EDX-Spectra Simulation Optimization of Excitation Conditions and Detection Limit Calculations in EPMA F. Eggert, Röntgenanalytik Apparatebau GmbH, Berlin

EDX-Spectra SimulationOptimization 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

• 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-deviation 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 The content of presentation is to show the benefits of spectra simulation to daily analytical work with Electron Microscope and EDX. work with Electron Microscope and EDX.

EDX – Spectra SimulationInroduction

IntroductionIntroduction

TheoryTheory of Simulationof Simulation

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EDX – Spectra Simulation Basics

• 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).

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l is the index of current channel during spectra calculation

• Calculation of the Bremsstrahlung deviation for all spectra channels taking into account the self-absorption Al

br and detector-absorption l in specimen. mass absorption coefficients (µ/) = f (Z , E) absorption jumps of (µ/) with energies EC

Lifshin Lifshin empiric 2.parameters

KramersKramersX

TheoryTheory of Simulationof Simulation

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

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


Bremsstrahlung + LinesBremsstrahlung + Lines

Escape + Artefacts (ICC)Escape + Artefacts (ICC)

Count statistics (Noise)Count statistics (Noise)

= Simulated Spectrum= Simulated Spectrum

________________________________________

(2000 cps, 3 minutes)(2000 cps, 3 minutes)

Atomic Data Library (Data Base)Atomic Data Library (Data Base)


To make the simulation possible, an atomic data library with fast access to all element specific data is necessary:

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 cruciallyThe accuracy of data base is crucially for quality of simulation!for quality of simulation!

The accuracy of data base is cruciallyThe accuracy of data base is crucially for quality of simulation!for quality of simulation!

Optimization Before the Measurement: EOptimization Before the Measurement: Eoo

EDX – Spectra Simulation Application

15 keV15 keV20 keV20 keV

30 keV30 keV25 keV25 keV

Verification: Excitation of Lines (EVerification: Excitation of Lines (Eoo))


Excitation of Au-L Excitation of Au-L lines (Sub-Shells !) lines (Sub-Shells !) with with different Edifferent Eoo

Optimization / Verification: Tilt-AngleOptimization / Verification: Tilt-Angle

AuAg-Alloy AuAg-Alloy

EEoo: 15 keV : 15 keV

tilt: -30tilt: -30oo...+30...+30oo

Simulation Simulation Absorption-Effects: Absorption-Effects: - Irregular Surfaces - Irregular Surfaces - Rough - Rough Specimen - Specimen - ParticleParticle


Optimization: Influence of Detector-ResolutionOptimization: Influence of Detector-Resolution

AuAg-Alloy: AuAg-Alloy: 125 eV vs. 165 eV125 eV vs. 165 eV


Verification of Possible Overlap-ProblemsVerification of Possible Overlap-Problems


5% Pd in Pb5% Pd in Pb

with/without Pdwith/without Pd

Element-Identification (Verification of Unknown Peaks)Element-Identification (Verification of Unknown Peaks)


Si in Specimen ?Si in Specimen ?

No !No !

...with Escape...with Escape

... without Escape... without Escape

Element-Identification (Comparison with Real Spectra)Element-Identification (Comparison with Real Spectra)

...measured spectrum...measured spectrum

...simulated spectrum...simulated spectrum

Compare !Compare !


• Additional elements ?• Improve data-base ?

• Additional elements ?• Improve data-base ?

Spectrum with BaSpectrum with Ba

EDX – Spectra Simulation

Application

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

15s Acq.time 15s Acq.time 2000 cps2000 cps

„„Acquisition“ Acquisition“ ready ... ready ...

Calculation of Detection LimitsCalculation of Detection Limits

EDX – Spectra Simulation 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)?

Detection Limit NDetection Limit NDLDLSignificance Level NSignificance Level NSS

... determination is ... determination is possiblepossible with spectra-with spectra-simulation !simulation !

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

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ProbabilityProbability ProbabilityProbability


Detection-Limit of an Element with Different SpecimensDetection-Limit of an Element with Different Specimens

MMDLDL for Pd in Te for Pd in Te

MMDLDL for Pd in Au for Pd in AuM L K


Detection-Limits with Varying ConditionsDetection-Limits with Varying Conditions

Al in CuAl in Cu

M L K

Al: 1 %Al: 1 %


Simulation of Spectra-Acquisition Near Detection-LimitsSimulation of Spectra-Acquisition Near Detection-Limits

MDL = 0.2 %MDL = 0.2 %

Al: 0.3 %Al: 0.3 %

Al: 0.15 %Al: 0.15 %

#1 #1 #2 #2 #3 #3

Yes You had luck ! Yes You had luck !

• Significant element presence !

• Concentration below the detection-limit !

• Is it really possible ... ?

• Significant element presence !

• Concentration below the detection-limit !

• Is it really possible ... ?


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

5 s: M5 s: MDLDL = 1.8% = 1.8%

10 s: M10 s: MDLDL = 1.3% = 1.3%

20 s: M20 s: MDL DL = 0.9 %= 0.9 %

50 s: M50 s: MDLDL = 0.6 % = 0.6 %

100 s: M100 s: MDLDL = 0.4 % = 0.4 %

detectable !detectable !

2000 cps

1% Zr in Sn ?

1% Zr in Sn ?

• 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:Future View:

Application of spectra simulation for interactive qualitative analysis Application of spectra simulation for interactive qualitative analysis (displacement of simple line-mark identification)(displacement of simple line-mark identification)

Calculation of entire spectrum for a visual comparison after quantitative Calculation of entire spectrum for a visual comparison after quantitative evaluation (reconstruction) for an improvement of final result-reliabilitiesevaluation (reconstruction) for an improvement of final result-reliabilities

EDX – Spectra Simulation Summary

Documents

EDX-Spectra Simulation Optimization of Excitation Conditions and Detection Limit Calculations in EPMA F. Eggert, Röntgenanalytik Apparatebau GmbH, Berlin