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The powder method

Now try a different experimental arrangement. Rather than using white radiation, take monochromatic X-radiation of one fixed wavelength and place the crystal in front of the beam. If one plane is set at exactly the correct value of $\theta$ to reflect, then we observe one and only one reflected beam from that crystal. Imagine now, still holding the crystal fixed at the angle $\theta$, we rotate the crystal around the direction of the incident X-ray beam so that the plane causing a reflection is still set at the angle $\theta$ relative to the X-ray beam. The reflected beam will describe a cone with the crystal at the apex of the cone. Now imagine the situation when we have not one crystal but we have a hundred crystals each of them set so that one plane is at exactly the right reflecting angle, to the incoming beam. We will now have a hundred reflected beams each giving us one observable point. Imagine now this clump of a hundred crystals is rotated about the axis of the incident X-ray beam. We will now have a hundred cones traced out by these reflected beams. Now consider what would happen if we had a powder of our material which may consist of a hundred million crystals. If the powdered sample is put into the beam of X-rays, there will be many crystals in that powder which will be in a position to reflect the incident beam and there will be enough of them to get the effect of not point reflections but of a continuous series of point reflections which will be lying along the arc of the cone that we previously imagined to exist. This is the basis of the so-called powder or Debye-Scherrer method which is probably the most common technique used in X-ray crystallography.


 
Figure 21
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\includegraphics {fig21.ps}
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The powder camera (Fig. 21) consists of a metal cylinder at the centre of which is the sample. The powdered material is often glued onto a glass rod with clear fingernail varnish. A strip of X-ray film is placed inside the cylinder. Punched into one side of the film is a hole for the beam collimator and punched into the other side 180$^\circ$ away, is another hole through which a beam catcher can be placed. The camera is closed by a light-tight lid and placed in front of the X-ray beam. The pattern on the film is shown on the right of Fig. 22.


 
Figure 22
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\includegraphics {fig22.ps}
\end{figure}

This technique can be adapted to photographing wires and sheets of metal. A flat film is also commonly used for recording reflections at small $\theta$ angles.


next up previous
Next: Moving crystal and moving film methods Up: Laue, Powder and Single Crystal Methods Previous: The Laue method

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