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XRF
A Basic Overview




     Marion E. Becker, M.S., B.S.
Introduction
    Presentation Outline

1.   X-Ray Radiation
2.   XRF Theory
3.   XRF Instrumentation
4.   XRF Hardware
5.   Summary
6.   Acknowledgements
Electromagnetic Radiation

                                                   1014Hz - 1015Hz
1Hz - 1kHz                  1kHz - 1014Hz
                                                                    1015Hz - 1021Hz
       Extra-Low    Radio          Microwave    Infrared      Ultraviolet       X-Rays,
       Frequency
         (ELF)                              Visible Light                   Gamma Rays




                   Low energy                               High energy
Theory

      A source X-ray strikes an
       inner shell electron. If at
       high enough energy (above
       absorption edge of
       element), it is ejected it
       from the atom.

      Higher energy electrons
       cascade to fill vacancy,
       giving off characteristic
       fluorescent X-rays.

      For elemental analysis of
       Na - U.
K & L Spectral Lines
                                  K - alpha lines: L shell e-
 L beta                            transition to fill a vacancy
     L alpha
                                   in K shell. Most frequent
                                   transition, hence most
                      K beta       intense peak.
                                  K - beta lines: M shell e-
                K alpha
                                   transitions to fill a vacancy
                                   in K shell.
K Shell

  L Shell
                                  L - alpha lines: M shell e-
                                   transition to fill a vacancy
            M Shell
                                   in L shell.
                 N Shell
                                  L - beta lines: N shell e-
                                   transition to fill a vacancy
                                   in L shell.
XRF Instrumentation

   The basic concept for all XRF
    spectrometers is a source, a sample, and a
    detection system.

   The source irradiates the sample, and a
    detector measures the fluorescence
    radiation emitted from the sample.
XRF Spectrometers




Wavelength Dispersive WDXRF Spectrometer




   Energy Dispersive EDXRF Spectrometer
XRF Primary Radiation Sources
For XRF, Instrumentation the source is an X-ray
tube or a sealed radioactive source:

Conventional X-Ray Tubes:

   End Window X-Ray Tubes
   Side Window X-Ray Tubes

Gamma Rays-Sealed Sources:

   Radioisotopes
Side Window X-Ray Tube

                                 Be Window
Glass Envelope                       Target (Ti, Ag,        HV Lead
                                     Rh, etc.)




                 Electron beam
                                     Copper Anode
            Filament



                                                 Silicone Insulation
End Window X-Ray Tube
X-Ray Tubes:

   Voltage determines which
    elements can be excited.

   More power = lower
    detection limits.

   Anode selection
    determines optimal source
    excitation (application
    specific).
Radioisotopes
  Isotope         Fe-55       Cm-244    Cd-109    Am-241    Co-57


  Energy (keV)    5.9         14.3,     22, 88    59.5      122
                              18.3
  Elements (K-    Al – V      Ti-Br     Fe-Mo     Ru-Er     Ba - U
  lines)
  Elements (L-    Br-I        I- Pb     Yb-Pu     None      None
  lines)



While isotopes have fallen out of favor they are still useful for
gauging applications.
Source Modifiers
Several Devices are used to modify the shape or
intensity of the source spectrum or the beam
shape.

            •   Source Filters
            •   Secondary Targets
            •   Polarizing Targets
            •   Collimators
            •   Focusing Optics
Detectors

   Si(Li) -EDXRF
   PIN Diode-EDXRF
   Silicon Drift Detectors-EDXRF
   Proportional Counters-WDXRF
   Scintillation Detectors-WDXRF
Detector Principles
A detector is composed of a non-conducting or semi-conducting material
between two charged electrodes.

X-ray radiation ionizes the detector material causing it to become
conductive, momentarily.

The newly freed electrons are accelerated toward the detector anode to produce
an output pulse.

An ionized semiconductor produces electron-hole pairs, the number of pairs
produced is proportional to the X-ray photon energy.
                         E
                    n
                         e

    where :     n   = number of electron-hole pairs produced
                E   = X-ray photon energy
                e   = 3.8ev for Si at LN2 temper atures
Wavelength Dispersive X-Ray
  Fluorescence (WDXRF)
Wavelength Dispersive XRF
Wavelength Dispersive XRF relies on a diffractive device such as
crystal or multilayer to isolate a peak, since the diffracted
wavelength is much more intense than other wavelengths that
scatter off the device.
                       Sample

                                                   Detector

                                     Collimators



                  X-Ray
                  Source
                                    Diffraction Device
Diffraction
The two most common diffraction instruments are the crystal and
multilayer. Both work according to Bragg’s formula: nλ = 2d · sinθ

 In XRF, the d-value of the analyzer crystal is known and we can solve
  Bragg's equation for the element characteristic wavelength (λ).

   n = integer
   d = crystal lattice or multilayer spacing
   θ = The incident angle
   λ= wavelength
Multilayers
While the crystal spacing is based on the natural atomic spacing
at a given orientation the multilayer uses a series of thin film
layers of dissimilar elements to do the same thing.



