The effect of lattice distortion around point defects on the scattering of long wavelength neutrons

In a crystalline element the scattering of long wavelength neutrons by a point defect and an atom are known to be identical. However, when lattice distortion around the point defect is also considered, this is no longer true; in particular the scattering of a point defect is now anisotropic and varies with wavelength. An expression for the cross section of a defect in a relaxed lattice has been derived and applied to f.c.c. and b.c.c. lattices, and in greater detail to copper and molybdenum where the magnitude of the relaxation around defects is known. It has been found that the scattering cross section is appreciably altered by the relaxation of the surrounding atoms, and in particular from those that are nearest neighbours to the defect. A neglect of this relaxation in the interpretation of a neutron scattering experiment could lead to an error of an order of magnitude in an estimate of the number of defects. Also a study of the variation in cross section with wavelength or scattering angle, wh...