spectrometers: Conceptual design, Qualitative analysis, advantages - - PowerPoint PPT Presentation

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, 4th June 2019

X-Ray Regime of Energies - Wavelengths: 0.2 keV - 98 keV 40 Å - 0.13 Å

E: energy (keV) λ : wavelength (Å)

Basic properties of X-rays

𝐹 𝑙𝑓𝑊 = 12.398 ሻ 𝜇(Å

Refractive index: phase term attenuation term

Andreas Karydas, ICTP, Tuesday, 4th June 2019

X-ray Scattering Interactions with atoms

E0>>Binding Energy Ei=E0 : Coherent (Rayleigh), it occurs mostly with inner-shell atomic electrons Ei < E0: Incoherent (Compton), it occurs mostly with outer, less bound electrons

Andreas Karydas, ICTP, Tuesday, 4th June 2019

X-ray Scattering Interactions with atoms

E0>>Binding Energy Ei=E0 : Coherent (Rayleigh), it occurs mostly with inner-shell atomic electrons Ei < E0: Incoherent (Compton), it occurs mostly with outer, less bound electrons

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Photoelectric interaction

Basic Condition: E0 >= Binding energy of the inner-shell electron

Photoelectric cross section: 𝜐 ~ Ε−3.5 𝜐 ~ Ζ3 𝑢𝑝 4

Andreas Karydas, ICTP, Tuesday, 4th June 2019

( )

x

C R

e I I

  + + −

 =

   

x , 

I I

Ratio photo./scat. ≈ 1000 - 10000 Photoelectric is the dominant process

X-rays interactions with matter

Beer-Lambert law

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Atomic Relaxation

hν

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Example: The fluorescence probability for Si is almost 5%, only 5 holes decay through emission of fluorescence K radiation over 100 primary ionizations

Fluorescence probability

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Emission of element ‘characteristic’ x-rays

Unique set of emission energies for each element X-ray spectroscopy within the energy range 1-30keV offers in principle the possibility to detect all the periodic table elements through their K, L or even M series of characteristic X-ray lines

Siegbahn/IUPAC notation:

Kα: K-L2+K-L3 Kβ: K-M2+K-M3 Lα: L3-M4+L3-M5 Lβ1: L2-M4 Lβ2: L3-N5

The emission of characteristic X-ray lines follows allowed electronic transitions between specific subshells LIII to K shell: EKα1 = UK- ULIII Moseley law:

2

) (

i ij ij

Z k E  −  =

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Κ L M

Nucleus

E0

Kα

Electron

Working principle: X-Ray Fluorescence

Working principle: 1) Photo-Ionization

• f atomic bound

electrons (K, L, M) /Photoelectric absorption 2) Electronic transition and emission

• f element

‘characteristic’ fluorescence radiation Incident photon Energy E0 should be adequate to ionize the atomic bound electrons E0 >= Inner shell binding energy

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Spectral interferences in XRF analysis

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Total-Reflection X-Ray Fluorescence Analysis and Related Methods, and Reinhold Klockenkämper, Alex von Bohlen

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 10

2

10

3

10

4

10

5

Zr Zn Cu Ti Nb Y Sr Br Ge Ni Co Mn Cr

Counts E (keV)

Ti Cr Mn Co Ni Cu Zn Ge KBr SrCO3 Y Nb Zr

XRF multielemental analysis: K-lines

Andreas Karydas, ICTP, Tuesday, 4th June 2019

K-Lines Spectra with Silicon Drift Detector

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, 4th June 2019

XRF spectra of nano-layered systems

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 10 10

1

10

2

10

3

10

4

10

5

Ti-L Cr-L N Ar Cl-K K-K Cl-K S Si Ni-L Al Cu-L Counts Energy (keV) Experiment PyMca Fit O

Cr/Al/Ni/Cu/Ti/ onto Si3N4 200 nm, each layer about 10 ug/cm2, ~10-40nm

1 2 3 4 5 6 7 8 9 10 10

1

10

2

10

3

10

4

Ti-KEP Cr-KEP Ni, Cu-L Ni-K Cu-K Cr-K K Cl-K Ti-K Cr-K Ni-K Cu-K Si-K Al-K

Counts Energy (keV) 10 keV excitation energy

Spectra measured at Elettra Sincrotrone Trieste, XRF beamline

Karydas et al., Journal of Synchrotron Radiation, (2018). 25, 189–203

Andreas Karydas, ICTP, Tuesday, 4th June 2019

1 2 3 4 5 6 7 8 9 10 10 10

1

10

2

10

3

10

4

10

5

V-K Al-K Ca-K Cu-K EP Ar-K Cu-Kb Cu-K Counts E (keV)

Detector Response function

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Peak shape of characteristic X-ray lines: Gaussians with tails and continuum

