MONTE CARLO CALCULATIONS FOR THE SHIELDING DESIGN OF THE
                            LINAC 4 BEAM DUMP AT CERN

                                  Jan Blaha
                                    CERN

Abstract:
The new linear accelerator Linac4, which is designed to provide a 160 MeV H-
beam with an average current of 40 mA and a mean power up to 2.8 kW, is
currently being constructed at CERN. The Linac4 is going to replace the present
50 MeV proton accelerator Linac2 as injector to the Proton Synchrotron Booster.
Thus the Linac4 will become an essential component of the whole CERN accelerator
complex, especially considering the future increase of the LHC luminosity. The
Linac4 is terminated by a dump collecting the beam which is not intended for
further utilization, i.e. during the accelerator commissioning phase, during the
measurement operation as well as in case of degraded situation. When beam ions
interacts with the material of the dump core (graphite), hadronic showers are
initialized and mixed radiation fields with large numbers of neutrons and highly
penetrating particles are produced. In addition the material of dump is highly
activated. Therefore an effective shielding surrounding the dump needs to be
established in order to limit activation of the surrounding structures and
protect personnel accessing the machine.

The aim of this study is to determine an optimal shielding design fulfilling the
radio-protection requirements for the Linac4 dump. The proposed shielding must
take into account different accelerator operational phases, the space
constraints inside the accelerator vault and the decommissioning of the
installation at the end of its lifetime. For this purpose a detailed Monte-Carlo
calculation using FLUKA particle transport and interaction code has been
performed and the relevant physics quantities, such as particle fluencies,
neutron energy spectra and prompt dose rates have been evaluated for different
irradiation profiles and shielding material combinations. Moreover, the residual
dose rate and induced activation have been calculated for several cooling times
in order to optimize the choice of shielding material following the ALARA
principle. Finally the airborne radioactivity induced by particles escaping the
shielding as well as the activation of the beam dump cooling water have been
calculated using FLUKA and simplified laminar flow models.