EDX-Spectra Simulation

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• EDX-Spectra Simulation
• Optimization of Excitation Conditions and
  Detection Limit Calculations in EPMA
• F. Eggert, Röntgenanalytik Apparatebau GmbH,
  Berlin
• Introduction
• Theory of simulation complete spectra
• Applications of spectra simulation
• Determination of detection limits with spectra
  simulation
• Summary

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EDX Spectra SimulationInroduction

• Introduction

• Today the standardless evaluation of measured
  spectra is an established methodology in Electron
  Probe Microanalysis (EPMA) with Energy Dispersive
  Spectrometer (EDX) in Scanning Electron
  Microscope (SEM)
• New developments offer the possibility to
  calculate complete spectra in dependence to
  analytical conditions (spectra simulation).
• The basics are
• - Exact knowledge about all X-ray lines of
  elements and about other atomic data
• - Knowledge about absolute cross-sections of
  both, the Characteristic X-rays and the
  Bremsstrahlung
• - Calculation of excitation and absorption of
  X-rays in specimen and detector
  (characteristic radiation and Bremsstrahlung)
• - Calculation of the entire Bremsstrahlung-devia
  tion as the main background and simulation of
  other background components
• - Simulation of detector-resolution influence
  and count-statistics to simulate realistic
  spectra
• The content of presentation is to show the
  benefits of spectra simulation to daily
  analytical work with Electron Microscope and EDX.

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EDX Spectra Simulation Basics

• Theory of Simulation

• The ratio of emitted counts of characteristic
  X-ray quanta to the counts of emitted
  Bremsstrahlung-quanta with same energy (in an
  specified energy region) is known as P/B-ratio
  (or P/U in this equation).

• Calculation of the Bremsstrahlung deviation for
  all spectra channels taking into account the
  self-absorption Albr and detector-absorption ?l
  in specimen. ? mass absorption coefficients
  (µ/?) f (Z , E) ? absorption jumps of (µ/?)
  with energies EC

X
Lifshin empiric 2.parameters
Kramers
l is the index of current channel during spectra
calculation
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EDX Spectra Simulation Basics

• Theory of Simulation

• All line- and shell- energies
• Relative emission rates of a single shell
• Excitation of sub-shells
• Coster-Kronig transitions
• Fluoresence yields

Bremsstrahlung Lines
Escape Artefacts (ICC)
Count statistics (Noise)
____________________
Simulated Spectrum
(2000 cps, 3 minutes)
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EDX Spectra Simulation Basics

• Atomic Data Library (Data Base)

To make the simulation possible, an atomic data
library with fast access to all element specific
data is necessary
The accuracy of data base is crucially
for quality of simulation!
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EDX Spectra Simulation Application

• Optimization Before the Measurement Eo

15 keV
20 keV
25 keV
30 keV
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EDX Spectra Simulation Application

• Verification Excitation of Lines (Eo)

Excitation of Au-L lines (Sub-Shells !)
with different Eo
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EDX Spectra Simulation Application

• Optimization / Verification Tilt-Angle

AuAg-Alloy Eo 15 keV tilt
-30o...30o
Simulation Absorption-Effects - Irregular
Surfaces - Rough Specimen - Particle
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EDX Spectra Simulation Application

• Optimization Influence of Detector-Resolution

AuAg-Alloy 125 eV vs. 165 eV
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EDX Spectra Simulation Application

• Verification of Possible Overlap-Problems

5 Pd in Pb
with/without Pd
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EDX Spectra Simulation Application

• Element-Identification (Verification of Unknown
  Peaks)

Si in Specimen ?
...with Escape
... without Escape
No !
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EDX Spectra Simulation Application

• Element-Identification (Comparison with Real
  Spectra)

Spectrum with Ba
...measured spectrum
...simulated spectrum

• Additional elements ?
• Improve data-base ?

Compare !
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EDX Spectra Simulation Application

• Teaching (Simulation of EDX X-Ray Acquisition
  Process)

15s Acq.time 2000 cps
Acquisition ready ...
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EDX Spectra Simulation Detection Limits

• Calculation of Detection Limits

• The question is, whether an element in specimen
  with expected concentration is detectable or not?
• If an element is detectable... How are the
  optimal measurement and excitation conditions
  (SEM and spectrometer parameters) and how long
  does it take (acquisition time)?

• The signal/background-ratio is the base for
  calculations of detection limits
  (? P/B-ratio)

... determination is possible with
spectra-simulation !
Counts
Counts
Probability
Probability
Detection Limit NDL
Significance Level NS
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EDX Spectra Simulation Detection Limits

• Detection-Limit of an Element with Different
  Specimens

MDL for Pd in Te
M L K
MDL for Pd in Au
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EDX Spectra Simulation Detection Limits

• Detection-Limits with Varying Conditions

Al in Cu
M L K
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EDX Spectra Simulation Detection Limits

• Simulation of Spectra-Acquisition Near
  Detection-Limits

? MDL 0.2

• Significant element presence !
• Concentration below the detection-limit !
• Is it really possible ... ?

Yes ? You had luck !
Al 0.15
Al 0.3
1
2
3
Al 1
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EDX Spectra Simulation Detection Limits

• Simulation of Spectra Acquisition / Detection
  Limit f (time)

5 s MDL 1.8
10 s MDL 1.3
20 s MDL 0.9
detectable !
50 s MDL 0.6
100 s MDL 0.4
2000 cps
1 Zr in Sn ?
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EDX Spectra Simulation Summary

• It is possible to calculate the entire
  EDX-spectrum with a standardless EPMA-correction
  model taking into account all effects of
  specimen- and detector-interaction.
• Spectra simlation is useful for a better
  understanding and interpretation of measured
  spectra.
• With spectra simulation all complex effects of
  excitation, absorption and detection are shown
  very descriptive and didactically (teaching,
  coaching, )
• The simulation of several excitation situations
  gives the possibility to optimize all conditions
  even before the actual specimen maesurement and
  data acquisition.
• With spectra simulation the analyst is able to
  make estimations for detection limits.
• Effects of counting-statistics are possible to
  verify.

• Future View
• ? Application of spectra simulation for
  interactive qualitative analysis
• (displacement of simple line-mark
  identification)
• Calculation of entire spectrum for a visual
  comparison after quantitative
• evaluation (reconstruction) for an
  improvement of final result-reliabilities