|
|
| Project
Home History
Concepts & Theory
Collecting Data
Lesson Materials |
Introduction
In order to begin to grasp the fundamental concepts of x-ray
crystallography, one must be familiar with the major accomplishments of
two scientists. They are Max von Laue and William Lawrence
Bragg. Many of the techniques and ideas that came out of their
experimentation are still relevant and applicable to modern
experimental techniques. |
 |
Max von Laue(1879-1960)
|
Background When von Laue started his work, the true nature of x-rays was still unknown. Many experiments had been completed related to their energy and polarization. Based on their energy, experimental evidence had suggested that if x-rays had a wavelength, it would have to be on the order of 0.1-1 Angstrom. Diffraction patterns, similar to those made by visible light had never been detected in x-rays. This is because the width of the slit, or diffraction grating, required to make x-rays interfere with one another was much too small to be created by humans. Hypothesis von Laue hypothesized that if x-rays were similar to other forms of wave-like radiation, they could be made to produce diffraction patterns when passed through a crystal. He predicted that the crystal could serve as the diffraction grating, because he knew that the distance between molecules in a crystal was thought to be around the same order of magnitude as the proposed wavelength of an x-ray. He focused his effort on developing the theoretical basis for how this type of interference would occur. In order to confirm his hypothesis in lab, von Laue would first need to be able to develop a mathematical model that could predict the relative location and intensity of the spots that would be produced as part of the diffraction pattern. Experimentation von Laue was in frequent contact with Arnold Sommerfeld, a medical doctor turned x-ray theorist. Sommerfeld was very skeptical of using a crystal as a diffraction grating. He theorized that the intrinsic motion within any crystal would prevent von Laue from observing any regular diffraction pattern. He reluctantly offered his own assistant, Walter Friedrich, to aid von Laue in his inquiry. Friedrich was a tremendous asset, as he had developed and used an apparatus to study x-rays while working under Sommerfeld. Paul Knipping, another researcher in the field, also agreed to contribute. As von Laue was fine tuning the mathematics that he would need to corroborate his hypothesis, his assistants started the experimentation. The apparatus consisted of an x-ray tube that focused a beam of x-rays through a small hole in a lead box that contained a crystal of copper sulfate. Below is a image from The Cathode Ray Tube Site of an x-ray tube that is similar to the one that von Laue's research team used in their experiment. I've labeled some of the most important parts.  The cathode is used to heat a
filament until it is "white hot" and starts to release electrons.
These electrons travel toward an anode made of tungsten. The
extremely high temperatures produced require that the anode be made of
a material with a high melting point. When conditions within the
glass tube are right, the beam of electrons causes the anode to release
x-rays in the path indicated with the blue line. A regulator is
used to adjust the pressure of the gas within the tube.
Photographic plates were used to monitor the x-rays that exited through another small hole in the back side of the box. Originally, the photographic plate was placed between the source of the beam and the crystal. The prediction was that a diffraction pattern would result from x-rays that were reflecting off of the crystal. When this failed to produce a pattern, Knipping placed plates on all sides of the crystal. A second attempt yielded a diffraction pattern on the plate behind the crystal. Here is an image that depicts the final experimental setup from the International Union of Crystallography (IUCr):  These images, also from IUCr, show the first successful diffraction photograph, as well as other images that were recorded in subsequent attempts:  These images confirmed von Laue's
hypothesis that x-rays were wave-like in nature and that they possessed
extremely small wavelengths that could be diffracted by a
crystal. It also meant that scientists could now study
these types of diffraction patterns to gain information about the
arrangement of atoms in a crystal. However, von Laue still had to
improve the mathematical models that he used to connect the spots made
on the photographic plate with the actual positions of the atoms within
the crystal.
|
|
 |
William Lawrence Bragg(1890-1971)
|
Background Even at a young age, Bragg was no stranger to the field of x-ray research. His father, William Henry Bragg, was a professor of mathematics and physics that eventually worked under JJ Thomson and went on to invent the x-ray spectrometer. It is widely reported that the first documented surgical use of the x-ray in Australia was when W.H. Bragg used it to examine his son's arm after he injured it while riding a tricycle. The younger Bragg excelled in academics at a young age and finished his undergraduate studies by the age of 18. He eventually shifted his focus from mathematics to physics and began researching the diffraction of x-rays by crystals at the University of Cambridge. Hypothesis The scientific community was still buzzing over the results of Max von Laue's experiment. However, von Laue's mathematical model was somewhat inconsistent and inaccurate. He attributed the spots on the photographic plate to the areas where multiple x-rays had diffracted and then combined constructively. However, nothing appeared in some of the areas where von Laue expected to see spots. He assumed that this meant that x-rays only possessed certain wavelengths and not others. Bragg was not satisfied with that explanation. He envisioned a continuous spectrum in the x-ray region and hypothesized that the crystal must have had a different structure than the one envisioned by von Laue and others. Bragg hypothesized that crystals must contain an atomic lattice. This view set him apart from other scientists of his day who thought that crystals were made up of a lattice of molecules. Bragg theorized that the path of x-rays shot through a crystal depended on planes and lattice points within the crystal. Lattice points occur when planes of atoms in the crystal intersect.
In order to test his hypotheses, Bragg
would need a
different apparatus from the one used by von Laue’s research team. He needed a source that produced x-rays of a
definite wavelength. In addition, he
needed to be able to easily change the position of the source to allow
the
x-rays to enter the crystal at various, controlled angles.
The optimal instrument for conducting his
experiments was the x-ray spectrometer that was invented by his father. For his role in the experimentation, the elder
Bragg shared the Nobel Prize with his son. A
picture of the instrument, from the Science Museum in
London, is
pictured below.
|
|
ReferencesCarroll, P. J., & Carroll, M. H. (n.d.). Structure Determination by X-ray Crystallography . In University of Pennsylvania X-Ray Crystallography Facility [Online Course]. Retrieved January 7, 2009, from http://macxray.chem.upenn.edu/course/index.html Dijkstra, H. (2009). Early villard x-ray tube [Data file]. Retrieved April 1, 2009, from http://members.chello.nl/h.dijkstra19/big/x-ray/villard-big.jpg Granqvist, G. (2009). The Nobel Prize in physics 1915 [Presentation speech]. Retrieved April 3, 2009, from http://nobelprize.org/nobel_prizes/physics/laureates/1915/present.html Granqvist, G. (2009). The Nobel Prize in Physics 1914 [Presentation speech]. Retrieved March 30, 2009, from Nobel Prize Foundation Web site: http://nobelprize.org/nobel_prizes/physics/laureates/1914/present.html Kapecki,
J. A.
(1972). An introduction to x-ray structure determination. Journal
of
Chemical Education, 49(4), 231-236. Science
Museum, London. (n.d.). X-ray spectrometer [Data file].
Retrieved April
9, 2009, from
http://www.sciencemuseum.org.uk/hommedia.ashx?id=11200&size=Small University of Cambridge. (2002). X-ray diffraction. In Cavendish Laboratory educational outreach. Retrieved March 30, 2009, from http://www-outreach.phy.cam.ac.uk/camphy/xraydiffraction/xraydiffraction_index.htm Wikipedia. (2006). Max von Laue [Data file]. Retrieved April 1, 2009, from http://en.wikipedia.org/wiki/File:Max_von_Laue.jpg#file Wikipedia. (2008). William Lawrence Bragg on TIME magazine- October 3, 1938 [Data file]. Retrieved April 1, 2009, from http://en.wikipedia.org/wiki/File:William_Lawrence_Bragg_on_TIME_Magazine,_October_3,_1938.jpg |