                                 Modern multilayers are more
                                 efficient than crystals and can be
                                 optimized for specific elements.

                                 Often used for low Z elements.
Focusing Optics
Simple collimation blocks unwanted x-rays and is a highly
inefficient method.

Focusing optics like polycapillary devices and other
Kumakhov lens devices were developed so that the beam could
be redirected and focused on a small spot. Less than 75 um
spot sizes are regularly achieved.




                            Source                   Detector
      Bragg reflection
      inside a Capillary             Polycapillary Optic
Collimators
Collimators are usually circular or a slit and restrict the size or
shape of the source beam for exciting small areas in either
EDXRF or µXRF instruments. They may rely on internal
Bragg reflection for improved efficiency.

                                Sample




                                     Collimator sizes range from 12
                                     microns to several mm
                       Tube
Soller Collimators
Soller and similar types of collimators are used to prevent beam
divergence. The are used in WDXRF to restrict the angles that
are allowed to strike the diffraction device, thus improving the
effective resolution.



                             Sample




                   Crystal
WDXRF Pulse Processing

    The WDXRF method uses the diffraction device and
     collimators to obtain good resolution, so The detector does
     not need to be capable of energy discrimination. This
     simplifies the pulse processing.

    It also means that spectral processing is simplified since
     intensity subtraction is fundamentally an exercise in
     background subtraction.

Note: Some energy discrimination is useful since it allows for rejection of low
     energy noise and pulses from unwanted higher energy x-rays.
Proportional Counter

                Window


          Anode Filament




Fill Gases: Neon, Argon, Xenon, Krypton
Pressure: 0.5- 2 ATM
Windows: Be or Polymer
Sealed or Gas Flow Versions
Count Rates EDXRF: 10,000-40,000 cps WDXRF: 1,000,000+
Resolution: 500-1000+ eV
Scintillation Detector

                     PMT (Photo-multiplier tube)
Sodium Iodide Disk                                 Electronics




       Window: Be or Al                                    Connector
       Count Rates: 10,000 to 1,000,000+ cps
       Resolution: >1000 eV
Cooling and Temperature Control

Many WDXRF Instruments use:

•X-Ray Tube Coolers

•Thermostatically Controlled Instrument Coolers

The diffraction technique is relatively inefficient and WDXRF
detectors can operate at much higher count rates.

WDXRF Instruments are typically operated at much higher
power than direct excitation EDXRF systems.

Diffraction devices are also temperature sensitive.
Energy Dispersive X-Ray
 Fluorescence (EDXRD)
Source Filters

 Filters perform one of two functions
 Background Reduction
 Improved Fluorescence




Source Filter

                                 Detector

                X-Ray
                Source
Secondary Targets
   Improved Fluorescence and lower
    background.

   The characteristic fluorescence of the
    custom line source is used to excite the
    sample, with the lowest possible background
    intensity.

   It requires almost 100x the flux of filter
    methods but gives superior results.
Secondary Targets

                         Sample


                                           Detector
       X-Ray Tube

                            Secondary Target

1. The x-ray tube excites the secondary target.
2. The secondary target fluoresces and excites the sample.
3. The detector detects x-rays from the sample.
Si(Li) Detector

            FET
Window


                              Super-Cooled Cryostat

                               Dewar
                               filled with
  Si(Li)                       LN2
              Pre-Amplifier
  crystal

    Cooling: LN2 or Peltier
    Window: Beryllium or Polymer
    Counts Rates: 3,000 – 50,000 cps
    Resolution: 120-170 eV at Mn K-alpha
Si(Li) Cross Section
EDXRF Si(Li) Detectors
   These consist of a 3-5 mm thick silicon
    junction type p-i-n diode with a bias of -
    1000 v across it. The lithium-drifted
    centre part forms the non-conducting i-
    layer.

   n-Type Region: negative electrons are
    the main charge carriers in such regions.

   p-Type Region: positive electron holes
    are the main charge carriers in such
    regions.

   i-Type Region: intrinsic-is supposed to
    be (undoped).

