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Energy of the x-rays depends on the molecule the atom's in (T/F)
Elements with Z<__ only produce a K series
How are x-rays produced?
If an inner electron is knocked out of an atom, an electron from a higher energy level will fall (cascade) to take its place. The energy difference is emitted as an x-ray.
Higher accelerating voltage is needed to knock electrons out of an element with a (lower/higher) atomic number
Higher (Greater positive charge on nucleus of bigger elements; therefore harder to knock electron away)
The name given to an electron dropping from M orbital (2 shells away) to K orbital
The name given to an electron dropping from an L orbital (adjacent orbital) to K orbital
3 ways x-rays are obtained
- 1. Bombardment of a metal target with a beam of high-energy electrons
- 2. Exposure of a primary beam of x-rays to generate a secondary fluorescent beam
- 3. Radioactive decay
What is an angstrom?
Duane Hunt Law
wavelength (in angstroms) = 12398/V
Accelerating voltage determines ________ used for analysis
In XRF, the emitted x-ray is (lower/higher) energy than the absorbed x-ray
The energy of the fluoresced X-rays gives you information about:
What element is present (qualitative analysis)
The number of the fluoresced X-rays produced gives you information about:
Concentration (quantitative analysis)
Wavelength Dispersive X-ray Fluorescence
WD-XRF uses a ___ to separate wavelengths of emitted X-rays
The angle between the incident beam and the scattered beam
2 Theta (the angle of maximum intensity) is also called
the Bragg angle
n(wavelength) = 2d sin theta
Lithium Fluoride (most common analyzer crystal)
Filters are often used between the ___ and the ____
In WDXRF the value for ___ is know, and the value for theta is measured in order to determine ___ and therefore the identity of the element
- d (slit width, since we know what crystal we're using)
- Wavelength emitted from the sample
The atomic number (Z) for the X-ray source must be ___ than Z for the elements you plan to analyze
After being produced the X-rays in the source pass through a ___ window
the ___ insures the beam of X-rays is parallel
The electron beam in the X-ray sourcce is generated by...
...heating a tungsten (W) filament
The term for the rotating table
Sample holders for liquids are typically made of...
plastic (because it is made up of low atomic number elements which are not affected much by X rays)
Name 4 types of detectors for XRD
- Ionization detector
- Scintillation Counter
How does a scintillation counter work?
- X-rays strike a crystal (NAI(Tl))
- The crystal emits many photons of visible light
- These photons are detected by a PMT
Why is the crystal of the scintillation counter shileded with aluminum?
To prevent contamination and water from destroying the crystal
When ionizing radiation passes through a semiconductor detector, it leaves behind a trail of +/- charges in the _____
2 main categories of X ray fluorescence analysis. Which is more expensive?
- Wavelength dispersive (more expensive)
- Energy dispersive
Which type of X-ray analysis gives us information about the crystal structure of your analyte?
The repeating group of a crystal
XRD peak position gives information about
dimensions fo the unit cell
XRD peak intensity gives information about
the content of the unit cell (% composition)
XRD peak broadening gives information about
strain on the crystal
Name 3 things that impact the intensity of an XRD peak
- crystallographic structure
- position of the atoms within the unit cell
- thermal vibration of the atoms
Two main designs for XRD. Which is older?
- Bragg-Brentano Geometry (uses collimator)
- Parallel beam optics (using Gobel mirrors and Soller slit)
The name for an equation that gives the minimum wavelength for X-rays produced by hitting a metal with electrons
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