Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
12 Cards in this Set
- Front
- Back
|
What is diffraction?
What is refraction? |
The apparent bending of waves around small objects and the spreading out of waves past small apertures.
On the other hand, refraction is the change in the direction of a wave due to a change in its speed. |
|
|
What is Bragg's Law of Diffraction (in words)?
|
When a collimated (light with parallel rays) strikes a pair of parallel lattice planes in a crystal, each atom acts as a scattering center and emits a secondary wave. All of these secondary waves interfere with each other to produce a diffracted beam.
|
|
|
What is the equation for Bragg's law?
|
n (lambda) = 2d sin (theta)
|
|
|
What are the three things you need in x-ray diffraction (XRD)?
|
1. Source of radiation (either monochromatic or variable wavelength)
2. Sample (single crystal, powder, or solid piece) 3. Detector (radiation counter or photographic film) |
|
|
What is the Debye-Sherrer method for powder XRD?
What are the setbacks of this method? |
The cones generated are recorded on a strip of film that is surrounding the sample. Each cone intercepts the film as two short arcs. You can convert these line positions on film to angles and intensities.
Setbacks: Long exposure times, poor resolution |
|
|
As you increase the value of theta for powder diffractograms, what happens to the d-spacing?
What is the minimum value of d? |
The d-spacing gets smaller
Minimum value of d: (lambda/2) |
|
|
What two things to peak positions depend on?
|
1. d-spacings of {hkl}
2. systematic absences |
|
|
What are peak positions determined by? What are peak intensities determined by? What are peak widths determined by?
|
Peak positions: size and shape of unit cell (d-spacings and systematic absences)
Peak intensities: atomic number and position of the atoms in the unit cell Peak widths: instrument parameters, temperature, crystal size, strain, and inhomogeneities |
|
|
How are x-rays created? Besides x-rays, what is the other big component generated?
|
Collision of high-energy electrons with metals
Other component: HEAT |
|
|
How does a rotating anode work?
|
100x higher powers are possible by spinning the anode at > 6000 rpm to prevent melting it (remember heat is generated)
brighter source |
|
|
Name two ways to generate MONOCHROMATIC x-rays
|
1. Filters (old way) - foil of the next lightest element can be used to absorb unwanted high energy radiation
2. Monochromator - use diffraction from a curved crystal to select x-rays of a certain wavelength |
|
|
Name 3 types of detectors for x-rays
|
1. point detectors (scintillation counters or gas proportional counters)
2. strip detectors 3. area detectors |
