unknown at first, these photons from innershell transitions have played a vital role in materials...
TRANSCRIPT
Unknown at first, these photons
from innershell transitions
have played a vital role in
materials analysis
X-Ray Generation
Where do X-Rays come from?
Source of X-rays as vacancy filled by cascade of electrons
from lower energy levels
X-Ray Generation
X-ray tube
Filament (Tungsten)
Target metal (Cu, Cr)
Electrons are accelerated by a
potential of about 55,000 Volts
Continuous X-Ray Spectrum
• 35 keV electrons strike the metal target
• They collide with the electrons in the metal
• Rapid deceleration results in emissions of proton
• Photons with a wide range of energies are emitted because the degree of deceleration is different
Characteristic X-Ray
• The incident e- collides with an e- from a cores level (K shell)
• An e- in the core level escapes
• The vacant K shell can be filled by a core electron from a higher energy level
• A photon is emitted during this transition
• Specific energies are emitted since the core e- energy levels are well-defined
X-Ray Diffraction
• Waves interact with crystalline structures whose repeat distance is about the same the wavelength.
• X-rays scattered from a crystalline structure constructively interferes and produces a diffracted beam.
Bragg’s Law
n = 2d sin
n = integer
= wavelength (Å)
d = interatomic spacing (Å)
= diffraction angle ()
Diffractometer
• A: Chiller• B: Regulator• C: Computer• D: Strip chart recorder
• E: X-ray source
• F: compensating slit
• G: Sample chamber
• H: Scintillation counter
• J: Goniometer
Diffraction Pattern
• Diffraction patterns are a plot of intensity vs
Sample Type
Single Crystal
• Sample is placed in a beam
and the reflections are
observed for specific
orientations
• Time consuming and
difficult to orient the
crystal
Powder Sample• Many small crystallites
with random orientations
• Much easier to prepare and one can see reflections in all directions
Analyzing a powder sample
X-Ray Fluorescence Spectrometry
• What is it?
• How does it work?
• Properties
• Advantages
• Disadvantages
X-Ray Fluorescence Spectrometry What is it?
• Instrumental method of qualitative and quantitative analysis for chemical elements
• Based on the measurement of the wavelength and intensities of element’s spectral lines emitted by secondary excitation
X-Ray Fluorescence Spectrometry How does it work?
• A beam of sufficiently short-wavelength X radiation irradiates the sample
• Excites each chemical element to emit secondary spectral lines
• Spectral lines have wavelengths characteristics
• This process is known as the secondary excitation
X-Ray Fluorescence Spectrometry How does it work? (continued)
• Sample can have practically any form
• Sample size and shape can be largely varied
• The material to be analyzed can be almost anything
X-Ray Fluorescence Spectrometry Properties
• The intensities of the resulting fluorescent X-rays are smaller
• The method is feasible only when high-intensity X-ray tubes, very sensitive detectors and suitable X-ray optics are available
• A certain number of quanta can reduce the statistical error of the measurement
X-Ray Fluorescence Spectrometry Properties (continued)
• Intensity influence the time that will be necessary to measure a spectrum
• The sensitivity of the analysis depend on the peak-to-background ratio of the spectral lines
• Few cases of spectral interference occur
X-Ray Fluorescence Spectrometry Advantages
• X-ray spectra is simple and regular
• Matrix effect in X-ray emission are systematic, predictable and readily evaluated
• X-ray fluorescence spectroscopy is non-destructive.
X-Ray Fluorescence Spectrometry Disadvantages
• Small surface layer contributes to the observed X-ray line intensity
• Not all of the elements in a sample can be measured using the same X-ray tube, crystals, and detector
X-ray Applications
• Electron Microprobe
• Scattering
• Absorptiometry
• Radiography
• Fluoroscopy
Electron Microprobe
• Nondestructive• determines
composition of tiny amounts of solids.
• Virtually all elements can be analyzed except hydrogen helium and lithium.
An Electron Microprobe
Scattering
• A fluorescence spectrometer is used on a gas, liquid, colloidal suspension or solid.
• Coherent and Incoherent scattering rays.
• Ratio of these rays are analyzed.
• Measures radius of gyrations.
• Widely used in proteins, viruses, catalysts, hardening and precipitation in alloys and lattice deformation.
Absorptiometry
• Chemical analysis is possible for gases, lipids or solids to measure densities porosities as well as coating, plating and insulation thickness.
• Most often applied to living patients in measurements of bone densities, iodine in the thyroid gland, liver diseases and other medical uses.
• Two types Single and Dual X-ray Absorptiometry.
Single X-ray Absorptiometry
• Single X-ray absorptiometry is used to measure the bone mineral content.
• Used for diagnosis of osteoporosis, providing reasonable accuracy and precision and low radiation exposure.
Dual X-ray Absorptiometry
• Used when single X-ray absorptiometry is not feasible.
• Used in areas with variable soft tissue and composition such as the spine, hip or the whole body.A Dual X-ray
Absorptiometry
Radiography
• Involves use of registration on film of the differential absorption of a beam passing through a specimen.
• Medical uses.
• Industrial uses.
• Nondestructive method.
Fluoroscopy
• Similar to radiography except the image is registered on a fluorescent screen.
• Instantaneous and permits observation of internal motions and other changes.
Auger electron spectrometer