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

A 4 pi silicon bail for detection and identification of light charged particles in large multidetector gamma-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 gamma-rays is minimized to the detector thickness of 300 or 500 mu m and a 0.63 mm ceramic backing. The geometrical coverage is designed for about 90% of 4 pi. A pulse shape discrimination method with totally depleted detectors working in the reverse mount allows identifying protons and alpha-particles above an energy threshold of about 2 MeV. The performances of the ball were rested at the tandem - booster accelerator combination of the MPI Heidelberg in two experiments using the high-recoil reaction of 228 MeV Ni-58 + Ti-46 and the low-recoil reaction of 95 MeV O-16 + Ni-58. The two-dimensional spectra of zero-crossing (ZC) versus energy confirmed an excellent discrimination of protons and alpha-particles in all the detectors at different angles. The energy spectra of protons and alpha-particles measured in the experiments are presented. too. The gamma-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 alpha-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. (C) 2000 Elsevier Science B.V. All rights reserved.