GEANT Optimisation of the Beam Shielding for the ALICE Dimuon Spectrometer

The detectors of the dimuon spectrometer have to be shielded against the enormous particle flux produced in heavy collisions. In a previous study (Internal Note 95-12 DIM), simulations showed that a thick tungstene beam shielding (and a C+W absorber) reduces background at a level of some 0.01 hits/cm2 on the first chambers. But the price of this shielding and the still too high beackground led to new studies about the beam shielding. The subject of this note is to present the results of both geometrical and material optimisations of the beam shielding. Four geometries have been studied: the closed geometry (note 95-12) and the new "open, single-cone and double-cone" geometries. Simulations showed that these three "exotic" shapes reduce the background by a factor of 1.5 to 2 compared to the "standard" geometry of the beam shielding. These new geometries decrease the amount of materials and allow the replacement of a part of the expensive tungstene by lead. But simulations showed that an important source of background (especially for the trigger chambers) comes from materials in the beam underpass (vacuum pompe, quadrupole). Then a special rear shielding is required with the beam shielding. In conclusion, these new geometries of the beam shielding should be some good alternatives to the geometry shown in the LOI.