Handheld and portable X-ray Fluorescence spectrometers: Conceptual design, Qualitative analysis, advantages and limitations Andreas Karydas Institute of Nuclear and Particle Physics NCSR “ Demokritos ” Agia Paraskevi Athenss, Greece karydas@inp.demokritos.gr
Outline • Principles of XRF analysis • Qualitative analysis • X-ray instrumentation: Sources, Detectors, optics • Optimization of Hand-held/portable XRF analysis Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Basic properties of X-rays X-Ray Regime of Energies - Wavelengths: 0.2 keV - 98 keV 40 Å - 0.13 Å 𝐹 𝑙𝑓𝑊 = 12.398 Refractive index: ሻ 𝜇( Å E: energy (keV) λ : wavelength ( Å ) attenuation term phase term Andreas Karydas, ICTP, Tuesday, 4 th June 2019
X-ray Scattering Interactions with atoms E i < E 0 : Incoherent (Compton), E i =E 0 : Coherent (Rayleigh), it occurs mostly with outer, less it occurs mostly with inner-shell bound electrons atomic electrons E 0 >>Binding Energy Andreas Karydas, ICTP, Tuesday, 4 th June 2019
X-ray Scattering Interactions with atoms E i < E 0 : Incoherent (Compton), E i =E 0 : Coherent (Rayleigh), it occurs mostly with outer, less it occurs mostly with inner-shell bound electrons atomic electrons E 0 >>Binding Energy Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Photoelectric interaction Basic Condition: E 0 >= Binding Photoelectric cross section: energy of the inner-shell 𝜐 ~ Ε −3.5 electron 𝜐 ~ Ζ 3 𝑢𝑝 4 Andreas Karydas, ICTP, Tuesday, 4 th June 2019
X-rays interactions with matter Beer-Lambert law , x I I 0 ( ) − + + = x I I e R C 0 Ratio photo./scat. ≈ 1000 - 10000 Photoelectric is the dominant process Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Atomic Relaxation h ν Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Fluorescence probability Example: The fluorescence probability for Si is almost 5%, only 5 holes decay through emission of fluorescence K radiation over 100 primary ionizations Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Emission of element ‘characteristic’ x -rays The emission of characteristic X-ray lines follows allowed electronic transitions between specific subshells Siegbahn/IUPAC notation: K α : K-L2+K-L3 K β : K-M2+K-M3 L α : L3-M4+L3-M5 L β 1 : L2-M4 X-ray spectroscopy within the L β 2 : L3-N5 energy range 1-30keV offers in principle the possibility to detect = − 2 E k ( Z ) Moseley law: all the periodic table elements ij ij i through their K, L or even M L III to K shell: E K α1 = U K - U LIII series of characteristic X-ray lines Unique set of emission energies for each element Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Working principle: X-Ray Fluorescence Working principle: E 0 Incident photon 1) Photo -Ionization Energy E 0 of atomic bound should be electrons K α adequate to (K, L, M) ionize the / Photoelectric L Κ M atomic absorption Nucleus bound electrons 2) Electronic transition E 0 >= and emission Electron Inner shell of element binding ‘ characteristic’ energy fluorescence radiation Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Spectral interferences in XRF analysis Total-Reflection X-Ray Fluorescence Analysis and Related Methods, and Reinhold Klockenkämper , Alex von Bohlen Andreas Karydas, ICTP, Tuesday, 4 th June 2019
XRF multielemental analysis: K-lines K-Lines Spectra with Silicon Drift Detector Ti Co Ti Cr Cu Mn Ge Br Cr Zn Sr Ni Y Nb Zr Mn 5 10 Co Ni Cu Zn 4 10 Counts Ge KBr SrCO 3 Y 3 10 Nb Zr 2 10 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 E (keV) Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Low Z XRF element analysis (down to Boron) Unterumsberger et al., dx.doi.org/10.1021/ac202074s, Anal. Chem. 2011, 83, 8623 – 8628 Andreas Karydas, ICTP, Tuesday, 4 th June 2019
XRF spectra of nano-layered systems Cr/Al/Ni/Cu/Ti/ onto Si 3 N 4 200 nm, each layer about 10 ug/cm 2 , ~10-40nm 10 keV excitation energy Cu-K 5 Cr-K Ni-K 10 Si Experiment PyMca Fit Ti-K 4 10 Al Cu-K 4 10 Si-K Cr-K Al-K Ni-L Cu-L K Cl-K 3 10 Cr-K EP 3 Ti-L Cr-L 10 Ti-K EP Counts Counts N O Cl-K 2 S Cl-K 2 10 10 K-K Ar 1 10 1 10 Ni-K Ni, Cu-L 0 10 0 10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1 2 3 4 5 6 7 8 9 Energy (keV) Energy (keV) Spectra measured at Elettra Sincrotrone Trieste, XRF beamline Karydas et al., Journal of Synchrotron Radiation, (2018). 25, 189 – 203 Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Detector Response function Cu-K 5 10 Cu-Kb 4 10 3 Cu-K EP 10 Counts Ar-K 2 V-K 10 Ca-K Al-K 1 10 0 10 1 2 3 4 5 6 7 8 9 10 E (keV) Peak shape of characteristic X-ray lines: Gaussians with tails and continuum Andreas Karydas, ICTP, Tuesday, 4 th June 2019
