Electron Microprobe Analysis X-ray spectrometry: 1. X-ray generation and emission 2. X-ray detection and measurement
X-ray energy and wavelength E=hν h : Planck's constant (6.626x10-34 Joule.sec or, 6.626x10-34 /1.6021x10-16 kev.sec) ν : frequency (= c/λ) (c : speed of light in vacuum = 2.99793x10 17 nm/sec λ : wavelength) λ (nm) = c/ν = hc/e = 1.2398/E (kev)
The electromagnetic spectrum UMα 3.17 kev 0.39 nm SiKα 1.74 kev 0.71 nm λ (nm) = 1.2398/E (kev)
Characteristic X-ray X generation Inner-shell ionization X-ray and electron transition Kα: L to K-shellK Lα: M to L-shellL Mα: N to M-shellM Kβ: M to K-shellK Lβ: N to L-shellL K-shell L-shell Ti KαK Fe KαK Flowchart for X-ray generation Ti KβK Fe KβK
Overvoltage U = E/E c where, E is the beam energy (usually 10-25 kev) E c : critical excitation energy or absorption edge of the shell Condition for inner shell ionization : U > 1
Kα Lα Mα kev
Inner-shell ionization cross-section section Q = 6.51x10-20 [(n s b s )/(UE 2 c )] ln(c s U ) Q : Cross section n s : # electrons in the shell b s,c s : constants
Inner-shell ionization cross-section section Al K-shell Best analytical condition, U 5 E c = 1.84 kev
X-ray production range An analogous expression to the electron range ( R = KE n 0 /ρ ) X-ray Range: R = K(E 0n - E n c )/ρ where, K = 0.064, n = 1.68 Always smaller than electron range
X-ray depth-distribution: distribution: φ(ρz) ρ = density z = depth φ(δρz) = X-ray intensity generated in a freestanding layer φ(ρz) at depth z = Intensity from depth z divided by φ(δρz)
Wavelength Dispersive Spectrometer (WDS) WDS WDS detector crystal
Bragg s s Law d = lattice spacing θ = angle of diffraction n = order of diffraction (any integer) nλ = 2 2d sin θ = path length ABC
WDS: Focusing geometry angle of diffraction take-off angle L = nλ.r/d
WDS: Analyzing crystals Name 2d (Å) Type Elements usually analyzed LDEC 98 Ni/C Multi-layer B-O (Kα), optimized for C analysis STE 100.4 Pb stearate B-O (Kα), optimized for C analysis LDE1 59.8 W/Si Multi-layer C-F (Kα), optimized for O analysis TAP 25.8 Thallium acid phthalate Na-P (Kα); Cu-Zr (Lα); Sm-Au (Mα) PET 8.742 Pentaerythritol S-Mn (Kα); Nb-Pm (Lα); Hg-U (Mα) LIF 4.028 Lithium fluoride Ti-Rb (Kα); Ba-U (Lα)
Curved diffracting crystals Johansson type bending curvature: 2R polished and ground to R R Johan type only bent to 2R, not ground FWHM of fully focusing Johansson-type crystal ~10 ev Some defocusing in Johan-type, but resolution is not compromised
WDS: Analyzing crystals Vertical, horizontal and tilted spectrometers
WDS: X-ray X detector (proportional counter) Flow proportional counter
WDS detector: Proportional counter Tungsten collection wire set at 1-31 3 kv bias Flow counter: 90% Ar +10% CH 4 (P-10); poly-propylene propylene window Sealed counter: Xe or Kr; Be window
Amplification in proportional counter Collection wire bias range (applied potential): 1-31 3 kv (Bias) Bias is set so that amplification is in the proportional region
Counting efficiency of gas in proportional counter Heavy elements Light elements Gas used for long wavelengths: 90% Ar +10% CH 4 (P-10) Gas used for short wavelengths: Xe or Kr
WDS: changing the angle of diffraction
Theoretical and actual limits of spectrometer movement
WDS signal processing Single channel analyzer (SCA) and pulse height analysis (PHA) window baseline Only pulses in this voltage interval are counted
WDS: PHA setup Single Channel Analyzer (SCA) scan
Imaging with X-rays: X compositional mapping Mg Na Ca Ti Beam-rastered image: electron beam rasters over the area to be imaged Stage-rastered image: electron beam is stationary, stage moves
X-ray image artifact: background Zn-Sn composite
Continuum X-rays X (background) Characteristic X-raysX Continuum X-rays Generated by deceleration of beam electrons in the Coulombic field of outer shells of target atoms. X-ray background (maximum energy = electron beam energy, E0) E
Continuum X-rays: X background artifact A composite made of 2 materials is being mapped: 1 2 Neither material contains Cr But background counts for Cr : in 1 in 2 Therefore, if a Cr X-ray X map is acquired, material 2 will show higher Cr than material 1
Defocusing in beam-rastered WDS X-ray X maps
Stage-rastered X-ray mapping parameters Resolution : # pixels (step size) Signal: beam current and dwell time/point Acquisition time increases as resolution increases as dwell time/point increases
Large area stage-rastered images BSE O Mg Ca Resolution: 20 μm/step Image size: 30mm x 15mm #steps (pixels): 1500x750 Dwell time: 30 msec/point Beam current: 30 na Voltage: 15 kv Avg stage speed: 0.71 mm/sec Acquisition time: 14 hrs Scale bar: 4 mm
MIT OpenCourseWare http://ocw.mit.edu 12.141 Electron Microprobe Analysis by Wavelength Dispersive X-ray Spectrometry January (IAP) 2010 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.