The geometry of the system is similar to a standard double-crystal-monochromator, but uses two different crystals, InSb(111) and InSb(220), to achieve an overall beam deflection. Rotation of this pair of crystals around the incident beam then allows the angle of incidence at the sample position to be varied.
You can view an animation of the motions involved in making a reflectivity measurement using the DCD together with the diffractometer.
The DCD can also be used in combination with the P2M detector for GISAXS measurements in combination with reflectivity. However because of the associated horizontal deflection, the position of the P2M detector is limited:
Full details of the design have recently been published: Journal of Synchrotron Radiation, 2012, 19, 3, p408-416.
This article is available here
The geometry of the beamline optics has a considerable influence on the intensity of the incident beam at the sample position. In particular, the flux transmitted through the DCD depends on the rotation angle φ (shown below), relative to the double-crystal monochromator (DCM) which always scatters in the vertical plane.
This arises from two factors: The first is the dependence of the Bragg reflectivity on the polarisation of the incident beam, while the second is because the DCM-DCD system selects different ranges of X-ray wavelengths and directions as φ is varied. The reasons for this behaviour are discussed in detail in the recently accepted paper. In particular this publication discusses the variation of intensity and polarisation of the transmitted beam with incident angle. This behaviour results from the particular geometry of the optical components of the beamline. The variation is energy dependent and results in a greater loss of flux at higher energies. This flux has recently been improved by replacing the original Si crystals with InSb (see below)
(a) Flux transmission as a function of φ (the rotation angle of the DCD about its axis) for four different energies covering the full operational range. The experimental data (crosses) has been arbitrarily scaled to match the theoretical prediction. (b) The proportion of the transmitted flux that is vertically polarised as a function of φ and energy
(a) The calculated flux transmission through the DCD as a function of energy at Qz = 0. For silicon the transmission is only 5-10% whereas this has been significantly improved by replacing the DCD crystals with InSb. (b) Maximum flux before the DCD, at a ring current of 300mA. Collimation and windows etc. can reduce this by up to half, which for example, gives a flux at the sample position of ~2x1012 photons/s/0.1% b/w (at 12.5keV and Qz = 0).
Below shows a fit (red) of reflectivity data from pure water measured at 12.5keV (blue) and 20keV (green). In both cases the background has been subtracted. Fit parameters: Background = 10-10 Roughness = 2.9Å, calculated using Motofit(Nelson, 2006).
Next is an example of reflectivity (and corresponding fit) from a lipid layer (DPPC) on water containing 0.1mM Ca2+ at a surface pressure of 30mN m-1 (measured by Roser et al.) Also shown is an example Pilatus image used for GIXD of DPPC at 35mN m-1 using the pin-hole geometry (Meron et al., 2009). The variation of this integrated peak as a function of surface pressures.
Nelson, A. (2006). Co-refinement of multiple-contrast neutron/X-ray reflectivity data using MOTOFIT, Vol. 39, Journal of Applied Crystallography, pp. 273-276.
Meron, M., Gebhardt, J., Brewer, H., Viccaro, J. P. & Lin, B. (2009). European physical Journal Special Topics 167, 137-142.