   The intrinsic region is doped with Li
    which creates a donor acceptor
    compensation giving the impure crystal
    the electrical properties intrinsic to the
    pure crystal.
Typical Si(Li) Detector Instrument




This has been historically the most common laboratory
grade EDXRF configuration.
PIN Diode Detector




Cooling: Thermoelectrically cooled (Peltier)
Window: Beryllium
Count Rates: 3,000 – 20,000 cps
Resolution: 170-240 eV at Mn k-alpha
Typical PIN Detector Instrument




This configuration is most commonly used in higher end
benchtop EDXRF Instruments.
Silicon Drift Detector- SDD




Packaging: Similar to PIN Detector
Cooling: Peltier
Count Rates; 10,000 – 300,000 cps
Resolution: 140-180 eV at Mn K-alpha
Function of Silicon Drift Detector

GND      UOR           UIR




                             1   2           3
               UBACK


                                 Radiation
Absorption of x-rays
Function of Silicon Drift Detector
          GND         UOR                      UIR




                                                     1             2        3
                                 UBACK

Charge Collection:          Signal
                                                                       t3


                                                t2

Event 1    signal 1

                            t1
Event 2    signal 2


Event 3    signal 3
                                     tDrift2
                                                         tDrift3
                                                                                Time
Detector Filters
Filters are positioned between the sample and detector
in some EDXRF systems to filter out unwanted X-ray
peaks.
                       Sample

                                   Detector Filter

                                  Detector

                   X-Ray
                   Source
Filter Vs. No Filter
Detector filters can dramatically improve the element of interest
intensity, while decreasing the background, but requires 4-10 times more
source flux.

They are best used with large area detectors that normally do not require
much power.

                       Unfiltered Tube target, Cl, and Ar
                       Interference Peak




                Illustrates the Hull or simple filter method.
Ross Vs. Hull Filters
 The previous slide was an
  example of the Hull or simple
  filter method.

 The Ross method illustrated
  here for Cl analysis uses
  intensities through two filters,
  one transmitting, one
  absorbing, and the difference is
  correlated to concentration.
  This is an NDXRF method
  since detector resolution is not
  important.
Energy Dispersive Electronics
Fluorescence generates a current in the detector.

In a detector intended for EDXRF, the height of the pulse
produced is proportional to the energy of the respective
incoming X-ray.

                                         Signal to Electronics
     Element
        A
    Element
       B
     Element
                     DETECTOR
        C
     Element
        D
Multi-Channel Analyser
   Detector current pulses are translated into counts
    (counts per second, “CPS”).

   Pulses are segregated into channels according to energy
    via the MCA (Multi-Channel Analyser).


                              Intensity
                              (# of CPS
                            per Channel)
                                           Channels, Energy
     Signal from Detector
Chamber Atmosphere
   Sample and hardware chambers of any XRF
    instrument may be filled with air.
   Air absorbs low energy x-rays from elements
    particularly below Ca, (Z=20).
   Argon sometimes interferes with measurements.
    Purges are often used.

The two most common purge methods are:

   Vacuum - For use with solids or pressed pellets.
   Helium - For use with liquids or powdered
    materials.
Changers and Spinners
   Other commonly available sample handling
    features are sample changers or spinners.

   Automatic sample changers are usually of the
    circular or XYZ stage variety and may have hold
    6 to 100+ samples.

   Sample Spinners are used to average out surface
    features and particle size affects possibly over a
    larger total surface area.
Summary
   X-ray Fluorescence (XRF) is a powerful quantitative and qualitative
    analytical tool for elemental analysis of materials.

   XRF is a fast, accurate, non-destructive, and usually requires only minimal
    sample preparation.

   WDXRF is capable of measuring Beryllium (Be, Z=4) to Uranium (U, Z=92)
    and beyond at trace levels and up to 100%.

   EDXRF systems detect elements between Sodium (Na, Z=11) and Uranium
    (U, Z=92).

   EDXRF and WDXRF involves use of ionizing radiation to excite the sample,
    followed by detection and measurement of X-rays leaving the sample that are
    characteristic of the elements in the sample.

   EDXRF spectrometer electronics digitize the signal produced by X-rays
    entering the lithium drifted detector, and send this information to the PC for
    display and analysis. The Na to U spectral data are in the 1-37 KeV range.
Acknowledgements
1.    www.skyrayinstrument.com
2.    www.panalytical.com
3.    www.wikipedia.com
4.    www.thermo.com
5.    www.rigaku.com
6.    www.xrfcorp.com
7.    www.bruker.com
8.    www.jordanvalley-apd.com
9.    www.moxtek.com
10.   www.amptek.com
11.   www.niton.com
12.   www.xos.com
13.   www.ketek.net
Thank You
Appendix
   XRF Chamber Atmosphere
   Evaluating XRF Spectra
   X-Ray Scattering
   Spectral Interferences
   Secondary Enhancement Effects
   XRF Sample Preparation
Evaluating Spectra
In addition to elemental peaks, other peaks appear in
the spectra:


             K & L Spectral Peaks
             Rayleigh Scatter Peaks
             Compton Scatter Peaks
             Escape Peaks
             Sum Peaks
             Bremstrahlung
K & L Spectral Peaks


                       K-Lines

  L-lines




       Rh X-ray Tube
Scatter


                             Some of the source X-rays
                             strike the sample and are
           Sample            scattered back at the
                             detector.

