X-Ray Fluorescence Spectroscopy

X-Ray Fluorescence Spectroscopy

• X-ray emission takes place approximately 10^-4 seconds after excitation.
• The energy of a characteristic X-ray depends on the difference in binding energies of the electron sub-shells involved in the transition.
• The energy of the characteristic X-ray is given by the equation: hy = E_K - E_L
• Characteristic X-ray spectrum shows peaks like K_α and K_β that relate to transitions between inner electrons shells of atoms.
• Continuous radiation is also present in the spectrum, with intensity decreasing with increasing wavelength.
• Absorption of X-rays by matter results in excited ions.
• Excited ions return to the ground state emit fluorescent X-rays.

Characteristic X-ray Emission and Absorption

• X-ray excitation comes from an incident X-ray beam
• The absorption of X-rays results in excited ions.
• The characteristic X-rays are emitted when the atom returns to its ground state.

Fluorescence Yield

• The fluorescence yield depends on the rate constants of excitation/recombination states.
• It estimates the relative effectiveness of X-ray emissions.
• For atomic numbers less than 4, the fluorescence yield is zero, with Auger processes dominating.
• K-series yield is more effective with higher intensity. Fluorescence yield for Ce (Z=58) atoms: K-line=0.91, L-line=0.14, M-line=0.001.

X-Ray Fluorescence Spectroscopy Methods

• Two main methods of XRF spectroscopy are: Wavelength-Dispersive (WDXRF) and Energy-Dispersive (EDXRF)
• WDXRF uses a crystal to separate X-rays by wavelength.
• EDXRF uses a semiconductor detector to measure X-rays by energy.

Advantages & Disadvantages of WDXRF

• Advantages:
  • High resolution: good spatial separation of X-ray lines. WDXF:10eV for 5.9keV peak.
  • Compositional analysis: O/P count is proportional to elemental concentration.
  • Good peak/background (signal/noise) ratio.
• Disadvantages:
  • Mechanical system errors during measurements
  • Peak/background need separate measurement.
  • High currents lead to large beam size causing specimen damage.

Advantages & Disadvantages of EDXRF

• Advantages:
  • Simple mechanical design (no moving parts)
  • Absence of collimators/monochromators leads to near 100% increase in radiation reading from detector.
  • Detector positioning closer to sample increases solid angle for X-ray collection.
  • Possible to use weaker radioactive sources enabling portable/field applications.
  • Higher count rates. Smaller volumes of material.
  • Almost 100% detector efficiency for wide X-ray range (3-20 keV)
  • Complete elemental spectrum measured simultaneously enables rapid analysis.
• Disadvantages:
  • Lower resolution at high wavelengths. Better resolution at lower wavelengths.
  • Equipment requires detector operated at 77K.
  • Low sensitivity to low Z elements (Z < 11) due to properties of semiconductor detector.
  • Stray signals/radiation affect measurement, increasing background.

Applications of XRF

• Analyzing solid/liquid samples of various shapes and sizes.
• Quality control in material preparation.
• Quantitative estimation of heavy elements.
• Determining film thickness (metal plating, oxide layers, paint layers).