Characterization and Modeling of Off-Specular Neutron Scattering for Analysis of Two Dimentionally Ordered Structures

Abstract

Off-specular neutron reflectometry is an instrumental technique which can be utilized for the characterization of thin-film systems in the depth and in-plane directions simultaneously. Currently, its use is limited both experimentally by the available neutron flux at modern neutron facilities and theoretically by a lack of widely available, user friendly, and open-source modeling software. This thesis describes work carried out on the development of a software package which utilizes currently available mathematical approximations to characterize model systems and evaluates the abilities and deficiencies of each algorithm. The evaluation will be carried out within the framework of a well-structured, object oriented, Python software package which is versatile and extendable. As new approximations and mathematical treatments are developed, they can be incorporated into the software infrastructure and tested with minimal effort.

We show that, at high q, the Born approximation can be used to qualitatively model off-specular scattering data; however, it does not capture any of the dynamic effects observed in real data. Some dynamical effects can be captured by perturbing the wavefunction by interactions with the substrate/incident media interface; however, low q scattering as well as scattering at the 'horizons' is still inaccurately represented. Currently, the best interpretation of the off-specular scattering can be accomplished with the complete distorted wave Born approximation. This is shown to produce theory functions which match quite well with scattering data.

Neutron coherence length is an important parameter in off-specular reflectometry as it dictates the number of feature periods being probed by the neutron beam. To determine the coherence length, a series of magnetic gratings were fabricated. Specular and off-specular measurements were used to evaluate the shape of the neutron wave packet work is still on going for a complete interpretation of these results.