34.9: Questions

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Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

Which of the following is not an effect of the small wavelength of the electron?

Answer: B.

Why might you get a diffraction spot where you thought there would be an absence?

Answer: C.

Where do higher order Laue zones come from?

Answer: A.

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

If you increase the camera length, what happens to the diffraction pattern? What about the Kikuchi lines?

Answer

The diffraction spots move further apart, because the spot spacing is related to the camera length by

\[r=\frac{\lambda L}{d}\]

The Kikuchi lines also move further apart, and stay in the same places with respect to the diffraction spots, as they are a result of diffraction from the same crystal.

Why would it not be possible to index a zero order Laue zone in a diffraction pattern from a cubic crystal, knowing the two points 111 and 111 ?

Answer: These two points are collinear and cannot be added to produce any points not in the line that passes through both of them - so we could not index any of the pattern not on that line!

Open-ended questions

The following questions are not provided with answers, but intended to provide food for thought and points for further discussion with other students and teachers.

We have seen how electron diffraction relates to X-ray diffraction; how do you think neutron diffraction compares to electron diffraction? (Assume that we have a sufficient vacuum to enable the neutrons to reach the sample and be diffracted, and that we have some means of detecting the diffracted neutrons.)

a	The angle of diffraction is very small
b	The reciprocal lattice points are elongated
c	The Ewald sphere is very large
d	The diffraction pattern is produced from a plane of reciprocal lattice points

a	The atoms moved across a little
b	Inelastic scattering
c	Some electrons were diffracted two or more times
d	There is a fault in the lens

a	The Ewald sphere cutting parts of reciprocal lattice planes other than the one tangent to the sphere
b	A different scattering mechanism
c	Internal reflection in the microscope
d	Small crystals on the edge of the "single" crystal