Study of the Background Magnetic Field Distribution for Neutron Transition Magnetic Moment Searches

Sammanfattning

At Oak Ridge National Laboratory (ORNL), where the High Flux Isotope Reactor (HFIR) GP-SANS beamline is located, an experiment searching for the neutron Transition Magnetic Moment (nTMM) was conducted. This experiment is dependent on the magnetic field homogeneity and requires a magnetic field scanning precision of at least 4 μT. A previous experiment performed at the same facility used powerful magnets, which magnetised parts of the steel support structure of the beamline. As a result, the background magnetic field in the experimental region was modified. The aim of this thesis was to determine the magnitude and homogeneity of the magnetic field inside the beamline, and to assess whether the variation of the field within the region of interest is larger or smaller than the required scanning precision. The magnetic field components were measured in the vicinity of the beamline. These data were used to reconstruct the magnetic scalar potential by fitting the measured field with spherical-harmonic gradients. The resulting expression in spherical harmonics was implemented as a boundary condition in the COMSOL simulation. The reconstructed field indicates that the variation in field magnitude across the region of interest is of the same order as the required scanning precision. This indicates that the residual background field may be relevant for the magnetic-field scan and should be accounted for when interpreting the experimental results.