Simulated Fast Neutron Instrument Backgrounds at a MW Spallation Source

Abstract: Like every megaproject, the European Spallation Source (ESS) currently under construction in Lund, Sweden, has had to make a number of inevitable compromises to remain within budget and on schedule. Some of these compromises carry technical consequences with a potential impact on the operational characteristics of the facility and the neutron scattering instruments in particular. In order to prepare for operation of the facility, it is timely to consider again the subjects of instrument backgrounds and shielding from the project design phase. The expected ranges of instrument backgrounds, and the relative strengths of each contributor, are estimated using multiple physics simulations. These are compared with other leading spallation sources. Finally, several suggestions are given to reduce the background contributions in each case, should they prove to be dominant in practice when the commissioning phase of the facility begins.