X-ray Diffraction- Powder Diffraction Components Flashcards
Effect of x-rays being slightly different wavelengths
Like Kα1 and Kα2. Energy difference between these transitions very small so doublet may not be resolved
What is a monochromator and what are the 2 types?
A device for selecting radiation of a single wavelength. Could be diffracted beam monochromator or primary beam monochromator
Diffracted beam monochromator
Curved graphite crystal. Removes Kβ but can’t separate Kα1, Kα2. Suppresses background and fluorescence. Preferred to filter. Essential for analysis of steels using Cu Kα radiation
Primary beam monochromator
Curved Ge or Si single crystal. Removes Kβ and Kα2 component. Suppresses background. Essential for high resolution work and structural analysis. Cost to diffracted beam intensity
How do single crystal monochromators work?
Use bent and cut crystals to focus x-rays making use of the Bragg law. Divergent source of x-rays diffracted by a bent crystal with set of diffracting planes parallel to its surface. Crystal bent to twice radius of focussing circle. Face of crystal then ground to the radius of focussing circle. X-rays meet lattice planes at same angle α. This angle is set to diffract only radiation if desired wavelength. The diffracted x-rays focus on opposite side of circle to source
Requirements of specimen for powder diffraction to work
Should be a truly random distribution of crystallites. Surface should be smooth and flat.
How to prepare polycrystalline samples
Need to back packed into specimen holder. Secure sample holder on frosted glass support. Load sample into cavity. Knock down powder with edge of spatula tip. Add more powder until cavity filled. Add small amount of sample and apply gentle pressure to pack sample. Cover back of sample, turn holder upside down, remove glass support
Why not front load specimen holder for polycrystalline samples?
Can cause preferred orientation of crystallites in solids with layered structures and weakly bound layers. Pressing down on specimen causes particles to preferentially align parallel to the surface so no longer random distribution of crystallites. Greater probability certain planes will be at correct angle for diffraction than for others. Some reflections enhanced in intensity whereas others diminished
3 types of detector
Scintillation counter
Gas proportional detector
Position sensitive detector
Scintillation counter
Has phosphor and photomultiplier tube. Phosphor converts each x-ray photon to photon of light. These cause ejection of electrons from the photocathode (photosensitive Cs-Sb alloy). Electrons drawn through series of dynodes each at potential of +100V with respect to last. Takes less than 1μs. Last dynode connected to measuring circuit. Gain at each dynode about 5. 10 dynodes means gain of 5^10. One x-ray photon yields large pulse of electrons
Gas proportional detector
Cylindrical metal cathode and wire anode. Detector filled with inert gas (Kr). X-ray photon causes ejection of photoelectron from a Kr atom. This is accelerated towards anode maintained at bias of +1000V. Photoelectron causes secondary ionisation of further Kr atoms meaning more photoelectrons generated and accelerated to anode, etc. This amplification means electron cascade impacting anode. Electron pulse momentarily charged a capacitor which is detected by measuring circuit
Position sensitive detector
Like gas proportional detector but filled with 10% CH4 and Ar. Photoelectron from Ar atom results in electron cascade. Electron pulse impacts wire anode (of high resistance). Time taken for pulse to reach end of wire measured. Position of pulse and angle of diffracted beam can be determined. Anode wire lies along focussing circle and allows angle of 6° to be continuously monitored. Don’t need receiving slit when using PSD
What are the factors for comparing detectors?
Efficiency: how many of incident photons are detected
Resolution: incident photon gives rise to range of output voltages (distribution defines resolution)
Linearity: should respond to every incident photon (but finite detector response time)
Robustness: how easily detector damaged
Comparison of efficiency, resolution, linearity and robustness for 3 types of detectors
Scintillation counter: excellent, poor, good, excellent
Gas proportional and PSD: moderate, excellent, excellent, poor