                                Sometimes called
                                “backscatter”

Detector            Source
Rayleigh Scatter
                     X-rays from the X-ray tube or
                      target strike atom without
                      promoting fluorescence.
                     Energy is not lost in collision. (EI =
                      EO)
                     They appear as a source peak in
                      spectra.
                     AKA - “Elastic” Scatter

                                EO


                           EI
Rh X-ray Tube
Compton Scatter
                     X-rays from the X-ray tube or
                      target strike atom without
                      promoting fluorescence.
                     Energy is lost in collision. (EI >
                      EO)
                     Compton scatter appears as a
                      source peak in spectra, slightly
                      less in energy than Rayleigh
                      Scatter.
                     AKA - “Inelastic” Scatter
                           EO


                      EI
Rh X-ray Tube
Sum Peaks
   2 photons strike the detector at the same time.

   The fluorescence is captured by the detector,
    recognized as 1 photon twice its normal energy.

   A peak appears in spectra, at: 2 X (Element keV).
Escape Peaks
                         X-rays strike the sample
                          and promote elemental
                          fluorescence.

                         Some Si fluorescence at
                          the surface of the detector
                          escapes, and is not
                          collected by the detector.

                         The result is a peak that
                          appears in spectrum, at:
                          Element keV - Si keV
1.74 keV                  (1.74 keV).
Rh X-ray Tube
Brehmstrahlung

Brehmstrahlung (or Continuum)
Radiation:

German for “breaking radiation”, noise that
appears in the spectra due to deceleration of
electrons as they strike the anode of the X-ray
tube.
Interferences

   Spectral Interferences

   Environmental Interferences

   Matrix Interferences
Spectral Interferences
                            Spectral interferences are peaks in
                             the spectrum that overlap the
                             spectral peak (region of interest) of
     220 eV Resolution
                             the element to be analyzed.
     140 eV Resolution

                            Examples:
                               K & L line Overlap - S &
                                Mo, Cl & Rh, As & Pb
                               Adjacent Element Overlap - Al
                                & Si, S & Cl, K & Ca...

                            Resolution of detector determines
                             extent of overlap.


Adjacent Element Overlap
Environmental Interferences

Al Analyzed with Si Target      Light elements (Na - Cl) emit weak
  Air Environment                X-rays, easily attenuated by air.
  He Environment                Solution:
                                   Purge instrument with He (less
                                      dense than air = less
                                      attenuation).
                                   Evacuate air from analysis
                                      chamber via a vacuum pump.
                                Either of these solutions also
                                 eliminate interference from Ar
                                 (spectral overlap to Cl). Argon (Ar)
                                 is a component of air.
Matrix Interferences
        Absorption/Enhancement Effects

   Absorption: Any element can absorb or scatter the
    fluorescence of the element of interest.

   Enhancement: Characteristic x-rays of one element excite
    another element in the sample, enhancing its signal.


    Influence Coefficients, sometimes called alpha corrections are
    used to mathematically correct for Matrix Interferences
Absorption-Enhancement Affects

                                          Sample
                                                   Red = Fe, absorbed
                                                   Blue = Ca, enhanced

                                                   Source X-ray



                       X-Ray Captured
                       by the detector.

   Incoming source X-ray fluoresces Fe.

   Fe fluorescence is sufficient in energy to fluoresce Ca.

   Ca is detected, Fe is not. Response is proportional to
    concentrations of each element.
Software

   Qualitative Analysis


   Semi-Quantitative Analysis


   Quantitative Analysis


   Extraction and Regression Methods
Semi-Quantitative Analysis
                               The algorithm computes both the
SLFP
                                intensity to concentration relationship
Standardless Fundamental
Parameters                      and the absorption affects

                               Results are typically within 10 - 20 % of
                                actual values.



                               The concentration to intensity
FP (with Standards)
                                relationship is determined with
NBS-GSC, NRLXRF, Uni-Quant,
TurboQuant, etc…                standards, while the FP handles the
                                absorption affects.

                               Results are usually within 5 - 10 % of
                                actual values
Quantitative Analysis
                           •XRF is a reference method,
                           standards are required for
                           quantitative results.

                           •Standards are analysed,
Concentration




                           intensities obtained, and a
                           calibration plot is generated
                           (intensities vs. concentration).

                           • XRF instruments compare
                           the spectral intensities of
                           unknown samples to those of
                           known standards.
Standards
   Standards (such as Certified Reference Materials) are
    required for Quantitative Analysis.

   Standard concentrations should be known to a better
    degree of precision and accuracy than is required for the
    analysis.

