Line integral corrections in spin-echo small angle neutron scattering instrument

Abstract

A good polarization of the neutron beam is required for the spin-echo small-angle neutron scattering (SESANS) instrument. SESANS is a novel kind of small-angle neutron scattering (SANS) technique. The measured quantity in a SESANS experiment is the depolarization of the neutron beam due to elastic scattering by a sample. The amount of precession of a neutron along a path is proportional to the line integral of the magnetic field intensity along this path. The inhomogeneities in the magnetic field intensity due to fringing of the magnetic field lines near the magnet edges give rise to line integral differences. This causes extra depolarization that decreases the sensitivity of the instrument. The requirement on the homogeneity of the line integral is calculated. In first order, the line integral increases quadratically towards the magnet poles. This quadratic increase is compensated by correction coils which have an opposite quadratic dependence.