A COMPUTER-SIMULATION OF THE COSMIC-MUON BACKGROUND INDUCTION IN A GEGAMMA-SPECTROMETER USING GEANT

A computer simulation of the cosmic-muon background induction in a HPG e gamma-spectrometer using GEANT3.15 is described. Relative contributi ons of muon interaction mechanisms in parts of a detector shield were investigated. The direct muon ionisation in the Ge-diode, production o f delta-electrons, direct e(-)e(+)-pair production, muon decay and muo n bremsstrahlung contribute to the induced lower-energy background by 2.5%, 81.2%, 10.0%, 4.0% and 2.3%, respectively. For muons with moment a below approximate to 0.7 GeV/c the muon decay is the most contributi ng mechanism. The contribution of the delta-electron production reache s a maximum at 1.7 GeV/c and extends to the highest momenta. Above app roximate to 70 GeV/c the direct e(-)e(+)-pair production dominates. Ab out 97% of muon primary interaction vertices are situated in shield ma terials. An inner shield layer that is 4.7 cm thick contains 90% of al l the vertices. A 3 mm thick Cd-Cu shield lining accounts for 15% of a ll events. The simulated spectrum agrees well with an experimental one . Limitations of the standard GEANT for simulating the background indu ction by cosmic rays are discussed as well.