Abstract
We report on the study of fast-neutron background for the BIFROST spectrometer at ESS. We investigate the effect of background radiation induced by the interaction of fast neutrons from the source with the material of the neutron guide and devise a reasonable fast, thermal/cold neutron shielding solution for the current guide geometry using McStas and MCNPX. We investigate the effectiveness of the steel shielding around the guide by running simulations with three different steel thicknesses. The same approach is used to study the efficiencies of the steel wall a flat cylinder pierced by the guide in the middle and the polyethylene layer. The final model presented here has a 3 cm thick steel shielding around the guide, 30 cm of polyethylene around the shielding, two 5 mm thick B4C layers and a steel wall at position Z = 38 m, being 1 m thick and 10 m in radius. The final model finally proves that it is sufficient to bring the background level below the cosmic neutron rate, which defines an order of magnitude of the lowest obtainable background in the instruments.
Highlights
The worlds strongest neutron source for the study of materials and biosystems will be the European Spallation Source (ESS) [1], which is presently under construction in Lund, Sweden
Prompt neutrons escaping the target monolith have energies reaching up to the energy of the initial proton beam, and the task of instrument shielding is completely different compared to the case of reactor sources, based on which most shielding experience relies
At neutron energies exceeding 10 MeV, the scattering cross section of most commonly used shielding materials drops dramatically meaning that the task of instrument shielding at the ESS is even more complex than what the proton beam power dictates
Summary
The worlds strongest neutron source for the study of materials and biosystems will be the European Spallation Source (ESS) [1], which is presently under construction in Lund, Sweden. At neutron energies exceeding 10 MeV, the scattering cross section of most commonly used shielding materials drops dramatically meaning that the task of instrument shielding at the ESS is even more complex than what the proton beam power dictates. While McStas is well-recognised for its capability to precisely describe neutron scattering instruments in terms of signal distributions, it does lack in the description of backgrounds. To remedy this challenge an exact one-to-one implementation of the instrument is developed MCNPX [8, 9] - which is the standard Monte Carlo tool used for shielding calculations. The aim of such study is to outline the shielding design prior to the instrument construction, hereby allowing to iterate the instrument and shielding design to a common optimum
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
