1. Instrument Parameters in WDXRF
Physics of X-rays
Instrument Parameters in WDXRF

2. Introduction Advantages of XRF
Solid and liquid samples can be analysed directly: large range of applications
Little or no sample preparation required
Analysis is non-destructive (for the sample)
Sampling-analysis result time is relatively short
Quantitative and qualitative analyses are possible
Accuracy and long term stability
Elemental range: (Be) Na to U
Linearity from ppm to 100%
September 19, 2018

3. Introduction Qualitative and Quantitative Analysis
The 4 stages in X-ray emission analysis are:
excitation of characteristic radiation from the specimen by bombardment with high energy photons, electrons, protons, etc.
selection of a characteristic emission line
detection and integration of the characteristic line to give a line intensity
conversion of the intensity to elemental concentration
Range: 0.2 to 20.0 Å ( keV)
Excitation levels: K, L and some M-series
Qualitative: position of the peak
Quantitative: intensity of the peak
September 19, 2018

4. Introduction WD-XRF Spectrometer - inner part
Z, i, E (kV)   SC
September 19, 2018

5. Physics of X-rays What are they?
XRF analysis covers the following energy- respective wavelength range:
E = 0.1 – 60.0 keV
l = – 0.02 nm
Elemental range from Berylium (Be) to Uranium (U)
September 19, 2018

6. Physics of X-rays Origin - General
X-rays originate from the energy loss associated with the interaction of high energy electrons or X-rays with atoms
The spectrum of an X-ray tube shows two types of X-ray radiation
Continuous radiation (white radiation or Bremsstrahlung)
Characteristic radiation (photoelectric effect)
Both types of radiation depend on the anode material
When a sample is irradiated by the X-rays generated in the X-ray tube, the photo-electric effect of each element present will be observed (if measurable) in the spectrogram
This effect is used for the analysis of the sample
The next slides will show the excitation principles and some spectrograms
September 19, 2018 6

7. Physics of X-rays Origin – Tube spectrum
Bohr’s atomic model
September 19, 2018

8. Physics of X-rays Origin – Tube spectrum
Continuous radiation (Bremsstrahlung)
September 19, 2018

9. Physics of X-rays Origin – Tube spectrum
Continuum and characteristic tube lines
Compton scattering of a H3BO3 sample
E= h∙c λ = 12.4 λ
Where:
lmin = wavelentgh in Å
h = Planck’s constant (6.626 × m2 kg/s)
c = velocity of light ( km/s)
kV = excitation potential
September 19, 2018

10. Physics of X-rays Origin – Photoelectric radiation
K radiation
September 19, 2018

11. Physics of X-rays Origin – Photoelectric radiation
L radiation
September 19, 2018

12. Physics of X-rays Origin – Siegbahn nomenclature
X-ray emission lines = electron transitions
September 19, 2018

13. Physics of X-rays Properties – Effects
l = l0
Absorption (by composite material)
Photo-electric effect
Scattering (coherent or incoherent)
Diffraction
I= I 0 ∙ exp −μ∙ρ∙x
I = absorbed intensity
I0 = original intensity
µ = attenuation coefficient
= density of the material
x = thickness of the material
r, m
I0 (l0)
I (l)
l > l0
September 19, 2018

14. Physics of X-rays Properties – Diffraction
coherent scatter from the different crystalline phases in the material
reinforcement of the scattered waves takes place
Bragg’s Law has to be fulfilled: nl = 2d sin
ANTI-PHASE
PHASE
September 19, 2018

15. Instrument Parameters The X-ray tube
Design: End-Window (or Side-Window: higher kV possible)
Anode material: in general Rh (other anodes available), characteristic lines
Kramer’s formula is applied for the continuum
I = intensity at wavelength l
K = constant
i = current (mA)
Z = atomic number of the anode
l = any wavelength
lmin = Short wavelength limit for applied voltage
𝐼()=𝐾𝑖𝑍   𝑚𝑖𝑛 −  2
λ min = h∙c kV
September 19, 2018

16. Instrument Parameters The Goniometer
September 19, 2018

17. Instrument Parameters The filters
Exclude tube lines e.g. Rh lines
Improve the peak to background ratio
Better limits of detection
To reduce the peak intensity
Avoid detector saturation
Suppression of spectral impurities from the tube spectrum e.g. Cu, Ni, Fe
Different types (Cu, Al,...) and varying thickness ( µm) of filters are available to select the optimum settings
September 19, 2018

18. Instrument Parameters The filters – exclusion of tube lines
200 µm Cu filter
exclude the Rh tube lines to analyze e.g. Cd and Ag
September 19, 2018

19. Instrument Parameters Sensitivity - The collimators
The collimator is mostly preceded by a collimator mask
The mask hides the sample cup radiation comes mainly from the sample
Different mask sizes (5, 8, 18, 23, 28 and 34mm):
different sample sizes
improve cup masking
Collimator:
diverges secondary X-ray beam
finer improves resolution, but less intensity
different sizes: 0.12, 0.23, 0.46, 1 and 2
September 19, 2018

20. Instrument Parameters Sensitivity - The collimators Influence on the resolution
September 19, 2018

21. POLYCHROMATIC MONOCHROMATIC
Instrument Parameters Sensitivity - The Crystals Principle of reflection
POLYCHROMATIC MONOCHROMATIC
𝐴𝐵𝐶 =2d∙sinθ
′AC′ d = sin θ
′𝐴𝐶′=d sin θ
′𝐴𝐶𝐵′=2d sin θ
′𝐴𝐶𝐵′=nλ
n = 1, 2, 3,… (reflection order)
d = interplanar lattice spacing
 = diffraction angle
nλ = 2d∙sinθ
September 19, 2018

22. Instrument Parameters Sensitivity - The Crystals Crystal types
September 19, 2018

23. Instrument Parameters Sensitivity - The Crystals Influence on the resolution
September 19, 2018

24. Instrument Parameters The Detectors
Detectors convert X - ray photons into measurable voltage pulses
A range of detectors is required to cover the wavelength range nm – 12 nm ( keV)
Gas proportional detectors 0.08 nm – 12 nm ( keV)
Scintillation detector nm – 1.5 nm ( keV)
Output pulse and the energy of the incoming X-ray photon are proportional identification
September 19, 2018

25. Instrument Parameters The Proportional Counter = flow detector
Incoming X-ray photons ionise Ar-atoms until their energy is expended
electrons (-) are attracted towards the anode wire (+)
small cloud of electrons cause a current flow and a drop in voltage
a pulse is registered in the amplifier
CH4 for recombining the Ar = quench gas
Concurrent/Side effect = escape effect
photons excite K electrons from the Ar
September 19, 2018

26. Instrument Parameters The Scintillation Counter
X-ray photons are converted into emitted radiation in the blue light region (+/- 410 nm) by a Thallium activated NaI crystal (2mm thick, hygroscopic, airtight by a Be window)
Fall onto a photo cathode  burst of electrons  multiplication by dynodes, each on a higher potential  anode is reached  a current is produced  drop in voltage  a pulse is created in the amplifier
Side effect: fluorescence of the Iodine from the NaI crystal
September 19, 2018

27. © Copyright Bruker Corporation. All rights reserved
19. September 2018 27
December 8, 2010
September 19, 2018