This document is an update to the proposal PR12-16-001 Dark matter search ina Beam-Dump eXperiment (BDX) at Jefferson Lab submitted to JLab-PAC44 in 2016reporting progress in addressing questions raised regarding the beam-onbackgrounds. The concerns are addressed by adopting a new simulation tool,FLUKA, and planning measurements of muon fluxes from the dump with its existingshielding around the dump. First, we have implemented the detailed BDXexperimental geometry into a FLUKA simulation, in consultation with expertsfrom the JLab Radiation Control Group. The FLUKA simulation has been compareddirectly to our GEANT4 simulations and shown to agree in regions of validity.The FLUKA interaction package, with a tuned set of biasing weights, isnaturally able to generate reliable particle distributions with very smallprobabilities and therefore predict rates at the detector location beyond theplanned shielding around the beam dump. Second, we have developed a plan toconduct measurements of the muon ux from the Hall-A dump in its currentconfiguration to validate our simulations.
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