Incoherent Neutron Scattering

Abstract

Hydrogen scatters neutrons strongly and incoherently, which makes neutrons very sensitive to the presence of hydrogen in materials (Squires, Introduction to the Theory of Thermal Neutron Scattering, 3rd edn. Cambridge University Press, New York, 2012). This strong incoherent scattering removes neutrons from a transmitted beam, so that hydrogen-rich regions appear as dark spots in neutron radiographs. The scattered neutrons are spread into all directions and appear as a contribution to the background in neutron diffraction patterns, often viewed as a nuisance. In this chapter, we first provide a brief introduction to coherent and incoherent neutron scattering. We then present two examples in which incoherent neutron scattering reveals knowledge of practical value. In the first example, we describe an experiment to determine the smallest change in hydrogen concentration that can be detected in nuclear fuels (~10 wt. ppm) by incoherent neutron scattering. Hydrogenous materials such as water and organic lubricants are often used in the processing of fuels for nuclear reactors, and residual lubricants trapped in the fabricated fuels can lead to complications in fuel performance. Neutron scattering is very sensitive to the presence of hydrogen in bulk specimens, and hence of water or hydrogen-rich organic lubricants. Furthermore, since neutron scattering is non-destructive and simple to apply, it is an ideal technique for quantifying hydrogen-bearing contaminants in fuels for the purpose of quality assurance. A brief description of the uncertainties associated with a typical neutron counting experiment is provided. Though the experiment deals with nuclear fuels, the method is straightforward and easily applicable to other materials. In the second example, we describe an in situ experiment in which both coherent and incoherent neutron scattering are used to study the bulk diffusion of hydrogen into a zirconium alloy in order to accurately determine the solubility limit of hydrogen. In CANDU® nuclear power reactors, pressurized heavy water coolant flows over fuel bundles in pressure tubes made of Zr–2.5Nb alloy. Over time, deuterium accumulates in the zirconium alloy, resulting in the precipitation of hydrides which can lead to various types of failure, such as Delayed Hydride Cracking (Puls, The Effect of Hydrogen and Hydrides on the Integrity of Zirconium Alloy Components: Delayed Hydride Cracking. Springer, New York, 2012). Accurate quantitative knowledge of the deuterium (hydrogen) concentration at which hydrides start to form is thus critical to inform regulations limiting the acceptable residence time of pressure tubes in power reactors. Again, though the experiment deals with a specific alloy, the technique is straightforward to apply and is applicable to a wide range of materials.