XRF Analytical Sensitivity

LoD: Limit of Detection

i

I

Fluorescent intensity (cps) Background intensity (cps)

B

I

i

c Analyte concentration

t t c I t I LoD

i i B i

   ) / ( / 3 ) (

(95%) CL

i i i i

N c LoD     3 ) (

B

N

i

N

LoD LoQ   3 . 3

B i

N  

Andreas Karydas, ICTP, Tuesday, 4th June 2019

XRF Information depth

Material X-ray line D (μm) Bronze 95% Cu, 5% Sn Cu-Kα 10 Sn-Kα 32 Gold 95% Au, 4.5 % Ag, 0.5% Cu Cu-Kα 1.4 Au-Lα 2 Ag-Kα 5 Egyptian Blue 20% + 80% binder Cu-Kα 270 Ca-Kα 37 Si-Kα 6

) ( 1

i T E

D    =

Critical thickness The information depth depends

• n:
• the sample matrix composition
• analyte energy
• incident beam energy

(spectrum)

• geometry (incident/outgoing

angles)

2 1

sin / ) ( sin / ) ( ) , (     

i s

• s

i

• T

E E E E +



Andreas Karydas, ICTP, Tuesday, 4th 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

• ptimised (30-50 W (air

cooled) ▪ Miniature X-ray tubes – geometry optimized (2W- 12W, 50kV)

• Anode material (Z)
• Window (Be, glass)
• Side or end window
• High Voltage
• Anode thickness (end-window)

Andreas Karydas, ICTP, Tuesday, 4th June 2019

2 4 6 8 10 12 14 10

9

10

10

10

11

Rh- L lines

15 kV Si excitation

Photons/(KeV x sr x mA x sec)

Energy (KeV)

2 4 6 8 10 12 14

10

9

10

10

10

11

Fe-K edge

Continuum exciting Fe

Rh-L lines

15 kV

Fe excitation Photons/(KeV x sr x mA x sec)

Energy (KeV)

Tube excited XRF analysis

4 8 12 16 20 24 28 32 36 40 10

9

10

10

40 kV

Cu excitation

Energy (KeV)

Photons/(KeV x sr x mA x sec)

4 8 12 16 20 24 28 32 36 40 44 10

9

10

10

continuum

exciting Ag-K 40 kV

Ag excitation

Energy (KeV)

Photons/(KeV x sr x mA x sec) Ag-K edge

15 kV Unfiltered 40 kV Filtered

Andreas Karydas, ICTP, Tuesday, 4th June 2019

XRF instrumentation: X-ray Sources-Detectors

Anode materials: Rh, Ag, Mo Focus spot size 50-150 μm Exposure < 0.5 mR/hr Oxford Model: XTF5011 Newton M47, 50kV 10W X-ray Source, 400 grs Moxtek end/side window tubes, 10W, 50kV Miniature X-ray detector

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Energy Dispersive Detector‘s technology

Central small anode contact surrounded by a number of concentric drift electrodes

Figure from Oxford Instruments Manual

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 cooling

CUBE preamplifier supports high-rate spectroscopy in XRF mapping applications, while preserving enough energy resolution at shorter shaping times. The use of short peaking times further reduces the impact of the detector leakage current on the total

• noise. Room temperature operation!

Andreas Karydas, ICTP, Tuesday, 4th June 2019

X-ray Optics in XRF analysis: Focusing

𝜘𝑑𝑠𝑗𝑢(𝑒𝑓𝑕𝑠𝑓𝑓𝑡ሻ ≈

1.651 𝐹(𝑙𝑓𝑊ሻ 𝑎 𝐵 𝜍( 𝑕 𝑑𝑛3ሻ

𝑜 ≈ 1 − 𝜀 𝜘𝑑𝑠𝑗𝑢 = 2𝜀

➢ Polycapillary full lens ➢ Collimator ➢ Curved crystals Divergence

P1

P2

R D

Andreas Karydas, ICTP, Tuesday, 4th June 2019

 

2 1 2 2 2

) ( ) ( ) ( f E FWHM R d E T E G

in col lens

   =   =

5 10 15 20 25 30 35 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Lens Transmission (a.u.) E (keV)

Characteristics of Polycappilary X-ray lenses

• Spot size –FWHM (E)
• Gain Factor – G(E)
• Focal distance

Lens Transmission efficiency

T(E)=transmission efficiency

Andreas Karydas, ICTP, Tuesday, 4th June 2019

• Spot size –FWHM (E)
• Gain Factor – G(E)
• Focal distance

2.4 keV 3.3 keV 3.7 keV 5.9 keV 10.5 keV 14.1 keV 6.9 keV 4.5 keV 8.6 keV

• T. Wolff et al, JAAS, 2009 24 669

Characteristics of Polycappilary X-ray lenses

Knife edge scan

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Portable milli-beam spot XRF spectrometer

