Optimization of neutron scattering instrumentation using neutron spin echo: Application to the discrimination of diffuse scattering in neutron reflectivity experiments

摘要

We describe a method, based on the neutron spin-echo technique, to achieve good resolution in many neutron-scattering experiments, without sacrificing signal intensity. By "good resolution" we do not mean the ～10~(-6) energy resolution usually associated with neutron spin-echo spectrometers, but rather the level of resolution traditionally obtained by collimation or monochromatization of neutron beams, often on specialized instruments, and almost always at some penalty in measured intensity. The method we discuss allows good resolution to be achieved in any chosen direction in the vector space defined by the neutron scattering wave vector, Q, and the energy transfer, E. Although the method has general applicability to many neutron scattering measurements, we discuss in detail its application to the problem of separating diffuse scattering from specular reflection in neutron reflectometry. The technological basis of the method is the availability of thin films of magnetic or easily magnetizable material that are very smooth and of uniform thickness and that do not depolarize neutrons. Enhancing resolution using this method can be as simple as adding a few such films to a spectrometer that already provides polarized neutrons. We have tested magnetizable permalloy films for this application and our experimental results show that components work as intended, although some further work is needed to optimize them. The apparatus we have used, which is inexpensive and simple to construct, will allow both in-plane and out-of-plane diffuse scattering to be effectively removed from the measurement of specular reflectivity with an accuracy comparable to that achievable with milliradian collimation. In contrast to traditional methods of achieving this level of resolution, however, the spin echo method permits the use of relatively poorly collimated beams and hence can provide substantial gains in signal intensity.