Chap. 1 (Introduction), Chap. 2 (Components and Circuits)

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Transcript Chap. 1 (Introduction), Chap. 2 (Components and Circuits)

Atomic X-Ray Spectrometry
• Emission, absorption, scattering, fluorescence
and diffraction
•
•
•
•
•
Fundamentals
Instruments
X-ray Fluorescence
X-ray Absorption
X-ray diffraction
8-1
Fundamentals
• X-rays
 X-ray wavelengths from 1E-5 angstrom to
100 angstrom
De-acceleration of high energy electrons
Electron transitions from inner orbitals
* Bombardment of metal with high
energy electrons
* Secondary x-ray fluorescence by
primary x-rays
* Radioactive sources
* Synchrotron sources
8-2
Continuum sources
• Cathode produced x-rays
 Short wavelength limit (lo)
 Dependent upon accelerating voltage, not
material
 X-ray produced by electron de-acceleration
 Line spectra
 Requires minimum voltage
 Electronic transitions amongst inner atomic
orbitals
* K and L lines
8-3
X-ray generation
8-4
8-5
Composite X-ray
8-6
8-7
Absorption Spectra
• Edge
keV
A
• K
115.6061
0.1072
• L-I
21.7574
0.5698
• L-II
20.9476
0.5919
• L-III
17.1663
0.7223
• M1
5.5480
2.2348
• M2
5.1822
2.3925
• M3
4.3034
2.8811
• M4
3.7276
3.3261
• M5
3.5517
3.4908
• N1
1.4408
8.6052
• N2
1.2726
9.7426
• N3
1.0449
11.8657
U absorption edges and scattering coefficients
8-8
Absorption
Ln (P0/P)=mx
x is sample thickness
Transmitted (P) and incident (P0) beam power
m is linear absorption coefficient
Linear combinations of elements
8-9
X-ray Fluorescence
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•
•
Emission of x-ray from excited elements

Absorption removes electron

Fluorescence involves inter
electron transfers
x-ray can either be absorbed by the atom
or scattered

x-ray absorbed by the atom by
transferring all of its energy to an
innermost electron is photoelectric
effect

if primary x-ray has sufficient
energy electrons are ejected from
inner shells, creating vacancies

electrons from outer shells are
transferred to the inner shells
resulting in characteristic x-ray
 energy is the difference
between the two binding
energies of the
corresponding shells

each element produces x-rays at a
unique set of energies
The process of emissions of characteristic
x-rays is X-ray Fluorescence (XRF)

innermost K and L shells are
involved in XRF detection
8-10
X-ray Fluorescence
8-11
Diffraction
• Interaction between x-rays and electrons in
sample
 Constructive and destructive interference
 Distance between scattering points are on
same order as radiation wavelengths
diffraction
8-12
Diffraction
• Bragg equation
 sin q defines observed angles
All others destructive interference
8-13
XRD
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•
•
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Fixed wavelength, vary angle
Powder specimen
Grains act as single crystal
Plot I vs angle
 At Bragg angle produce
angle
8-14
Data analysis
Normalize data to 1st sin^2theta
Clear fractions
Speculate on hkl
Know wavelength from source, solve for a
8-15
Si glass XRD
8-16
Transducers
• Photon counting
 Gas filled counters
 Ionization due to photon interaction with gas
* Ionization chambers
* Geiger counters
* Proportional counters
 Scintillation counters
 Solid state
 Ge
 NaI
8-17
Gas filled counters
8-18
Solid state detector
4000
3500
Charactaristic X-ray
Counts
3000
2500
Photopeak
2000
Lead X-ray
1500
Backscattering
1000
Compton
Edge
500
0
0
100
200
300
400
500
Channel Number
137-Cs
8-19