Handheld

1 2 3 4 6 5 5 6

• 1. X-ray source,
• 2. X-ray detector
• 3. Beam shutter
• 4. Rotatable filter wheel
• 5. Lasers for sample

positioning

• 6. Collimating parts

Source Detector Sample

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Portable Micro XRF spectrometer

Customized design

• f the micro-XRF

probe by IFG based

• n ARTAX model by

Bruker-AXS

X-ray Detector Pointer

Headed Eagle lapis lazuli and gold 3000 B.C. Early Bronze Age Archaeological Museum of Damascus October 2007

X-ray lens

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Fluorescence production: Selective excitation

Optimizing the energy of the exciting beam for maximizing the produced characteristic X- ray intensity XRF K-shell fluorescence cross section

KX K

• K
• KX

F E E   =    ) ( ) (

K-shell photoelectric cross section K-shell fluorescence yield Transition probability for Kα emission

) (

• KX E



Andreas Karydas, ICTP, Tuesday, 4th June 2019

ψ2 dx ψ1 I0(Eo) dI(Εi)

1 – Rate of incident photons at depth x 2 – Probability of production of a vacancy in the atomic shell s (s=K, L1, L2, L3, …) of the element i across the path dx/sin ψ1 3 – Probability of emission of a photon of energy EF of the element i among the family of emitted photons corresponding to transitions to the atomic shell 4 – Transmission of the fluorescent radiation in the outgoing path towards the detector 5 – Overall detection efficiency for EF photons

x dΩ

Fluorescence Emission Rate

d

1 2 3 4 5

𝑒𝐽𝑗 𝐹𝑗 = 𝐽0 exp − 𝜈 𝐹0 𝜍𝑦 sin 𝜔1 ⋅ 𝐷𝑗𝜐𝑗𝑡 𝐹0 sin 𝜔1 𝜍𝑒𝑦 𝜕𝑗𝑡𝑆𝑗𝑡(𝐹𝐺ሻ exp − 𝜈(𝐹𝑗ሻ𝜍𝑦 sin 𝜔2 𝑒Ω 4𝜌 𝜁𝐸(𝐹𝑗, Ωሻ

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Monochromatic excitation, Emission angle, detection efficiency constant within the solid angle

𝐽𝑗(𝐹𝑗, 𝐹0ሻ = 𝐽0𝐷𝑗𝜕𝑗𝑡𝑆𝑗𝑡(𝐹𝐺ሻ 𝜐𝑗𝑡(𝐹0ሻ 𝜈(𝐹0ሻ + 𝜈(𝐹𝐺ሻ sin 𝜔1 sin 𝜔2 1 − exp − 𝜈(𝐹0ሻ sin 𝜔1 + 𝜈(𝐹𝑗ሻ sin 𝜔2 𝜍 𝑒 Ω 4𝜌 𝜁𝐸(𝐹𝑗ሻ

Fluorescence Emission Rate: Approximations

Thick sample: 𝜍 𝑒 ≫ 1

𝐽𝑗(𝐹𝑗, 𝐹0ሻ = 𝐽0𝐷𝑗𝜕𝑗𝑡𝑆𝑗𝑡(𝐹𝐺ሻ𝜐𝑗𝑡(𝐹0ሻ 𝜈(𝐹0ሻ + 𝜈(𝐹𝐺ሻ sin 𝜔1 sin 𝜔2 Ω 4𝜌 𝜁𝐸(𝐹𝐺ሻ

Thin sample: 𝜍 𝑒 ≪ 1 AND those elements not detected also contribute to the attenuation within the whole matrix!

𝐽𝑗(𝐹𝑗, 𝐹0ሻ = 𝐽𝑝 ∙ 𝐷𝑗 ∙ 𝑆𝑗𝑡(𝐹

𝑔ሻ ∙ 𝜐𝑗𝑡(𝐹𝑃ሻ ∙ 𝜍𝑒

𝑡𝑗𝑜𝜔1 Ω 4𝜌 𝜁𝐸(𝐹

𝑔ሻ

Tube excitation: 𝐽𝑗 𝐹𝑗 = ׬

𝐹=𝑉𝑌𝑗 𝐹=𝑉𝑝 𝐽 𝐹𝑗, Ε dE

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Source Detector Sample R2 R1 D1 D2

Fluorescence intensity depends on: ✓ Tube intensity, optimized spectral distribution for different elements ✓ Geometry: 1/R1

