The Sin2 Ψ-method in Pulsed Neutron Transmission

The transmission spectrum of thermal neutrons through polycrystalline samples as a function of wavelength displays sudden increases in intensity known as Bragg edges. These edges occur whenever the wavelength ceases to fulfil the Bragg condition for coherent scattering at Bragg angle 2Θ = 180°. The shape and position of these edges contain information about the stress state present in the sample. The transmission method has two key advantages over conventional neutron diffraction. For samples with essentially one or two-dimensional stress-fields the use of pixellated detector arrays enables simultaneous, therefore very fast, spatial imaging of strains. At the same time it allows for the determination of the unstrained lattice spacing as well as stresses in the sample by evaluating a set of strain images taken at different sample inclinations, similar to the sin2 Ψ-technique of X-ray diffraction. We show how this technique has been successfully employed to determine accurately the unstrained lattice parameter and stresses in bi-axially stressed samples.