1. Introduction to X-Ray Fluorescence NON DESTRUCTIVE CHEMICAL ANALYSIS Notes by: Dr Ivan Gržetić, professor University of Belgrade – Faculty of Chemistry

2. What are X-Rays? Photons with a wavelength between 10 -9 and 10 -12 m

3. Application fields for X-Rays X-RAYS Radiography Simultaneous XRD EDX Sequential ANALYTICAL XRF WDX MEDICAL Sim / Seq XRF : X-RAY Fluorescence XRD : X-RAY Diffraction WDX : Wavelength Dispersive X-RAY EDX : Energy Dispersive X-RAY SECURITY Airport Cargo

4. Limits of Detection and Dynamic Ranges

6. 84 Elements of Periodic Table H Na Li K Rb Cs Fr Mg Be Ca Sr Ba Ra Sc Y La Ac Ti Zr Hf V Nb Ta Cr Mo W Mn Tc Re Fe Ru Os Co Rh Ir Ni Pt Cu Ag Au Zn Cd Hg Al B Ga In Tl Si C Ge Sn Pb P N As Sb Bi S O Se Te Po Cl F Br I At He Ar Ne Kr Xe Rn CePrNdPmSmEuGdTbDyHoErTmYbLu ThPaU PuAmCmBkCfEsFm Md NoLw Np Not measurable by XRF Rare Gases Unstable Elements Standard Elements for EDX and WD-XRF Ultra light Elements require special Crystals Pd Elements that require a Primary Beam Filter to suppress the Rh Tube Spectrum Dmitri Ivanovitch Mendelejev 1834-1907

7. Oils Metals, Slags Ores and Raw Materials Chemicals Food Products Glass Polymers Ceramic What can be analysed by X-Ray Fluorescence?

8. Typical Samples for X-Ray Fluorescence

9. Production of X-Rays – The X-Ray Tube The cathode is heated by a current of several amperes. Between the cathode and the anode, which is made of a characteristic metal, like Rh, W, Mo, etc., a high voltage is applied. By thermo-ionic effect, the cathode emits electrons that are accelerated by the high voltage (e.g. 40- 60kV).

10. Production of X-Rays – The X-Ray Tube When these high energy electrons hit the anode, the anode emits a spectrum of X-rays. As a vacuum exists within the tube, the produced current is a few milliamps. 99% is dissipated as heat.

11. The Minimum Wavelength or Maximum Energy and the Maximum Intensity and its position of the X- Ray tube spectrum depends on the high voltage applied. The X-ray spectrum produced by the tube consists of a Continuum (Bremsstrahlung) and characteristic lines corresponding to the anode element (in this case Rhodium). Output of the X-ray tube - Spectrum produced by a Rh X-Ray Tube 11

12. Excitation of the sample X-Ray Tube Spectrometer Secondary X-Rays or X- Ray Fluorescence which is characteristic for the elemental composition of the sample Sample

13. Principle of the excitation by X-Rays  An Incoming X-Ray photon strikes an electron, the electron breaks free and leaves the atom.

14. Principle of the excitation by X-Rays  This leaves a void that must be filled by an electron from an outer shell.  The excess energy from the new electron is released (fluorescence) in the form of an x-ray photon. X-Ray Photon X-Ray Fluorescence

15. Excitation and Interactions between shells Layer K Layer L Layer N Layer M

16. X-Ray Emission Energies (Lines) Transitions

17. Atomic Shell Structure Energy Min.Max. Nucleus Q P O N M L K Shells The farther from the nucleus the higher is the energy

18. Useful Transitions between Energy Levels KK LL LL MM MM O N M L K Shells KK

19. Absorption edge Plot of the X-ray absorption curves for Fe (black), Co (red), and Ni (blue), showing the Ka absorption edges. The locations of the Ka lines for these elements is also shown. Note than Ni-Ka x-rays have sufficient energy to fluoresce Fe-Ka x-rays. If the energy of primary radiation is less then the absorption edge of the element X ray line of interest no characteristic X ray emission can be obtained.

20. X-Ray Emission Energies Fluorescence Yield (ω) = gives the efficiency of the fluorescence process. It is the ratio of photons emitted to photons absorbed.