2, D1 2, 1/R2 2, D2 2

D2= diameter of detector crystal or collimator ✓ X-ray lens features (micro-XRF, spot size, focal distance, gain) ✓ Incident/outgoing sample angles ✓ Absorption in air paths, detector windows Background radiation ✓ Set-up geometry (scattering angle) ✓ Degree of exciting spectrum monochromaticity – Use of filters ✓ Proper collimation of exciting/fluorescence radiation to avoid parasitic lines

Optimization of pXRF analysis

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Optimization of excitation spectrum

X-ray tube optimization

➢Anode material ➢High voltage ➢Current ➢Incident/Exit angle ➢Type and thickness of tube window ➢Side/End window tube ➢Incident beam aperture diameter and distance

e- z

α

ε

z 2

• E

Anode thickness

Power consumption

Andreas Karydas, ICTP, Tuesday, 4th June 2019

10 20 30 40 50 1E8 1E9 1E10

Intensity (ph/s)

X-ray energy (keV)

30kV 40kV 45kV 50kV

10 20 30 40 50 1E9 1E10 1E11

X-ray intensity (ph/s) High Voltage (kV)

K L L

X-ray tube optimization – High Voltage

Rh anode, 50kV, side window, standard 78/12

Andreas Karydas, ICTP, Tuesday, 4th June 2019

5 10 15 20 25 30 35 40 45 50 0.00E+000 5.00E+009 1.00E+010 1.50E+010 2.00E+010

X-ray intensity (ph/s)

X-ray energy (keV) Rh W Cu Cr

20 30 40 50 60 70 80 1E9 1E10 1E11 1E12

X-ray intensity (ph/s) Anode atomic number

K L L

X-ray tube optimization – Anode material

Operation at 50kV, side window

Andreas Karydas, ICTP, Tuesday, 4th June 2019

• 5

5 10 15 20 25 30 35 40 45 50 55

0.00E+000 5.00E+009 1.00E+010 1.50E+010

X-ray intensity (ph/s) X-ray energy (keV)

Incidence = 6 degrees Incidence = 12 degrees

1 2 3 4 5 6 7 8 9 10

• 2.00E+009

0.00E+000 2.00E+009 4.00E+009 6.00E+009 8.00E+009 1.00E+010 1.20E+010 1.40E+010

X-ray intensity (ph/s)

X-ray energy (keV)

Be 125 um Be 250 um

X-ray tube – Incidence angle/Window

Rh anode, 50kV, side window

Andreas Karydas, ICTP, Tuesday, 4th June 2019

X-ray tube geometry – Side window

Andreas Karydas, ICTP, Tuesday, 4th June 2019

https://xray.oxinst.com/products/x-ray-tubes/

X-ray tube geometry – End window

Andreas Karydas, ICTP, Tuesday, 4th June 2019

http://www.newtonscientificinc.com/

• 5

5 10 15 20 25 30 35 40 45 50

1E7 1E8 1E9 1E10

X-ray intensity (ph/s)

X-ray energy (keV)

Reflection, 50 kV, 1mA, R1=33 mm, D1=2mm Transmission, 50kV, 0.1 mA, R1=5mm, D2=2mm 50 Watt versus 10 Watt!

X-ray tube geometry optimization

Andreas Karydas, ICTP, Tuesday, 4th June 2019

X-ray detectors optimization: Window/thickness

Andreas Karydas, ICTP, Tuesday, 4th June 2019

www.Amptek.com

1 2 3 4 5 0.0 0.2 0.4 0.6 0.8 1.0

Transmission Energy (keV)

Air 5 mm Air 10 mm Air 20 mm He 20 mm Be 1/3 mil Be 0.5 mil Be 1 mil

X-ray detection environment

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Filter modifier of the exciting beam

5 10 15 20 25 30 10 10

1

10

2

10

3

40 kV

5 10 15 20 40 60 80 100

Counts Energy (keV)

Andreas Karydas, ICTP, Tuesday, 4th June 2019

@ 19 keV: 3-4 times @ 5-15 keV: 10-100 times

  sin 2   = d n

Bragg’s Law

) ( 2398 . 1 ) ( , nm keV E hc E   = =

Filtered vs Unfiltered excitation: An example

Rh anode tube, 40 kV, low atomic number scatterer

Andreas Karydas, ICTP, Tuesday, 4th June 2019

Optimizing Det. collimation: Requirements

The detector collimation ensures:

• Detected radiation occurs only from the

sample itself

• Same solid angle is defined for low and high

energy fluorescent X-ray lines (Fe-Ka, Sn-Ka)

• Blank spectra are free of collimator material

X-rays

• Elimination to certain degree of detector

materials fluorescence

• Improved

P/B ratio by focusing only to central area of the crystal (actual use of part of the crystal effective area)

Andreas Karydas, ICTP, Tuesday, 4th June 2019

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Thank you for your attention!!!