Pulsed Neutron Diffraction in Special Sample Environments

Abstract

Neutron diffraction is a powerful tool for structural studies of samples in special sample environments because of the high penetrating power of neutrons compared to X-rays. The neutrons readily penetrate special sample containers, heat shields, pressure vessels, etc., making it unnecessary in most cases to compromise the effectiveness of the sample environment system by providing windows for the incident and scattered neutrons. Pulsed neutrons obtained from an accelerator-based pulsed neutron source offer the additional advantage that many diffraction experiments can be done at a single, fixed scattering angle by the time-of-flight technique. Thus, if windows are needed (e.g., in an especially thick-walled sample vessel such as a pressure cell), they need to cover only a limited angular range. More importantly, in the fixed-angle scattering geometry, shielding and collimation can be optimized in order to access the largest possible sample volume with neutrons while completely avoiding scattering from the surrounding sample vessel. Thus, the data are free from unwanted background scattering which could hamper data interpretation. In this paper, the basic principles of neutron diffraction in special sample environments are discussed and examples of apparatus used for neutron diffraction measurements at low temperature, high temperature, and high pressure are presented. The concepts are illustrated with several unique scientific results taken from the published literature.