Lesson plan
X unit
: For the software testing tools, see xUnit. The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10 −13 m). It is used to quote the wavelength of X-rays and gamma rays. Originally defined by the Swedish physicist Manne Siegbahn (1886–1978) in 1925, the x unit could not at that time be measured directly; the definition was instead made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus. One x unit was set at of the spacing of the (200) planes of calcite at 18 °C. In modern usage, there are two separate x units, which are defined in terms of the wavelengths of the two most commonly used X-ray lines in X-ray crystallography: * the copper x unit (symbol xu(Cu Kα 1 )) is defined so that the wavelength of the Kα 1 line of copper is exactly 1537.400 xu(Cu Kα 1 ); * the molybdenum x unit (symbol xu(Mo Kα 1 )) is defined so that the wavelength of the Kα 1 line of molybdenum is exactly 707.831 xu(Mo Kα 1 ). The 2006 CODATA recommended values for these units are: : 1 xu(Cu Kα 1 ) = , : 1 xu(Mo Kα 1 ) = .
Article summary
: For the software testing tools, see xUnit. The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10 −13 m). It is used to quote the wavelength of X-rays and gamma rays. Originally defined by the Swedish physicist Manne Siegbahn (1886–1978) in 1925, the x unit could not at that time be measured directly; the definition was instead made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus. One x unit was set at of the spacing of the (200) planes of calcite at 18 °C. In modern usage, there are two separate x units, which are defined in terms of the wavelengths of the two most commonly used X-ray lines in X-ray crystallography: * the copper x unit (symbol xu(Cu Kα 1 )) is defined so that the wavelength of the Kα 1 line of copper is exactly 1537.400 xu(Cu Kα 1 ); * the molybdenum x unit (symbol xu(Mo Kα 1 )) is defined so that the wavelength of the Kα 1 line of molybdenum is exactly 707.831 xu(Mo Kα 1 ). The 2006 CODATA recommended values for these units are: : 1 xu(Cu Kα 1 ) = , : 1 xu(Mo Kα 1 ) = .
Key claims
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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?
Bibliography
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Key claims and bibliography are generated from the same tracked claims and sources as the article page.