XRF Analytical Sensitivity LoD: Limit of Detection 3 i ( LoD ) c (95%) CL i i N i N i B I Fluorescent intensity (cps) i N i I Background intensity (cps) B N B Analyte concentration c i I / t t LoQ 3 . 3 LoD B ( LoD ) 3 i ( I / c ) t i i Andreas Karydas, ICTP, Tuesday, 4 th June 2019
XRF Information depth Material X-ray line D ( μ m) The information depth depends Bronze Cu-K α 10 on: 95% Cu, 5% Sn Sn-K α 32 • the sample matrix composition Gold Cu-K α 1.4 • analyte energy 95% Au, 4.5 % Ag, Au-L α 2 • incident beam energy 0.5% Cu Ag-K α 5 (spectrum) Egyptian Blue Cu-K α 270 • geometry (incident/outgoing 20% + 80% binder Ca-K α 37 angles) Si-K α 6 Critical thickness 1 = D T E ( ) i + ( E , E ) ( E ) / sin ( E ) / sin T o i s o 1 s i 2 Andreas Karydas, ICTP, Tuesday, 4 th June 2019
X-ray sources Synchrotron radiation High brilliance, low divergence, high polarization: Micro/Nano- XRF (< 1µm) X-ray tubes ▪ High power (~ kW) diffraction x-ray tubes ▪ Micro focus (~ 50- 100µm) anode size - Brilliance • Anode material (Z) optimised (30-50 W (air • Window (Be, glass) cooled) • Side or end window ▪ Miniature X-ray tubes – • High Voltage geometry optimized (2W- • Anode thickness (end-window ) 12W, 50kV) Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Tube excited XRF analysis Fe excitation Si excitation Photons/(KeV x sr x mA x sec) 15 kV 15 kV Photons/(KeV x sr x mA x sec) Rh-L lines 11 11 Rh- L lines 10 10 Continuum exciting Fe 15 kV Unfiltered 10 10 10 10 Fe-K edge 9 10 9 10 2 4 6 8 10 12 14 2 4 6 8 10 12 14 Energy (KeV) Energy (KeV) Ag excitation Cu excitation Photons/(KeV x sr x mA x sec) 40 kV Photons/(KeV x sr x mA x sec) 40 kV continuum exciting Ag-K 40 kV 10 10 10 10 Filtered Ag-K edge 9 9 10 10 0 4 8 12 16 20 24 28 32 36 40 44 4 8 12 16 20 24 28 32 36 40 Energy (KeV) Energy (KeV) Andreas Karydas, ICTP, Tuesday, 4 th June 2019
XRF instrumentation: X-ray Sources-Detectors Oxford Model: XTF5011 Anode materials: Rh, Ag, Mo Focus spot size 50-150 μ m Exposure < 0.5 mR/hr Newton M47, 50kV 10W X-ray Source, 400 grs Moxtek end/side Miniature X-ray detector window tubes, 10W, 50kV Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Energy Dispersive Detector‘s technology Silicon Drift Detector - Principle: The charge is drifted from a large area into a small read-out node with low capacitance, independent of the active area of the sensor. Thus, the serial noise decreases and shorter shaping time can be used Two advantages: 1) Faster counting is enabled 2) Higher leakage current can be accepted, drastically reducing the need for Central small cooling anode contact CUBE preamplifier supports high-rate spectroscopy surrounded by a in XRF mapping applications, while preserving number of enough energy resolution at shorter shaping times. concentric drift The use of short peaking times further reduces the electrodes impact of the detector leakage current on the total Figure from Oxford Instruments Manual noise. Room temperature operation! Andreas Karydas, ICTP, Tuesday, 4 th June 2019
X-ray Optics in XRF analysis: Focusing 𝑜 ≈ 1 − 𝜀 𝜘 𝑑𝑠𝑗𝑢 = 2𝜀 1.651 𝑎 𝜘 𝑑𝑠𝑗𝑢 (𝑒𝑓𝑠𝑓𝑓𝑡ሻ ≈ 𝐵 𝜍( 𝑑𝑛 3 ሻ 𝐹(𝑙𝑓𝑊ሻ ➢ Polycapillary full lens ➢ Collimator ➢ Curved crystals P2 P1 D Divergence R Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Characteristics of Polycappilary X-ray lenses • Spot size – FWHM (E) Lens Transmission efficiency 1.1 • Gain Factor – G(E) 1.0 Lens Transmission (a.u.) • Focal distance 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 5 10 15 20 25 30 35 E (keV) 2 2 ( ) T E d R = = lens in G ( E ) T(E)=transmission efficiency 2 2 FWHM ( E ) f col 1 Andreas Karydas, ICTP, Tuesday, 4 th June 2019
Characteristics of Polycappilary X-ray lenses • Spot size – FWHM (E) • Gain Factor – G(E) • Focal distance 2.4 keV 3.7 keV 3.3 keV 5.9 keV 4.5 keV 6.9 keV 8.6 keV 10.5 keV 14.1 keV T. Wolff et al, JAAS, 2009 24 669 Knife edge scan Andreas Karydas, ICTP, Tuesday, 4 th June 2019
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