   Standards should be of the same matrix as samples to be
    analyzed.

   Number of standards required for a purely empirical
    method, N=(E+1)2, N=# of standards, E=# of Elements.

   Standards should vary independently in concentration
    when empirical absorption corrections are used.
Sample Preparation
Powders:
 Grinding (<400 mesh if possible) can minimise scatter affects due to
  particle size. Additionally, grinding insures that the measurement is
  more representative of the entire sample, vs. the surface of the sample.
  Pressing (hydraulically or manually) compacts more of the sample into
  the analysis area, and ensures uniform density and better
  reproducibility.

Solids:
 Orient surface patterns in same manner so as minimise scatter affects.
   Polishing surfaces will also minimise scatter affects. Flat samples are
   optimal for quantitative results.

Liquids:
 Samples should be fresh when analysed and analysed with short
   analysis time - if sample is evaporative. Sample should not stratify
   during analysis. Sample should not contain precipitants/solids,
   analysis could show settling trends with time.

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XRF Basic Principles

  • 1. XRF A Basic Overview Marion E. Becker, M.S., B.S.
  • 2. Introduction  Presentation Outline 1. X-Ray Radiation 2. XRF Theory 3. XRF Instrumentation 4. XRF Hardware 5. Summary 6. Acknowledgements
  • 3. Electromagnetic Radiation 1014Hz - 1015Hz 1Hz - 1kHz 1kHz - 1014Hz 1015Hz - 1021Hz Extra-Low Radio Microwave Infrared Ultraviolet X-Rays, Frequency (ELF) Visible Light Gamma Rays Low energy High energy
  • 4. Theory  A source X-ray strikes an inner shell electron. If at high enough energy (above absorption edge of element), it is ejected it from the atom.  Higher energy electrons cascade to fill vacancy, giving off characteristic fluorescent X-rays.  For elemental analysis of Na - U.
  • 5. K & L Spectral Lines  K - alpha lines: L shell e- L beta transition to fill a vacancy L alpha in K shell. Most frequent transition, hence most K beta intense peak.  K - beta lines: M shell e- K alpha transitions to fill a vacancy in K shell. K Shell L Shell  L - alpha lines: M shell e- transition to fill a vacancy M Shell in L shell. N Shell  L - beta lines: N shell e- transition to fill a vacancy in L shell.
  • 6. XRF Instrumentation  The basic concept for all XRF spectrometers is a source, a sample, and a detection system.  The source irradiates the sample, and a detector measures the fluorescence radiation emitted from the sample.
  • 7. XRF Spectrometers Wavelength Dispersive WDXRF Spectrometer Energy Dispersive EDXRF Spectrometer
  • 8. XRF Primary Radiation Sources For XRF, Instrumentation the source is an X-ray tube or a sealed radioactive source: Conventional X-Ray Tubes:  End Window X-Ray Tubes  Side Window X-Ray Tubes Gamma Rays-Sealed Sources:  Radioisotopes
  • 9. Side Window X-Ray Tube Be Window Glass Envelope Target (Ti, Ag, HV Lead Rh, etc.) Electron beam Copper Anode Filament Silicone Insulation
  • 10. End Window X-Ray Tube X-Ray Tubes:  Voltage determines which elements can be excited.  More power = lower detection limits.  Anode selection determines optimal source excitation (application specific).
  • 11. Radioisotopes Isotope Fe-55 Cm-244 Cd-109 Am-241 Co-57 Energy (keV) 5.9 14.3, 22, 88 59.5 122 18.3 Elements (K- Al – V Ti-Br Fe-Mo Ru-Er Ba - U lines) Elements (L- Br-I I- Pb Yb-Pu None None lines) While isotopes have fallen out of favor they are still useful for gauging applications.
  • 12. Source Modifiers Several Devices are used to modify the shape or intensity of the source spectrum or the beam shape. • Source Filters • Secondary Targets • Polarizing Targets • Collimators • Focusing Optics
  • 13. Detectors  Si(Li) -EDXRF  PIN Diode-EDXRF  Silicon Drift Detectors-EDXRF  Proportional Counters-WDXRF  Scintillation Detectors-WDXRF
  • 14. Detector Principles A detector is composed of a non-conducting or semi-conducting material between two charged electrodes. X-ray radiation ionizes the detector material causing it to become conductive, momentarily. The newly freed electrons are accelerated toward the detector anode to produce an output pulse. An ionized semiconductor produces electron-hole pairs, the number of pairs produced is proportional to the X-ray photon energy. E n e where : n = number of electron-hole pairs produced E = X-ray photon energy e = 3.8ev for Si at LN2 temper atures
  • 15. Wavelength Dispersive X-Ray Fluorescence (WDXRF)
  • 16. Wavelength Dispersive XRF Wavelength Dispersive XRF relies on a diffractive device such as crystal or multilayer to isolate a peak, since the diffracted wavelength is much more intense than other wavelengths that scatter off the device. Sample Detector Collimators X-Ray Source Diffraction Device
