Bragg Edge Determination for Accurate Lattice Parameter and Elastic Strain Measurement

The transmission spectrum of thermal neutrons through a polycrystalline sample displays sudden, well-defined increases in intensity as a function of neutron wavelength. These steps, known as Bragg edges, occur at the point at which the neutron wavelength exceeds the Bragg condition for coherent scattering from the respective lattice planes, and can be easily observed using the time-of-flight method. Accurate location of these edges and determination of their magnitude and shape can be used to extract information about the stress state, texture and phases present in the material. This paper describes a method for analysing these edges singly and collectively, using a Pawley-type refinement. Furthermore, experimental trials are presented which demonstrate the utility of the method for the accurate measurement of lattice spacings, and thus strain. These trials include measuring the lattice parameter in Cu/Zn alloys as a function of Zn content, and the determination of elastic strain of an iron rod under tensile/compressive straining. In the former case the results are compared with Bragg diffraction peak measurements made on HRPD and in the latter case with conventional strain gauge measurements.