Precision Magnetic Field Direction Measurements for Neutron Spin Structure Studies*

The experiments E12-06-110 and E12-06-121 at Jefferson Lab require the helium-3 target polarization direction to be measured with a precision of ± 0.1° in a fixed Hall C coordinate system to correct for the contribution of longitudinal electron asymmetry to the total measured asymmetry. A novel air-floated compass was conceptualized and constructed at the University of Kentucky that achieved the required level of precision in the horizontal magnetic field direction measurements. The magnetic field direction was determined using an optical method by reflecting a laser beam off mirrors mounted on a compass, aligned perpendicular to the magnetic axis of the compass magnet as precisely as possible. The compass calibration techniques and the results from the magnetic field direction measurements in Hall C will be presented.