  • 17. Diffraction The two most common diffraction instruments are the crystal and multilayer. Both work according to Bragg’s formula: nλ = 2d · sinθ  In XRF, the d-value of the analyzer crystal is known and we can solve Bragg's equation for the element characteristic wavelength (λ).  n = integer  d = crystal lattice or multilayer spacing  θ = The incident angle  λ= wavelength
  • 18. Multilayers While the crystal spacing is based on the natural atomic spacing at a given orientation the multilayer uses a series of thin film layers of dissimilar elements to do the same thing. Modern multilayers are more efficient than crystals and can be optimized for specific elements. Often used for low Z elements.
  • 19. Focusing Optics Simple collimation blocks unwanted x-rays and is a highly inefficient method. Focusing optics like polycapillary devices and other Kumakhov lens devices were developed so that the beam could be redirected and focused on a small spot. Less than 75 um spot sizes are regularly achieved. Source Detector Bragg reflection inside a Capillary Polycapillary Optic
  • 20. Collimators Collimators are usually circular or a slit and restrict the size or shape of the source beam for exciting small areas in either EDXRF or µXRF instruments. They may rely on internal Bragg reflection for improved efficiency. Sample Collimator sizes range from 12 microns to several mm Tube
  • 21. Soller Collimators Soller and similar types of collimators are used to prevent beam divergence. The are used in WDXRF to restrict the angles that are allowed to strike the diffraction device, thus improving the effective resolution. Sample Crystal
  • 22. WDXRF Pulse Processing  The WDXRF method uses the diffraction device and collimators to obtain good resolution, so The detector does not need to be capable of energy discrimination. This simplifies the pulse processing.  It also means that spectral processing is simplified since intensity subtraction is fundamentally an exercise in background subtraction. Note: Some energy discrimination is useful since it allows for rejection of low energy noise and pulses from unwanted higher energy x-rays.
  • 23. Proportional Counter Window Anode Filament Fill Gases: Neon, Argon, Xenon, Krypton Pressure: 0.5- 2 ATM Windows: Be or Polymer Sealed or Gas Flow Versions Count Rates EDXRF: 10,000-40,000 cps WDXRF: 1,000,000+ Resolution: 500-1000+ eV
  • 24. Scintillation Detector PMT (Photo-multiplier tube) Sodium Iodide Disk Electronics Window: Be or Al Connector Count Rates: 10,000 to 1,000,000+ cps Resolution: >1000 eV
  • 25. Cooling and Temperature Control Many WDXRF Instruments use: •X-Ray Tube Coolers •Thermostatically Controlled Instrument Coolers The diffraction technique is relatively inefficient and WDXRF detectors can operate at much higher count rates. WDXRF Instruments are typically operated at much higher power than direct excitation EDXRF systems. Diffraction devices are also temperature sensitive.
  • 26. Energy Dispersive X-Ray Fluorescence (EDXRD)
  • 27. Source Filters Filters perform one of two functions Background Reduction Improved Fluorescence Source Filter Detector X-Ray Source
  • 28. Secondary Targets  Improved Fluorescence and lower background.  The characteristic fluorescence of the custom line source is used to excite the sample, with the lowest possible background intensity.  It requires almost 100x the flux of filter methods but gives superior results.
  • 29. Secondary Targets Sample Detector X-Ray Tube Secondary Target 1. The x-ray tube excites the secondary target. 2. The secondary target fluoresces and excites the sample. 3. The detector detects x-rays from the sample.
  • 30. Si(Li) Detector FET Window Super-Cooled Cryostat Dewar filled with Si(Li) LN2 Pre-Amplifier crystal Cooling: LN2 or Peltier Window: Beryllium or Polymer Counts Rates: 3,000 – 50,000 cps Resolution: 120-170 eV at Mn K-alpha
  • 32. EDXRF Si(Li) Detectors  These consist of a 3-5 mm thick silicon junction type p-i-n diode with a bias of - 1000 v across it. The lithium-drifted centre part forms the non-conducting i- layer.  n-Type Region: negative electrons are the main charge carriers in such regions.  p-Type Region: positive electron holes are the main charge carriers in such regions.  i-Type Region: intrinsic-is supposed to be (undoped).  The intrinsic region is doped with Li which creates a donor acceptor compensation giving the impure crystal the electrical properties intrinsic to the pure crystal.
  • 33. Typical Si(Li) Detector Instrument This has been historically the most common laboratory grade EDXRF configuration.
  • 34. PIN Diode Detector Cooling: Thermoelectrically cooled (Peltier) Window: Beryllium Count Rates: 3,000 – 20,000 cps Resolution: 170-240 eV at Mn k-alpha
