In the nEDM@SNS experiment, the nEDM is measured by detecting the scintillation light emitted from the capture of polarized ultracold neutrons (UCN; E < 165 neV) on 3He. Polarized 8.9Å neutrons from the Fundamental Neutron Physics beamline will downscatter off phonons in superfluid 4He to produce the UCN. Unfortunately, neutrons can also scatter and be absorbed by other materials surrounding the measurement cells that contain the isotopically purified 4He. If the materials activate, b-delayed emission products can travel into the measurement cells and mimic the signal produced in the UCN-3He capture events. At the desired sensitivity reach of the experiment, even low levels of activating contaminants in materials can be troublesome.To quantify the effect of neutron activation background rates, SULI student Devon Loomis carried out MCNP simulations of the central detector, varying parameters such as neutron window thickness and exploring different shielding geometries. The output of these simulations was the neutron fluence impinging on different components. These data were combined with measurements of material composition, tabulated beta and gamma emission energies, and previous GEANT simulations of response versus b and g energy to determine the expected background rates for different experimental configurations. These results provide design guidance for minimizing the background rates (thin windows, material purity, local neutron shielding, minimize presence of chlorine-containing epoxy). A detailed write-up of these background rate studies, as well as the results of a GEANT simulation of the nEDM@SNS light collection system is being prepared.
