Understanding X-rays: The electromagnetic spectrum

Transcription

1 Understanding X-rays: The electromagnetic spectrum 1 ULa kev 0.09 nm BeKa 0.11 kev nm E = hn = h c l where, E : energy, h : Planck's constant, n : frequency c : speed of light in vacuum, l : wavelength E (kev) = h c l = /l (nm) or, l (nm) = h c E = /E (kev) Examples: l BeKa = nm; Hence, E BeKa = /11.27 = 0.11 kev E ULa = kev; Hence, l ULa = /13.61 = 0.09 nm

2 X-ray spectrum 2 Ti Ka Characteristic X-rays Fe Ka Intensity Bremmstrahlung (continuum) X-rays Ti Kb Fe Kb Wavelength Energy

3 Characteristic X-ray generation 3 Overvoltage, U = E/E c, > 1 E : electron beam energy E c : critical excitation energy (or, ionization energy) of shell in target atom Inner shell ionization through inelastic scattering (Ka)

4 cross-section of ionization Condition for ionization: Overvoltage 4 Best analytical condition, U 5

5 X-ray energies 5 X-ray Electron transition Energy Ka L II+III to K I E Ka = E c(ki ) - E c(lii+iii ) Kb M III to K I E Kb = E c(ki ) - E c(miii ) La M IV+V to L III E La = E c(liii ) - E c(miv+v ) Ma N VII to M V E Ma = E c(mv ) - E c(nvii )

6 Characteristic X-ray energy and critical excitation energy 6 The energy required to generate UKa must be higher than the critical excitation energy of the U K-shell, E c(k), i.e., overvoltage E/E c(k) > 1 To calculate E c(k) : Start E Ka = E c(k) - E c(l) Rearrange E c(k) = E Ka + E c(l) E c(k) kev Required energy > kev Substitute E c(l) = E La + E c(m) = E Ka + (E La + E c(m) ) Substitute E c(m) = E Ma + E c(n) = E Ka + E La + (E Ma + E c(n) ) Therefore, E c(k) E Ka + E La + E Ma

7 Maximum x-ray production depth (range) 7 (Castaing s formula) R X-ray = x-ray range (maximum depth) E = electron beam energy E c = critical excitation energy of target atomic shell A = atomic weight r = density Z = atomic number

8 Maximum x-ray production depth (range) 8 Characteristic X-ray range increases as E increases, and decreases as r and rz increase

9 Electron range versus X-ray range 9 characteristic x-ray range electron range The characteristic x-ray range is always smaller than the electron range E = beam energy E c = critical excitation energy of sample atomic shell Z = atomic number A = atomic weight r = density

10 X-ray depth-distribution: the f(rz) function 10 f(drz) = intensity from a free standing layer of thickness z f(rz) at depth z = intensity from depth z divided by f(drz) where, r = density, and z = depth

11 Continuum X-ray generation 11 Electron beam Produced by deceleration of beam electrons in the electrostatic field of target atoms Energy lost by beam electrons is converted to x-ray (Maximum energy of continuum x-rays = electron beam energy)

12 Continuum X-rays: background intensity 12 Low-Z sample (Ca-Fe poor) Low background High-Z sample (Ca-Fe rich) High background Increases with sample atomic number

13 Wavelength Dispersive Spectrometer (WDS) 13 detector crystal

14 Wavelength Dispersive Spectrometer (WDS) 14 sin q = L 2R q: angle of incidence or diffraction L: distance between sample and crystal R: radius of focusing (Rowland) circle for n=1, ABC = 1l q A B q C Bragg s Law: nl = 2d sin q l d L-value : L = nl R d n: order of diffraction l: wavelength of X-ray d: lattice spacing in diffracting crystal q: angle of incidence or diffraction

15 L-value 15 Example 1. Example 2. Si Ka U Ma Energy, E = 1.74 kev Energy, E = 3.17 kev l (nm) = E (kev) l (nm) = E (kev) Wavelength, l = = nm Wavelength, l = = nm L (mm) = n l (nm) R (mm) d (nm) For n =1, R = 140, and d TAP = , L TAP = 1 x x = mm L (mm) = n l (nm) R (mm) d (nm) For n=1, R = 140, and d PET = , L PET = 1 x x = mm

16 WDS operation: detecting a specific l 16 Radius of focusing circle (R) is fixed Different wavelengths (l 1, l 2 ) can be diffracted using appropriate incidence angles (q 1, q 2 ) by changing the L-value (L 1, L 2 )

17 Diffraction angle 17 l 1 (Element 1) Different elements l 2 (Element 2) q 1 q 2 nl 1 = 2d sinq 1 nl 2 = 2d sinq 2 Wavelength being diffracted changes with the incidence angle (for the same order of diffraction, n)

18 First and second order diffractions 18 Same element n=1 n=2 q 1 q 2 D F A C B 1l = 2d sinq 1 = ABC E 2l = 2d sinq 2 =DEF path DEF = 2* path ABC Same wavelength is being diffracted at different diffraction angles; sinq 2 = 2sinq 1

19 Spectrometer movement 19 q q sample

20 Theoretical limits of spectrometer movement 20 L = 280 mm q = 90 o L = 210 mm q = 48.6 o Theoretical limits: 2R L mm L 0 mm 90 o q 0 o L = 140 mm q = 30 o L = 70 mm q = 14.5 o L = 0 mm q = 0 o

