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X-Ray Spectroscopy
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X-Ray Spectroscopy
Lecture 3
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1
Question
What is the primary mechanism behind the generation of characteristic X-rays in an atom?
Answer
Characteristic X-rays are generated when an electron is knocked out from the inner shells of an atom, creating a vacancy. An electron from an outer shell then falls into this lower energy state, and the energy difference is released as a characteristic X-ray photon.
2
Question
Explain Moseley's law and its significance in X-ray spectroscopy.
Answer
Moseley's law defines the wavelength (λ) of the characteristic X-ray emitted by an element in terms of its atomic number (Z), where B and σ are constants.
3
Question
Sensitivity of characteristic X-rays to atomic number (Z) formula
Answer
B: empirical constant Z: Atomic number of the element σ: Screening constant E: Energy of the emitted X-ray photon h: Planck’s constant c: Speed of light in vacuum
4
Question
What are the selection rules for electron transitions that generate characteristic X-rays?
Answer
5
Question
Describe the difference between Wavelength Dispersive Spectroscopy (WDS) and Energy Dispersive Spectroscopy (EDS) in detecting characteristic X-rays.
Answer
WDS uses Bragg diffraction from a single crystal to detect the wavelength of X-rays by measuring angles of diffraction, while EDS uses a photon detector to directly measure the energy of incoming X-rays, without wavelength dispersion.
6
Question
What is fluorescent yield (ω)?
Answer
Quantifies how likely it is that a vacancy in an inner shell electron will be filled by an electron from a higher shell, leading to the emission of a characteristic X-ray photon, rather than an Auger electron. Auger electron: after a vacancy is filled, the leftover energy ejects an electron from the outer shell.
7
Question
High vs Low CPS (counts per second)
Answer
• High counts per second (CPS) can improve throughput, but reduces the energy resolution (as processing time reduces). • Low CPS can improve resolution while losing on throughput and the ability to detect trace elements.
8
Question
How does low counts per second (CPS) affect energy resolution and throughput in EDS?
Answer
Low CPS improves energy resolution by allowing longer processing time per event, but it reduces throughput and the ability to detect trace elements.
9
Question
Define 'sum peaks' in the context of EDS and their cause.
Answer
Sum peaks occur when two different X-rays arrive simultaneously at the detector at high CPS, causing pulse pile-up that the detector cannot separate; this results in an artificial peak at the energy equal to the sum of the two X-ray energies.
10
Question
What two factors affect the spatial resolution of EDS?
Answer
11
Question
What is the formula for spatial resolution in EDS?
Answer
E = energy (keV) p = local density
12
Question
How does increasing acceleration voltage (Vacc) affect the spatial resolution of EDS?
Answer
increases the electron beam's penetration depth and excitation volume, which reduces spatial resolution because X-rays are generated from a larger volume.
13
Question
What are the three quantitative correction factors that must be considered when measuring element concentrations with EDS?
Answer
The atomic number factor (Z), the X-ray absorption factor (A), and the fluorescence factor (F).
14
Question
Why does the atomic number factor (Z) affect X-ray generation in EDS?
Answer
Because higher atomic number leads to increased backscattering which reduces X-ray generation, but the X-rays produced have higher energy and more reach the detector.
15
Question
Describe the significance of X-ray absorption inside the sample during EDS analysis.
Answer
X-rays generated inside the sample are partially absorbed before exiting, with absorption depending on sample density (ρ), mass absorption coefficient (μ), and the path length (t), affecting measured intensity.
16
Question
Write the equation that describes X-ray intensity attenuation due to absorption in the sample.
Answer
where (I_x) is the attenuated intensity, (I_0) is the initial intensity, (µ) is the mass absorption coefficient, (ρ) is density, (t) is path length.