CH08 - X Ray Spectroscopy Flashcards
XRF, XRA, XRE
Every day life
Security, Medicine, Astronomy, Chemistry
The emission maxima depends on…
the element (exclusively)
Emission spectrum results from transitions of internal electrons. Therefore it does not depend on ionization state (unlike AES) or bonding.
Incoming radiation hits which electron?
the lowest energy electron (K)
what happens to this electron?
it gets kicked out
formula for change in E
change in E= E-E0
E0 is
K orbital (lowest energy)
E in E-E0 is
the incoming radiation
What happens to E0 when it is kicked out?
Another electron from a higher energy will take its place
formula for K alpha line energy
change in E= E1-E0 = K alpha line energy
The first xray emission line is from
Relaxation of whichever electron is taking the place of the electron being kicked out
The second E in E-E0 is…
Whichever orbital the electron being kicked off is located
Remembering E formula
Where it’s currently at - where its going ,= letter orbital where its going
Moseley’s law
The square root of the frequency of the x-ray emitted by an atom is proportional to its atomic number
Xray Absorption formula
I=I0e ^ -umpx
“Beer’s law” for XRA
Meaning of I in absorption formula?
transmitted power
Meaning of I0 in absorption intensity formula?
incident power
um=
mass absorption coefficient (units: (cm2)/gr)
- Characteristic value depends on element and absorbed wavelength.
- Does not depend on chemical or physical state of element.
p=
(It’s actually “rho”)
sample density (units: gr/cm)
“rho”
x=
sample thickness (units:cm)
Fluorescence
sample placed in a beam of Xrays. Some absorbed.
qualitative analysis with XRF?
xrays of characteristic wavelengths emiited
quantiative analysis with XRF?
Emission intensity proportional to element concentration
Bragg Equation
nλ=2dsinθ
n - differaction order
lamba - x ray wavelength
d - interplanar spacing
theta - x-ray angle of incidence
unit cell
smallest volume element thats completely represenative of the whole crystal
Miller Indices
integers of any family of parallel planes. the integers are recipricals of the coordinates in which the planes intersect x, y and z axes.
Given the set of Miller indices (4,4,0), (3,3,0), (2,2,0) for parallel crystal planes, which is the closest to the origin of
the set of planes?
(4,4,0), because indices are reciprocals of actual intersection points on axes.
essential components
source of monochromatic xrays (Cu anode),sample holder (spins),detector,data recording & processing
goniometer
heart of diffractometer\XRF spectrometer. keeps source, sample, & detector in correct relative positions & permits scattering angle to be controlled
xray tubes must have
Source of electrons,high voltage source to accelerate electrons ,metal target. components are encased in a vacuum tube.
filament tubes cathode filament
tungsten
filament tubes anode
Cu
filament tubes - what emits xrays
focal spot on anode
how do filament tubes work
- The cathode (filament) is heated and emits electrons by thermionic emission.
- Electrons are accelerated toward cathode by a sufficiently high voltage between the cathode and anode (2-100kV)
- Upon striking it, the electrons
give up their energy at the metallic surface of the anode. If the electrons have been accelerated
to a high enough velocity, energy
is released as X-rays (0.1–100 Å) - xray escape from windows made of material of low coefficient of absorption (Be, Al, mica)
2-100kV is the voltage range that can emit electrons. tube voltage is usually 4-50kV.
filament tubes disadvs
expensive, limited life time
filament tubes operate at approx
4-50kv
Kb & Ka
numerical subscripts relate to relative intensities ,Kb never resolved, only Ka noticeable ,Kb removed by filter
what b filter used for Cu tube
Ni b filter
what is used as a filter
the element preceding the target element in the periodic table
beta filter may be inserted in
either primary beam or secondary beam
backscattered electrons - the higher the atomic number of the atoms,
the more backscattered electrons are bounced back out
xray spectrum shows distribution of xray intensity as a function of either…
wavelength - wavelength dispersive spectrum WDS,or quantum energy - energy dispersive spectrum EDS
wavelength dispersive spectrometers
radiation source -> sample -> wavelength selector -> detector
energy dispersive spectroscopy
radiation source -> sample -> detector
EDS
quick & simple analysis & data collection ,complete spectrum of energies obtained simultaneously
WDS
spectrum acquired sequentially as full wavelength range is scanned ,takes longer
ED detectors advs
fast qualitative analysis ,analyses all elements at once ,less expensive, simpler
ED detectors disadvs
low count rates,poor resolution ,limited detection limits,poor light element detection
WD detectors advs
high count rates,good resolution,good detection limits ,excellent quantitative analysis & light element detection
WD detectors disadvs
slow qualitative analysis ,analyses 1/few elements at a time ,more expensive & complex
WDS uses a crystal of
known d spacing to disperse xrays according to their wavelengths (analyzing crystal). Serves as a monochromator for X Rays.
