AXP Research group Analytical X-ray Physics

Transcription

1 Research group Analytical X-ray Physics X-ray Fluorescence Spectrometry Wolfgang

2 and BLiX

3 Team

4 Our Current Activities 3D Micro-XRF 3D Micro-XANES High resolution X-ray emission spectroscopy Characterisation of X-ray optics and sources Analytic for photovoltaics Laser-Plasma-Sources for the soft X-ray regime X-ray microscopy with laser-plasma-source Profesional courses fields of application: cultural heritage, solar cells, geology, bio-medical

5 X-rays as a probe

6 Selection rules and emission line energy shells quantum numbers n l j 1 0 1/2 K Kα1 Kα2 Kβ1 Kβ /2 1 1/2 LIII 2 1 3/2 MI /2 1 1/2 1 3/2 MV selection rules l=± 1 j = 0, ± 1 satellites LI LII Kα2: Siegbahn Notation K-L3: IUPAC Notation

7 Outline - Survey on XRF Instrumentation and Methods - Quantitative X-ray Fluorescence Analysis - An Application of Micro-XRF

8 Scheme of a XRF-spectrometer detector source beam conditioner geometry

9 X-ray detectors Wavelength Dispersive Spectrometer - Crystal or multilayer source - proportional counter, scintillation counter detector - Quantitative analysis - Light element analysis beam conditioner Energy Dispersive Spectrometer - Solid state detector (Si, Ge) - Small, leightweight detectors (SDD, PIN)geometry - Qualitative and quantitative analysis - Range: (B) Na - U

10 X-ray sources Syncrotron radiation source - High brillance - Mikro/Nano-XRF (< 1µm) detector X-ray tubes - Micro focus X-ray tubes beam conditioner beam conditioner - Brillance optimised - Miniature X-ray tubes - Portable instrumentation (Handheld) geometry Other - Radioactive sources

11 Spectrometer geometry source Standard: Ψ 45 detector Gracing Incidence XRF - Surface sensitive Gracing Exit XRF beam conditioner - Surface sensitive Total reflection XRF geometry

12 Outline - Survey on XRF Instrumentation and Methods - Quantitative X-ray Fluorescence Analysis - An Application of Micro-XRF

13 Why is quantification an issue? Intensity plots for a 2-element sample 1. No matrix effects 2. Apsorption by matrix dominates 3. Absorption by analyte dominates 4. Analyte line is enhanced by matrix

14 Routes and Steps of Quantification Primärintensität Peakidentifikation Selbstabsorption Peaküberlagerung Leichte Matrix Untergrund Sekundärfluoreszenz Detektoreffekte n e tr n k e Sp uatio l a v e Spektrum Netto intensitäten Fun d Par ament ame ale ter M. Em p M. irische Zusammensetzung der Probe

15 Spectrum evaluation Spectra of solid state det. And of wavelength systems detail

16 Solid State Detectors spectrum fitting

17 Enhanced uncertainty due to overlap with escape peak

18 Routes and Steps of Quantification Primärintensität Peakidentifikation Selbstabsorption Peaküberlagerung Leichte Matrix Untergrund Sekundärfluoreszenz Detektoreffekte n e tr n k e Sp uatio l a v e Spektrum Netto intensitäten Fun d Par ament ame ale ter M. Em p M. irische Zusammensetzung der Probe

19 X-ray fluorescence intensity of bulk samples Complex radiation transport is simplified to primary fluorescence intensity + secondary fluorescence intensity

20 Sherman s approach for intensity calculation Equation di, monochromatic Ω dn i = wiε i 4 π Model for X-ray fluorescence production Integration over sample thickness µ S ( E0 ) ρx µ S ( Ei ) ρx τi ρdx jiωi pi N I 0 exp sin Ψ0 sinψ 0 sinψ det

21 Sherman s approach for intensity calculation Equation di, monochromatic Integration over excitation spectrum NIo Sample is flat and homogeneous τi N i = wi K i * N I 0 µi with µ = w * i j ( µ0 j j sinψ 0 + µ ij sinψ det )

22 Use of Sherman s equation for quantification wi = K iτ i N I0 µ i* Ni (1 + S i ) but µ = f (w ) * i Initial guess for w Iterative solution

23 Stratified materials Sketch primary intensity ( ) 1 exp µ Q N i = wi K i τ i N I0 * µi * i

24 Stratified materials Primary fluorescence intensity + intra-layer enhancenment + inter-layer enhancenment

25 Use of Sherman s equation What is the information depth of my sample? Is the homogeneity of my sample sufficient?

26 Information depth Equation di Definition effective mass absorption coefficient Example figures Definition of information depth: Half of the total intensity comes from above Simplifications: * Only consider attenuation of fluorescence line * 1/2 1/e Mean free path length 1/µρ is an estimate for the information depth

27 Information depth Equation di Definition effective mass absorption coefficient Example figures Mean free path in µm Fe Sn SiO2 (2 g/cm3) 70 3,500 O (1 g/cm3) ,000

28 Calculation of transmission

29 Sample inhomogeneity Respective LambertBeer equations exp( ( wa µ A + wb µ B )Q ) Use Q = exp( wa µ A Q ) exp( wb µ BQ ) exp( wa µ A Q ) + exp( wb µ BQ ) Q = ρd

30 Sample inhomogeneity particle size «particle mean free path length information depth» sample mean free path length

31 Routes and Steps of Quantification Primärintensität Peakidentifikation Selbstabsorption Peaküberlagerung Leichte Matrix Untergrund Sekundärfluoreszenz Detektoreffekte n e tr n k e Sp uatio l a v e Spektrum Netto intensitäten Fun d Par ament ame ale ter M. Em p M. irische Zusammensetzung der Probe

32 Empirical methods for quantification Standard addition, internal standard, etc. Fingerprint Influence coefficients method

33 Influence coefficients ( ) + m N (1 + m I ) wi = w0i + mi N i 1 + j α ij w j wi = w0i i i j ij j Best precision achievable (< 1%) Two reference materials per coefficient Applied in analysis of steel, gold, concrete production

34 Fin Part 1