X-ray Fluorescence Physics of XRF Characteristics Caveats Instrumentation

The physics of XRF Step 1: An external x-ray impinges on an atomic electron Step 2: The atomic electron is ejected leaving a hole Step 3: Another atomic electron occupies the hole releasing its energy as an x- ray photon

The physics of XRF Electrons can be emitted from different levels Electrons from L or M shells fills K vacancy emitting an x-ray Electron from M or N shells fills L vacancy emitting an x-ray Auger electrons can also be emitted in the process In: x-ray Out: 1 photoelectron (always) 1 X-ray photon or 1 Auger electron

The physics of XRF Nomenclature of transitions Examples: Kα1 Electron from sub-level LIII to the K-shell Kα2 Electron from sublevel LII to the K-shell Kα1,2 if neither line is resolved by the spectrometer Kβ1 Electron from sublevel M to the K-shell Lα1 Electron from sublevel M to the L-shell

The physics of XRF Nomenclature of transitions Notice that some transitions are forbidden

The physics of XRF Energy range of x-rays XRF measures the following range of energy or wavelengths: E = 0.11 – 60 keV λ = 11.3 – 0.02 nm The radiation intensity is the number of X-ray photons emitted or measured per second, cps (counts per second) or kcps (kilocounts per second). Energy range (keV) Wavelength rangeName < 10-7cm to kmRadio waves < 10-3µm to cmMicrowaves < 10-3µm to mmInfra-red – to 750 nmVisible light – to 380 nmUltra-violet 0.11 – to 12 nmX-rays 10 – to 0.12 nm Gamma radiation

The physics of XRF K and L energies

The physics of XRF Capturing the x-rays and measuring its energy can be used to identify the emitting elements Characteristic X-rays of lead K electrons have BE = keV Kβ lines correspond to transitions from M and N shells with energies of 85 and 87 keV. Kα lines are from transitions from L shells with energies of 72 and 75 keV. L lines are from transitions to the L shell with energies of 10 to 15 keV. M lines are from transitions to the outer M shells and have energies around 2.5 keV.

The physics of XRF Characteristic X-rays of silver Silver has a lower atomic number and BE = 25.5 keV K shell x-rays are emitted at much lower energies. K α1 and K α2 lines, which were distinct for Pb, are not resolvable for Ag. Link to table:

The physics of XRF K lines for several elements Energies of the characteristic X-ray lines depend only on the atomic energy levels so are always the same. Intensity of an emission line depends on number of atoms in sample which were excited but on many other parameters as well. The probability of X-ray emission depends on the element and on the energy of the excitation source. The probability of detecting the X-rays depends on the detector sensitivity, attenuation in the sample itself, etc.