Multiple Angle Grazing Incidence K (vector)
a reflectometer/diffractometer for measuring subsurface structures
The problem: how can we measure in-plane structure in thin-film samples? In some ways, traditional small-angle neutron scattering (SANS) instruments are perfect for measuring in-plane structure in thin films - for a sample mounted perpendicular to the beam, all the scattering is in-plane scattering! But then we notice that the scattering volume (and the integrated cross-section) in this geometry is tiny. On the other hand, specular reflectivity is ideal for measurements of the structure (nuclear and magnetic) along a direction perpendicular to the plane of a thin film, in part because the instruments are designed with one relaxed collimation axis which points in-plane, which doesn't affect purely specular measurements at all (while increasing the intensity on the sample by orders of magnitude). The trick to getting in-plane information from a reflectometry-type geometry is simple - just stop requiring the scattering angle to be twice the incident angle. By measuring a number of scattered-beam angle simultaneously (with a position-sensitive detector), we can build up a picture of the scattering cross-section quite quickly - this is off-specular reflectivity.
Analysis: While taking the data isn't infinitely more difficult than in the case of specular reflectivity or SANS, analyzing the results is a unique problem. One must bridge the theory gap between the perfectly-solvable one-dimensional wave problem seen in specular reflectivity and the perfectly calculable Born approximation used in SANS modeling. Several approaches are being developed, from taking the Born approx. wherever it is appropriate, to doing a full Bloch-wave expansion of the in-plane potential and solving the 3-d scattering problem exactly, to doing a perturbation of the 1-d specular solutions with the 2-d Born approximation for the in-plane bits.
Last modified 23-August-2012 by website owner: NCNR (attn: John Copley)