21 Actual limits of spectrometer movement 21 Actual limits: 60 mm L 260 mm 12.4 o q 68.2 o Recall sinq = L L, so L = 2Rsinq and q = sin-1 2R Hence, 2R Typically, 70 mm L 230 mm 14.5 o q 55.2 o for L = 60 mm, q = 12.4 o and L = 260 mm, q = 68.2 o for q = 15 o, L = mm and q =55 o, L = mm

22 2d of x-ray diffractors 22 Crystal lattices l (nm) For n=1, ~ 0.5d < l < 1.6d (L mm or q o ) Layered structures l of BeKa = nm. So, BeKa can be diffracted only by 2d > nm diffractors, e.g., LDE3H with L= mm, and LDEB or LDEBH with L=217.6 mm

23 Curved diffracting crystals 23 Johansson type bending radius: 2R polished and ground to R R Johan type only bent to 2R, not ground Peak resolution with fully focusing Johansson-type crystal: FWHM ~10 ev Some defocusing in Johan-type, but resolution is not compromised

24 X-ray focusing ellipsoid 24

25 Spectral resolution 25 Full-Width Half-Maximum (FWHM)

26 WDS vs. EDS spectral resolution 26 Peak overlaps in EDS spectrum Peak resolution with WDS (FWHM ~10 ev) is an order of magnitude better than with EDS (FWHM ~150 ev)

27 WDS detector: Proportional counter 27 Tungsten collection wire at 1-3 kv voltage Pulse voltage generated is proportional to the voltage in the collection wire Signal is amplified through a chain of outer-shell ionizations in the gas by the incoming X-ray Flow counter: P-10 gas (90% Argon + 10% methane quenching agent) Polypropylene window Sealed counter: Xenon gas Beryllium window

28 (for pulse voltage) Signal amplification 28 (Voltage) Typical voltage range in the proportional counter region for a W wire: V The amplification factor is proportional to the voltage in the collection wire in the proportional counter region

29 Quantum efficiency of counter gas 29 Highest when the incoming X-ray is least absorbed by the gas Decreases when the X-ray is absorbed by ionizing an inner shell of the gas atom, generating ArKa or XeLa Lowest when E X-ray is slightly higher than the E c(ar K-shell) or E c(xe L-shell) absorption edges Heavier elements Lighter elements Argon: long wavelength (low energy) detection Xenon: short wavelength (high energy) detection

30 Proportional counter setup 30 Proportional counter output: Voltage pulses from noise and x-ray signal de baseline window A Single Channel Analyzer (SCA) allows only pulses from x-rays to pass through the energy window DE An SCA scan shows the variation in count rate as a small voltage window (de) is moved across the voltage range Baseline and window voltages (DE) are set to filter out noise DE is determined through Pulse Height Analysis (PHA) using an SCA scan

31 Pulse voltage in SCA scan 31 SCA scan Energy of SiKa (1.739 kev) is ~1.39 times the energy of MgKa (1.253 kev) If the pulse for MgKa is at 4 V, the pulse for SiKa will be at 4 x 1.39 = 5.56 V Pulse voltage is proportional to energy of the X-ray being detected

32 Escape peak in SCA scan 32 Escape peaks fluoresced by incoming X-ray: P-10 counter: ArKa Energy difference between incoming X-ray and ArKa or XeLa SCA scan Xenon counter: XeLa If the pulse for NiKa (7.47 kev) is at 5.20 V, the XeLa (4.11 kev) escape peak will be at ( ) = 1.84 V

33 Proportional counter window 33 Mylar has lower transmittance than polypropylene, especially for light element x-rays Thin windows are better for light elements 1 mm thick polypropylene window transmits ~60% of the F Ka 6 mm thick polypropylene window transmits only ~5% of the F Ka

34 Detector slit 34 Positioned at focal point of diffracted x-rays on the Rowland (focusing) circle Cuts off stray x-rays and electrons Open: LDE P-10 flow counter Very light elements (very low E, very long l) mm: PET or LIF Xe sealed counter Heavy elements (high E, short l) 300 mm: TAP P-10 flow counter Light elements (low E, long l) 300 mm PET or LIF P-10 flow counter Heavy elements with Mylar film: (high E, short l)

35 Semi-quantitative analysis 35

36 Compositional imaging with X-rays: elemental mapping 36 Beam-rastered image: electron beam rasters over the area to be imaged Stage-rastered image: electron beam is stationary, stage moves

37 Background in x-ray spectra 37 Ti Ka Peak intensity Characteristic X-rays Intensity Fe Ka Peak minus Background Bremmstrahlung (continuum) X-rays Ti Kb Fe Kb Background intensity Wavelength Energy

38 Background in x-ray image 38 Zn-Sn composite Background image Zn-rich phase (low Z) Sn-rich phase (high Z)

39 X-ray defocusing in beam-rastered image 39

40 Image quality of x-ray maps 40 Two factors: Image resolution: number of points measured within the imaged area X-ray Signal: beam current and counting (dwell) time per point

41 Combined WDS and EDS X-ray mapping 41

42 Combined BSE, CL and X-ray mapping 42