X-ray diffraction is coherent elastic scattering of x-rays by atoms
or ions in a crystal. Because the wavelength of photons with energy of
order 10 KeV is a little smaller than the spacing of atoms in solids, a
crystal will act as a sort of diffraction grating for x-ray. As a crystal
is three dimensional, the diffraction conditions are more stringent than
for a two-dimensional grating. For certain alignments of the crystal
and detector relative to the x-ray beam, all atoms in the crystal scatter
in phase*. This is called Bragg diffraction (see references).
[* for a simple lattice with just one atom per primitive cell. The primitive cell is the smallest structure which can be translated periodically to generate the crystal.]
The principal quantum number of the vacancy (n = 1, 2, 3,...) is designated by K, L, M, ... respectively. The principal quantum number of the upper level of the transition, relative to the vacancy, is indicated by a subscript a, b ,d ,... Fine structure levels are indicated by a further numerical subscript. Thus Ka1 and Ka2 designate x-rays due to vacancies in the lowest core level (n=1) being filled by a transition from the next level (n=2), specifically from the P3/2 and P1/2 angular momentum states of n=2, respectively.
The wavelengths for copper are:
An "unknown" sample "X" is provided (something from the cubic class). Obtain evidence for its crystal structure and determine the lattice constant.
Look at Pb-Sn solder. Compare to diffraction expected for pure Pb and pure Sn. What does this tell you about the alloy?
Other things to check: Separation of CuKa1
and CuKa2 peaks. Effect of Ni filter:
If you remove it, do you see CuKb
peaks? Effects of scan rate; peak widths and precision with which centers
of peaks can be located. Calculate the resulting uncertainty in the lattice
parameter, as calculated from a peak at small and at large angle, and from
all the peaks combined. [i.e., do the error analysis!]
A. Starting Procedure
2. Turn on the water at the ball valve on the wall.
3. Turn on power to the stepper motor controller, detector electronics, and strip chart recorder. Position the goniometer at the desired starting angle (You could do this manually when the enclosure is open).
Turn on the high voltage generator "control power" (bottom button).
4. Set the strip chart recorder speed and sensitivity (push button). Set the pen on a cm line by moving the paper manually. Record necessary information on the chart.
5. Set the desired stepper motor rate. Select "Scan".
6. Turn on high voltage (top button: "x-ray on"). The generator should automatically ramp up to 40 kV, 20 ma (or as preset ) within 1 min.
7. Open the shutter: Turn on the power switch of the external black box and then push the
"x-ray on" button.
8. Push the "start" button on stepper motor controller to start scan and strip chart recorder.
9. Enter starting time & other information in the log book.
10. Calculate the completion time of scan and be present to stop the
scan. Do not depend on the limit switches.
1. Close the shutter by pushing the "x-ray OFF" button on the external box.
3. Open & latch the enclosure door. [If you fail to do steps 1 or 2, they will be done by the interlock system, but only after the door is partially open].
4. Install new sample.
5. Close the enclosure door.
6. Proceed from step A.4.
2. Turn off the high voltage by pushing the middle button on the HV generator: "X-ray OFF".
3. Turn off the high voltage generator "control power" (bottom button).
5. Open the enclosure. Remove your sample. Turn off the two interlock power switches inside enclosure. Return the goniometer to a typical starting position.
6. Turn off all other power switches.
7. Close the water valve on the wall.
This is not part of the normal operating procedure and should be done only by persons authorized by Larry Suddarth.
Likewise, do not bypass the water flow interlock without Larry's knowledge.