Improving sensitivity of the nEDM@SNS experiment with a redesigned neutron guide*

One of the foremost problems in modern physics is the baryonic asymmetry of the universe, as the standard model does not predict sufficient violation of CP-symmetry to account for it. Beyond standard model theories that propose additional CP-violating sources almost invariably predict a neutron electric dipole moment (nEDM) several orders of magnitude larger than predicted by the standard model. The nEDM@SNS experiment will measure the nEDM by detecting the scintillation created by capture of ultracold neutrons (UCN) on polarized ³He. The UCN are generated by downscattering 8.9 Å neutrons from the Fundamental Neutron Physics Beamline at Oak Ridge National Laboratory’s Spallation Neutron Source inside a measurement cell filled with superfluid ⁴He. The SNS provides the highest flux of pulsed cold neutrons making it an ideal location for the experiment, but care must be taken in the design of the neutron guide to maximize the number of neutrons reaching the measurement cells. By focusing our efforts on curating an optimal beam profile, instead of transmission, through each component of the guide we can substantially increase the number of cold neutrons delivered to the ⁴He. Using McStas and independent ray tracing software we present a redesigned neutron guide capable of delivering 23% more neutrons to the measurement cells than the current design. Additionally, through OpenMC, we have designed radiation shielding capable of reducing all regions near the beam guide to below the 0.25 mRem/hr safety requirement.