Neutron Veto Inefficiency Studies with FLUKA for the LDMX Hadronic Calorimeter

Sammanfattning: Dark matter accounts for about 85% of all matter in the Universe, yet its particle nature remains unknown. The Light Dark Matter eXperiment (LDMX) is a proposed fixed-target missing-momentum experiment that intends to probe the predominantly experimentally uncharted MeV-GeV mass range for "light dark matter". A significant challenge for LDMX is to efficiently veto rare photo-nuclear background events resulting in high energy (∼3 GeV) neutral hadrons. The preliminary design of the sampling hadronic calorimeter (HCal), which is responsible for detecting neutral hadrons and consists of alternating layers of steel absorber plates and polystyrene scintillator bars, has been developed using GEANT4 simulations to realise the required veto inefficiency. To validate the GEANT4 results, it was thus of interest and the aim of this project to perform a comparative study of neutron veto inefficiency between GEANT4 and other Monte Carlo codes, in this case FLUKA. By modelling a simplified LDMX HCal geometry using the FLUKA Advanced InteRface (FLAIR) the neutron veto inefficiency as a function of HCal depth was found. This was done for 0.1 GeV, 0.5 GeV, 1.0 GeV and 3.0 GeV incident neutrons with a threshold of 1 MeV and 10 MeV. Results indicate that FLUKA and GEANT4 are comparatively similar and independent of the thresholds. This indicates that the current design of the HCal with a depth of ∼3 m, is sufficient so as to ensure the veto performance for the relevant photo-nuclear backgrounds.