WDS crystal examples
LiF,PET,quartz
WDS detector examples
scintillation counter ,sealed or gas flow proportional counter (the latter are gas filled detectors)
EDS uses charge generated by
xray photons within a semiconductor wafer (Si(Li) or Ge type)
what does EDS require
pulse processor ,multi channel analyser MCA
EDS - some detectors need to be
cooled with liquid Ni or He to function correctly
line spectrum
pattern of variably intense emissions at definite wavelengths
characteristic xrays
xrays forming part of a line spectrum of an element
Manifests as sharp lines on brehmsstrahlung.
real xray spectra combine
bremsstrahlung with characteristic xrays
characteristic xrays result when
beam electrons eject inner shell electrons of specimen atoms
continuum (bremsstrahlung) xrays result when
beam electrons interact with nucleus of specimen atoms.
Each electron generally
undergoes a series of collisions.
Each collision resulting in a photon of slightly different energy, creating emissions on a continuum of wavelengths.
Each electron
EDS plot - duane-hunt limit
high energy value where continuum goes to 0
Continuum radiaiton intensity vs wavelength (or energy) plot, like figure 8.11 (a)
bremsstrahlung
braking radiation, loss in energy as electron brakes emitted as photon
Differences between XRF and XRE
XRF = X Ray Fluorescence -Electrons are ejected using radiation (X-ray of higher energy).
XRE = X Ray Emission
-Electrons are ejected by incident particles such as high energy electrons and ions (do they have to be charged?)
K Line
X-ray emission lines from electron transitions terminating in the K shell.
Similar definition exists for L shell
Auger Electron
The energy released when a core electron vacancy is filled knocks an electron out of the M shell. This electron is called an Auger Electron.
Origin of absorption edges?
- As the wavelength of the X-ray decreases, its energy increases, its penetrating power increases, and the percent absorption decreases (downward slope).
- As the wavelength
decreases further, the X-ray eventually has sufficient energy to displace electrons from the K shell => abrupt increase in absorption => K absorption edge is observed.
Goniometer setup for WDXRF\XRD\XRA?
Analyzing crystal angle: θ
Detector angle: 2θ
The wavelengths of the absorption edges and of the corresponding emission lines do not coincide. Why? Which one is longer?
- energy required to dislodge an electron from an atom (absorption edge energy) is greater than the energy released when the dislodged electron is replaced by anouter shell electron (emitted X-ray energy).
- (absorption edge wavelength) < (emmited X-ray wavelength)
What is a primary X-ray beam?
In XRF, the beam of exciting X-rays is called the primary beam
What is a secondary X-ray beam?
In XRF, the X-rays emitted from the sample (as a result of excitation by a primary x-ray beam) are called secondary X-rays
The primary X-ray beam must have a λmin that is ____
than the absorption edge of the element to be excited.
Shorter - so it is energetic enough to eject an electron (be absorbed).
XRD (X Ray Differaction)
Determines crystal structure from diffraction pattern of X-Rays from a crystal.
Name 3 X-Ray Sources
- X-ray tube (filament tube): Most common source in XRD, XRF. Beam of high-energy electrons. Produces a broadband continuum of x-radiation (brehmsstrahlung from impact with a metal target) and element-specific X-ray lines
(from ejecting inner core electrons from target metal) - “XRF source”: a primary X-ray beam to eject inner core electrons to produce a secondary X-ray beam.
- Gamma rays from a radioactive isotope: the gamma rays are high energy X-rays.
