Precision Magnetic Field Mapping for the 3He Neutron Spin Filter

3He neutron spin filters require a homogeneous magnetic field, otherwise the nuclear polarization of the 3He gas will easily be lost by small magnetic field gradients. Magnetic cavities with field gradients of 10−4/cm or better are often in demand, and many such cavities - Helmholtz coils, solenoids, permanent magnet circuits, etc - have been developed [1]. These magnetic cavities are designed mostly by using simulations, but the actual magnetic fields may differ from the calculations because of the presence of the environmental magnetic field as well as nearby magnetic materials. To evaluate the real magnetic fields, a precision magnetometry system with a precision of 10-5 was developed.