  • 35. Typical PIN Detector Instrument This configuration is most commonly used in higher end benchtop EDXRF Instruments.
  • 36. Silicon Drift Detector- SDD Packaging: Similar to PIN Detector Cooling: Peltier Count Rates; 10,000 – 300,000 cps Resolution: 140-180 eV at Mn K-alpha
  • 37. Function of Silicon Drift Detector GND UOR UIR 1 2 3 UBACK Radiation Absorption of x-rays
  • 38. Function of Silicon Drift Detector GND UOR UIR 1 2 3 UBACK Charge Collection: Signal t3 t2 Event 1 signal 1 t1 Event 2 signal 2 Event 3 signal 3 tDrift2 tDrift3 Time
  • 39. Detector Filters Filters are positioned between the sample and detector in some EDXRF systems to filter out unwanted X-ray peaks. Sample Detector Filter Detector X-Ray Source
  • 40. Filter Vs. No Filter Detector filters can dramatically improve the element of interest intensity, while decreasing the background, but requires 4-10 times more source flux. They are best used with large area detectors that normally do not require much power. Unfiltered Tube target, Cl, and Ar Interference Peak Illustrates the Hull or simple filter method.
  • 41. Ross Vs. Hull Filters  The previous slide was an example of the Hull or simple filter method.  The Ross method illustrated here for Cl analysis uses intensities through two filters, one transmitting, one absorbing, and the difference is correlated to concentration. This is an NDXRF method since detector resolution is not important.
  • 42. Energy Dispersive Electronics Fluorescence generates a current in the detector. In a detector intended for EDXRF, the height of the pulse produced is proportional to the energy of the respective incoming X-ray. Signal to Electronics Element A Element B Element DETECTOR C Element D
  • 43. Multi-Channel Analyser  Detector current pulses are translated into counts (counts per second, “CPS”).  Pulses are segregated into channels according to energy via the MCA (Multi-Channel Analyser). Intensity (# of CPS per Channel) Channels, Energy Signal from Detector
  • 44. Chamber Atmosphere  Sample and hardware chambers of any XRF instrument may be filled with air.  Air absorbs low energy x-rays from elements particularly below Ca, (Z=20).  Argon sometimes interferes with measurements.  Purges are often used. The two most common purge methods are:  Vacuum - For use with solids or pressed pellets.  Helium - For use with liquids or powdered materials.
  • 45. Changers and Spinners  Other commonly available sample handling features are sample changers or spinners.  Automatic sample changers are usually of the circular or XYZ stage variety and may have hold 6 to 100+ samples.  Sample Spinners are used to average out surface features and particle size affects possibly over a larger total surface area.
  • 46. Summary  X-ray Fluorescence (XRF) is a powerful quantitative and qualitative analytical tool for elemental analysis of materials.  XRF is a fast, accurate, non-destructive, and usually requires only minimal sample preparation.  WDXRF is capable of measuring Beryllium (Be, Z=4) to Uranium (U, Z=92) and beyond at trace levels and up to 100%.  EDXRF systems detect elements between Sodium (Na, Z=11) and Uranium (U, Z=92).  EDXRF and WDXRF involves use of ionizing radiation to excite the sample, followed by detection and measurement of X-rays leaving the sample that are characteristic of the elements in the sample.  EDXRF spectrometer electronics digitize the signal produced by X-rays entering the lithium drifted detector, and send this information to the PC for display and analysis. The Na to U spectral data are in the 1-37 KeV range.
  • 47. Acknowledgements 1. www.skyrayinstrument.com 2. www.panalytical.com 3. www.wikipedia.com 4. www.thermo.com 5. www.rigaku.com 6. www.xrfcorp.com 7. www.bruker.com 8. www.jordanvalley-apd.com 9. www.moxtek.com 10. www.amptek.com 11. www.niton.com 12. www.xos.com 13. www.ketek.net
  • 49. Appendix  XRF Chamber Atmosphere  Evaluating XRF Spectra  X-Ray Scattering  Spectral Interferences  Secondary Enhancement Effects  XRF Sample Preparation
  • 50. Evaluating Spectra In addition to elemental peaks, other peaks appear in the spectra:  K & L Spectral Peaks  Rayleigh Scatter Peaks  Compton Scatter Peaks  Escape Peaks  Sum Peaks  Bremstrahlung
  • 51. K & L Spectral Peaks K-Lines L-lines Rh X-ray Tube
  • 52. Scatter Some of the source X-rays strike the sample and are Sample scattered back at the detector. Sometimes called “backscatter” Detector Source
  • 53. Rayleigh Scatter  X-rays from the X-ray tube or target strike atom without promoting fluorescence.  Energy is not lost in collision. (EI = EO)  They appear as a source peak in spectra.  AKA - “Elastic” Scatter EO EI Rh X-ray Tube
