Monte Carlo calculations for the ATLAS cavern background

A new application for simulating the ATLAS cavern background was developed. This was done using FLUGG, software that allows Geant4 geometry to be used within the FLUKA simulation framework. A Geant4 description of the ATLAS detector including its cavern was built from scratch for this application. In order to gain computing performance, our geometry is less detailed than that of GeoModel which is used in the full detector simulation, but good enough for the investigation of cavern background. Our geometry can also be used in a standalone Geant4 simulation. Thus it is possible to perform unbiased comparisons between Geant4 and FLUKA using the same complex geometry. We compared neutron and photon fluxes using the FLUKA-FLUGG application with the result of Geant4 simulations based on the QGSP_BERT and QGSP_BERT_HP physics lists. In all cases the same set of initial collision 4-vectors produced by the PHOJET event generator was used. The result from the QGSP_BERT_HP physics list, which uses the High Precision (HP) neutron model, is similar to the result of FLUKA-FLUGG and the differences in the fluxes between them are within 40% in most regions of the ATLAS cavern. The result from the QGSP_BERT physics list, which does not include the HP model, does not agree with either of the previous two results.