles above an energy threshold of about 2MeV. The performances of the ball were tested at the
tandem - booster accelerator combination of the MPI Heidelberg in two experiments using the high-recoil
reaction of 228MeV 58Ni+46Ti and the low-recoil reaction of 95MeV 16O+58Ni. The two-dimensional spectra of
zero-crossing (ZC) versus energy confirmed an excellent discrimination of protons and -particles in all the
detectors at different angles. The energy spectra of protons and -particles measured in the experiments are
presented, too. The -spectra measured in coincidence with various combinations of emitted particles showed a
high selectivity of the ball. The reduced total efficiency for protons of 59% and 55% and -particles of 44% and
32% measured in a nuclear spectroscopy application is analyzed in a Monte-Carlo simulation (GEANT). It is
due to a combined influence of a thick target needed to stop the recoiling residual nuclei and thick absorbers
needed to protect the Si-detectors from scattered beam. The results along with the GEANT extrapolation to
optimum experimental conditions confirm that RoSiB is a highly efficient and selective device for identification
of rare reaction channels with heavy ions.