Section 2.9.5. Experimental methodology

Neutron reflectivity measurements can be carried out in two principal ways: either (1) with a monochromatic incident beam of narrow angular divergence in the plane of reflection (defined by and where λ is constant, and Q is varied by changing the glancing angle of incidence, θ, relative to the sample surface; or (2) using a pulsed polychromatic incident beam, also of narrow angular divergence at fixed , and obtaining data over a range of Q values simultaneously by performing time-of-flight analysis on the reflected neutrons. For either method, the instrumental resolution is simply given as where Δ is the angular divergence of the reflected beam, and Δλ is the wavelength spread. In the case of a steady-state source, the wavelength resolution is determined by the monochromator, whereas the timing and moderator characteristics determine the wavelength resolution on a time-of-flight instrument. Although the second term in equation (2.9.5.1) is standard in scattering, it has a unique characteristic, in that the angular divergence of the reflected beam determines the resolution. This is the case because the sample is a δ-function scatterer, so that the angle of the incident beam can be determined precisely by knowing the reflected angle (Hamilton, Hayter & Smith, 1994). For a more complete description of both types of neutron reflectometry instrumentation, see Russell (1990).