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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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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 work with Electron Microscope and EDX. EDX – Spectra Simulation Inroduction Introduction Introduction

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Theory of Simulation Theory of Simulation 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). 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 A l br and detector-absorption l in specimen. mass absorption coefficients (µ/ ) = f (Z, E) absorption jumps of (µ/ ) with energies E C Lifshin Lifshin empiric 2.parameters Kramers X

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

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Atomic Data Library (Data Base) Atomic Data Library (Data Base) EDX – Spectra Simulation Basics 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! for quality of simulation! The accuracy of data base is crucially for quality of simulation! for quality of simulation!

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Optimization Before the Measurement: E o Optimization Before the Measurement: E o EDX – Spectra Simulation Application 15 keV 20 keV 30 keV 25 keV

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Verification: Excitation of Lines (E o ) Verification: Excitation of Lines (E o ) EDX – Spectra Simulation Application Excitation of Au-L lines (Sub-Shells !) with different E o

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Optimization / Verification: Tilt-Angle Optimization / Verification: Tilt-Angle AuAg-Alloy E o : 15 keV tilt: -30 o o Simulation Absorption-Effects: - Irregular Surfaces - Rough Specimen - Particle EDX – Spectra Simulation Application

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Optimization: Influence of Detector-Resolution Optimization: Influence of Detector-Resolution AuAg-Alloy: 125 eV vs. 165 eV EDX – Spectra Simulation Application

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Verification of Possible Overlap-Problems Verification of Possible Overlap-Problems EDX – Spectra Simulation Application 5% Pd in Pb with/without Pd

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Element-Identification (Verification of Unknown Peaks) Element-Identification (Verification of Unknown Peaks) EDX – Spectra Simulation Application Si in Specimen ? No !...with Escape... without Escape

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Element-Identification (Comparison with Real Spectra) Element-Identification (Comparison with Real Spectra)...measured spectrum...simulated spectrum Compare ! EDX – Spectra Simulation Application Additional elements ? Improve data-base ? Additional elements ? Improve data-base ? Spectrum with Ba

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EDX – Spectra Simulation Application Teaching (Simulation of EDX X-Ray Acquisition Process) Teaching (Simulation of EDX X-Ray Acquisition Process) 15s Acq.time 2000 cps „Acquisition“ ready...

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Calculation of Detection Limits Calculation 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 N DL Significance Level N S... determination is possible with spectra-simulation ! The signal/background-ratio is the base for calculations of detection limits ( P/B-ratio) Counts Counts ProbabilityProbability

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EDX – Spectra Simulation Detection Limits Detection-Limit of an Element with Different Specimens Detection-Limit of an Element with Different Specimens M DL for Pd in Te M DL for Pd in Au M L K

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EDX – Spectra Simulation Detection Limits Detection-Limits with Varying Conditions Detection-Limits with Varying Conditions Al in Cu M L K

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Al: 1 % EDX – Spectra Simulation Detection Limits Simulation of Spectra-Acquisition Near Detection-Limits Simulation of Spectra-Acquisition Near Detection-Limits MDL = 0.2 % Al: 0.3 % Al: 0.15 % #1 #2 #3 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... ?

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EDX – Spectra Simulation Detection Limits Simulation of Spectra Acquisition / Detection Limit = f (time) Simulation of Spectra Acquisition / Detection Limit = f (time) 5 s: M DL = 1.8% 5 s: M DL = 1.8% 10 s: M DL = 1.3% 10 s: M DL = 1.3% 20 s: M DL = 0.9 % 20 s: M DL = 0.9 % 50 s: M DL = 0.6 % 100 s: M DL = 0.4 % 100 s: M DL = 0.4 % detectable ! 2000 cps 1% Zr in Sn ? 1% Zr in Sn ?

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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) (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 evaluation (reconstruction) for an improvement of final result-reliabilities EDX – Spectra Simulation Summary

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