21. Bragg’s Law X-Ray Diffraction n = 2d sin  n = 1,2,3,4, … (Diffraction Order) Sir William Lawrence Bragg 1890 - 1971

22. Bragg’s Law

23. Crystal 2d (Angströms) LiF 4201.8 LiF 2202.848 LiF 2004.0267 NaCl5.641 Ge 1116.532 InSb7.48 PET8.742 ADP10.642 TLAP25.8 Crystals used in WD-XRF Pentaerythritol (PET) Thallium acid phthalate (TAP or TLAP) Ammonium dihydrogen phosphate (ADP)

24. Goniometer for wavelenth dispersive spectrometry

25. ARL Goniometer Sample Holder X-Ray Tube Digital 1  - Crystal – Drive Digital 2  - Detector – Drive Detectors Crystal Changer 7 Primary Beam Filters 4 Collimators

26. Collimator Divergence Divergence of collimators depends on blade spacing and length of collimator Therefore the divergence should always be expressed as degrees of divergence and not as millimeters of spacing Φ1Φ1 Φ2Φ2

27. Detectors The detectors are used to transform the X- Ray Fluorescence radiation into electrical pulses. There are three types of detectors (counters) used in WD-XRF instruments : ◦ FPC : Flow Proportional Counter ◦ SC : Scintillation Counter ◦ PC : Proportional Counter

28. Flow Proportional Counter (FPC) working principle The detector is a metal cylinder (connected to ground) filled with a gas (Ar + CH 4 ). In the center of the cylinder, there is an insulated wire at a positive potential (1000 to 3000V ). X-Rays enter through a very thin window (Polypropylene). Because the window is very thin, the gas diffuses trough the window and has to be replaced constantly.

29. Flow Proportional Counter (FPC) working principle In a proportional counter the fill gas of the chamber is an inert gas which is ionised by incident radiation, and a quench gas to ensure each pulse discharge terminates; a common mixture is 90% argon (fill gas), 10% methane (quench gas), known as P-10. An ionizing X-Ray photon entering the gas collides with a molecule of the inert gas and ionise it to produce an electron and a positively charged atom, commonly known as an "ion pair". As the charged particle travels through the chamber it leaves a trail of ion pairs along its trajectory, the number of which is proportional to the energy of the particle if it is fully stopped within the gas. In some cases ionisation could even escalate, causing a so-called current "avalanche" which, if prolonged, could damage the counter tube. Quenching of the ionisation with methane is therefore essential. So, FPC detects X-ray photon but it amplifies it proportionally to its incoming energy as well.

30. Flow Proportional Counter (FPC) working principle When an X-Ray hits an atom of argon, it is ionized (Ar + - e). The electron in turn ionizes another atom and so on. The total number (M) ion-electron pairs produced by a photon (X-Ray) is given by :

31. Flow Proportional Counter (FPC) working principle  ε = Energy required to produce an electron-ion pair ≈ 27eV for Argon.  E = Energy of the entering X-Ray Photon [eV]  A = gas gain (The gas amplification factors) 10 3 ÷ 10 5. Example : CuK α Radiation 1.5418Å

32. Flow Proportional Counter (FPC) working principle Ar + ions are moving towards the cylinder (ground) and the electrons are moving towards the central wire (positive high voltage). The electrons accumulate and discharge suddenly on a very small section of wire which produces an electric pulse. This pulse is then amplified, processed and counted. Important : The amplitude (voltage) of the pulse output is directly proportional to the X-Ray photon energy that produced this pulse. For this reason, the counter is called proportional counter.

33. Proportional Counters Proportional or Sealed Counters are filled with Kr, Ar, Ne depending on the wavelengths to be detected. They are mostly used for Monochromators (Fixed Channels) in Simultaneous Instruments (Industrial heavy duty instruments).

34. Scintillation Counter (SC) The X-Ray Photon hits a NaI crystal doped with Thallium. Iodine fluorescence emits a certain amount of violet light. The intensity of this light is proportional to the X-Ray Photon energy having struck the crystal. This light strikes a photocathode and snatches up a number of electrons. This electron is directed to the dynode whose potential is 100 V above the photocathode and snatches up two electrons (or more).

35. Scintillation Counter (SC) These two electrons are directed towards a second dynode, which is at +100 V compared to the first. They pull more electrons that are directed towards a third dynode and so on. As for the flow gas counter, the voltage pulse produced is proportional to the X-Ray photon energy having struck the crystal.

36. Choice of Collimators, Crystals & Detectors FPC = flow proportional counter AX03 - AX20 = multilayer pseudo crystals for diffraction of long wavelengths

37. Choice of Collimators, Crystals & Detectors

38. Commonly used Units in XRF Wavelength λ : ◦ Angström (Å) = 10 -10 m ◦ Nanometer (nm) = 10 -9 m ◦ 1 Å = 0.1 nm or 1 nm = 10 Å Energy: ◦ Kilo Electronvolt (keV) = 1000 Electronvolts CONVERSION example: ◦ CuK α Radiation: λ = 1.5418Å ; Intensity: ◦ Counts per Second (cps)

39. The End THANK YOU