Mosley’s law describes the relationship between ______ number and wavelengths of ______, while Duane-Hunt law describes minimum wavelength of _____ as a function of _____.
- Atomic Number
- Characteristic Lines
- All emmited X rays (including background brehmsstrahlung)
- Accelerating voltage
In XRF analysis, the target element of the primary source should have a _____ atomic number than the elements being examined in the sample
Greater
(Sample atoms are excited by characteristic lines, because those are more intense. Wavelengths of characteristic lines depend on Z, according to Moseley’s law)
In XRF, sample atoms are excited by _____, because those are more intense.
characteristic lines
In Moseley’s law, Why is the wavelength inversely dependent of Z?
Greater Z => more protons in nucleus => electrons are pulled down by a stronger force toward the nucleus => K_alpha transition emits more energy (shorter photon wavelength)
How does a gas filled X ray detector work?
- X Rays fall on He gas in the tube, causing the following interaction:
hν+He→He+ + e- + hν - The ejected photoelectron has a very high kinetic energy (which is proportional to the X Ray wavelength).
- The photoelectron loses this kinetic energy by colliding with and ionizing many additional gas molecules as it moves to
the center wire (by applied potential). - The number of electrons created in 4 is proportional to the electron’s kinetic energy and therefore to the photon’s energy.
- The gas filled tube is operated within the “proportional counter” voltage range, where the current pulse is proportional to the energy of the incoming X-ray photon (in the “geiger counter zone” it is no longer proportional).
What are escape peaks (in a gas filled proportional counter)?
- An X ray photon ejects an inner shell electron of the filler gas and loses some of its energy (instead of transferring it as kinetic energy to photoelectrons).
- As a result, the photon creates an artefact peak at a wavelength corresponding to the energy:
E’=E(incoming X ray)-E(filler gas characteristic X Ray)
This peak is called an Escape Peak
How does a scintillation detector work?
(1) formation of a photoelectron in the NaI(Tl) crystal after an X-ray photon hits the crystal
(2) emission of visible light photons from an excited state in the crystal, (3) production of photoelectrons from the cathode in the photomultiplier
(4) electron multiplication.
Why is a PMT detector not enough for X rays?
PMT’s do not react to wavelengths in X ray (only UV Vis). The X rays must be first converted to UV Vis by exciting crystal electrons (NaITl crystal), as in a scintillation detector.
What are sum peaks in EXRF and what causes them?
A single peak is registered at an energy that is the sum of the two peaks.
Cause: two high-intensity photons arrive too close in time, signal processing electronics cannot separate them.
What causes escape peaks in EDXRF?
X rays from fluorescing analyte ejects an electron from K level of detector element (e.g. Si), “loses” part of its energy and creates an absorption line at lambda(analyte element)-lambda(detector element)
In XRA, X-rays emitted from a particular element will be absorbed by elements with ______ atomic number.
A lower
In XRD, it is possible to draw an
infinite number of crystal planes in three dimensions, but only those planes with ____ on
them reflect X-rays.
electron density
(reflections occur as a result of interactions between incident EM radiation and electrons on reflecting surface)
_____ can be used to measure d, the distance between planes of electron density
in crystals and is the basis of X-ray crystallography.
Bragg equation nλ=2dsinθ
Can you use XRD to characterize structure of liquid, gases and amorphous polymers?
No, because these lack an ordered structure.
In XRF, Energy of the emitted X-ray is independent of _____ of the element; therefore, XRF is
generally considered to be an ______.
- The chemical state
- Elemental analysis method
In general, X rays wavelengths are indepnendent of the chemical state of the atom..
Low-Z elements (3< Z < 11) such as Be cannot be analyzed in ____.
Air (Low Z=>long fluorescence lambda=> X-rays absorbed by air)
XRF allows _____ analysis.
Multielement
In multielement analysis, why is it important to have a high acceleration voltage (as high as 50kV) in X ray tube?
High acceleration voltage => electrons which excite target atoms are higher in energy => target atoms emit X rays in broader emission spectra
Not all of the electrons’ energies go to the target atoms so it’s better to set them with a high energy to begin with…
