Correction of Neutron Scattering Lengths for Electromagnetic Interactions

Abstract

The scattering of thermal neutrons in non-magnetic materials is due almost entirely to the strong (i.e. nuclear) interaction between the neutrons and the nuclei, and the primary role of electromagnetic interactions is in magnetic materials, where the magnetic dipole interaction between the neutrons and the magnetic electrons gives a contribution to the scattering that is normally comparable in magnitude to that from the nuclear interaction. Nevertheless, even in a non-magnetic material, there exist other kinds of electromagnetic interactions between the neutrons and the atoms which give a small (typically 1%) contribution to the effective bound coherent scattering length. This contribution is, however, wave-vector dependent and is often one or two orders of magnitude larger than the current experimental uncertainty in the nuclear part of the scattering length. The purpose of this article is to call attention to the existence of these other electromagnetic interactions, to demonstrate that they are important in many kinds of scattering experiments and should be taken into account in the analysis of the data, and to describe two simple ways to correct for them, either of which should be adequate for most purposes.