An instrument with a single field box with a 1 mT magnetic field along the y-axis.
A neutron beam polarized aling the x-axis is emitted with central wavelength such
that the polarization is rotated pihalfturns radians in the field.
Parameters in boldface are required;
the others are optional.
beam polarization (for the cetral wavelength) is rotated pihalfturns by the field