  • 54. Compton Scatter  X-rays from the X-ray tube or target strike atom without promoting fluorescence.  Energy is lost in collision. (EI > EO)  Compton scatter appears as a source peak in spectra, slightly less in energy than Rayleigh Scatter.  AKA - “Inelastic” Scatter EO EI Rh X-ray Tube
  • 55. Sum Peaks  2 photons strike the detector at the same time.  The fluorescence is captured by the detector, recognized as 1 photon twice its normal energy.  A peak appears in spectra, at: 2 X (Element keV).
  • 56. Escape Peaks  X-rays strike the sample and promote elemental fluorescence.  Some Si fluorescence at the surface of the detector escapes, and is not collected by the detector.  The result is a peak that appears in spectrum, at: Element keV - Si keV 1.74 keV (1.74 keV). Rh X-ray Tube
  • 57. Brehmstrahlung Brehmstrahlung (or Continuum) Radiation: German for “breaking radiation”, noise that appears in the spectra due to deceleration of electrons as they strike the anode of the X-ray tube.
  • 58. Interferences  Spectral Interferences  Environmental Interferences  Matrix Interferences
  • 59. Spectral Interferences  Spectral interferences are peaks in the spectrum that overlap the spectral peak (region of interest) of 220 eV Resolution the element to be analyzed. 140 eV Resolution  Examples:  K & L line Overlap - S & Mo, Cl & Rh, As & Pb  Adjacent Element Overlap - Al & Si, S & Cl, K & Ca...  Resolution of detector determines extent of overlap. Adjacent Element Overlap
  • 60. Environmental Interferences Al Analyzed with Si Target  Light elements (Na - Cl) emit weak Air Environment X-rays, easily attenuated by air. He Environment  Solution:  Purge instrument with He (less dense than air = less attenuation).  Evacuate air from analysis chamber via a vacuum pump.  Either of these solutions also eliminate interference from Ar (spectral overlap to Cl). Argon (Ar) is a component of air.
  • 61. Matrix Interferences Absorption/Enhancement Effects  Absorption: Any element can absorb or scatter the fluorescence of the element of interest.  Enhancement: Characteristic x-rays of one element excite another element in the sample, enhancing its signal. Influence Coefficients, sometimes called alpha corrections are used to mathematically correct for Matrix Interferences
  • 62. Absorption-Enhancement Affects Sample Red = Fe, absorbed Blue = Ca, enhanced Source X-ray X-Ray Captured by the detector.  Incoming source X-ray fluoresces Fe.  Fe fluorescence is sufficient in energy to fluoresce Ca.  Ca is detected, Fe is not. Response is proportional to concentrations of each element.
  • 63. Software  Qualitative Analysis  Semi-Quantitative Analysis  Quantitative Analysis  Extraction and Regression Methods
  • 64. Semi-Quantitative Analysis  The algorithm computes both the SLFP intensity to concentration relationship Standardless Fundamental Parameters and the absorption affects  Results are typically within 10 - 20 % of actual values.  The concentration to intensity FP (with Standards) relationship is determined with NBS-GSC, NRLXRF, Uni-Quant, TurboQuant, etc… standards, while the FP handles the absorption affects.  Results are usually within 5 - 10 % of actual values
  • 65. Quantitative Analysis •XRF is a reference method, standards are required for quantitative results. •Standards are analysed, Concentration intensities obtained, and a calibration plot is generated (intensities vs. concentration). • XRF instruments compare the spectral intensities of unknown samples to those of known standards.
  • 66. Standards  Standards (such as Certified Reference Materials) are required for Quantitative Analysis.  Standard concentrations should be known to a better degree of precision and accuracy than is required for the analysis.  Standards should be of the same matrix as samples to be analyzed.  Number of standards required for a purely empirical method, N=(E+1)2, N=# of standards, E=# of Elements.  Standards should vary independently in concentration when empirical absorption corrections are used.
  • 67. Sample Preparation Powders:  Grinding (<400 mesh if possible) can minimise scatter affects due to particle size. Additionally, grinding insures that the measurement is more representative of the entire sample, vs. the surface of the sample. Pressing (hydraulically or manually) compacts more of the sample into the analysis area, and ensures uniform density and better reproducibility. Solids:  Orient surface patterns in same manner so as minimise scatter affects. Polishing surfaces will also minimise scatter affects. Flat samples are optimal for quantitative results. Liquids:  Samples should be fresh when analysed and analysed with short analysis time - if sample is evaporative. Sample should not stratify during analysis. Sample should not contain precipitants/solids, analysis could show settling trends with time.