Lesson plan
Diffraction topography
Diffraction topography (short: "topography") is an imaging technique based on Bragg diffraction. Diffraction topographic images ("topographies") record the intensity profile of a beam of X-rays (or, sometimes, neutrons) diffracted by a crystal. A topography thus represents a two-dimensional spatial intensity mapping (image) of the X-rays diffracted in a specific direction, so regions which diffract substantially will appear brighter than those which do not. This is equivalent to the spatial fine structure of an Laue reflection. Topographs often reveal the irregularities in a non-ideal crystal lattice. X-ray diffraction topography is one variant of X-ray imaging, making use of diffraction contrast rather than absorption contrast which is usually used in radiography and computed tomography (CT). Topography is exploited to a lesser extent with neutrons, and is the same concept as dark field imaging in an electron microscope. Topography is used for monitoring crystal quality and visualizing defects in many different crystalline materials. It has proved helpful e.g. when developing new crystal growth methods, for monitoring growth and the crystal quality achieved, and for iteratively optimizing growth conditions. In many cases, topography can be applied without preparing or otherwise damaging the sample; it is therefore one variant of non-destructive testing.
Article summary
Diffraction topography (short: "topography") is an imaging technique based on Bragg diffraction. Diffraction topographic images ("topographies") record the intensity profile of a beam of X-rays (or, sometimes, neutrons) diffracted by a crystal. A topography thus represents a two-dimensional spatial intensity mapping (image) of the X-rays diffracted in a specific direction, so regions which diffract substantially will appear brighter than those which do not. This is equivalent to the spatial fine structure of an Laue reflection. Topographs often reveal the irregularities in a non-ideal crystal lattice. X-ray diffraction topography is one variant of X-ray imaging, making use of diffraction contrast rather than absorption contrast which is usually used in radiography and computed tomography (CT). Topography is exploited to a lesser extent with neutrons, and is the same concept as dark field imaging in an electron microscope. Topography is used for monitoring crystal quality and visualizing defects in many different crystalline materials. It has proved helpful e.g. when developing new crystal growth methods, for monitoring growth and the crystal quality achieved, and for iteratively optimizing growth conditions. In many cases, topography can be applied without preparing or otherwise damaging the sample; it is therefore one variant of non-destructive testing.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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