X-Ray Fluorescence Spectrometry X-Ray History Who Discovered X-rays? What’s an X-Ray? How X-rays...

X-Ray X-Ray Fluorescence Fluorescence SpectrometrySpectrometry

X-Ray History X-Ray History

Who Discovered X-rays? Who Discovered X-rays?

What’s an X-Ray? What’s an X-Ray?

How X-rays are Produced? How X-rays are Produced?

X-Ray GenerationX-Ray Generation

Content :

INSTRUMENT COMPONENTS:

1) Sources

2) Filters for X-Ray Beams

3) Wavelength Dispersion with Monochromators

4) X-R.y Detectors and Signal Processors

5) Signal Processes and Readout DeVices

The Coolidge Tube Radioisotopes Secondary Fluorescent Sources. synchrotron

a) photographic emulsions

b) Gas-Filled Detectors c) Scintillation Counters d) Semiconductor Detectors

a) Energy Dispersive Systems b) Counters and Scalers

The Geiger TubeProportional CountersIonization Chambers

X-rayX-ray emission (including fluorescent emission)

X-rayX-ray absorption

X-ray X-ray Diffraction

X-ray FluorescenceX-ray Fluorescence

Electron MicroscopesElectron Microscopes

X-RAY Photoelectron Spectroscopy And ……..

X-Ray MethodsX-Ray Methods

X-Ray Fluorescence X-Ray Fluorescence SpectrometrySpectrometry

What is it?What is it?

How does it work?How does it work?

PropertiesProperties

AdvantagesAdvantages

DisadvantagesDisadvantages

Who Discovered X-rays?Who Discovered X-rays?

1895 – Wilhelm Roentgen 1895 – Wilhelm Roentgen discovered X-rays while studying discovered X-rays while studying luminescence produced by cathode luminescence produced by cathode tubes. tubes.

Because he didn’t know what they Because he didn’t know what they where he called them X-rayswhere he called them X-rays

(X for unknown)(X for unknown)

He also noticed that the rays caused He also noticed that the rays caused photographic plates to darkenphotographic plates to darken

X-ray photographs revealed the X-ray photographs revealed the inner structure of objectsinner structure of objects

Definition : :

Basically the same thing as visible light raysBasically the same thing as visible light rays

Both are wavelike forms of electromagnetic Both are wavelike forms of electromagnetic energy carried by particles called photonsenergy carried by particles called photons

The difference is the The difference is the energy levelenergy level of the of the individual photons - expressed as the individual photons - expressed as the wavelengthwavelength of the rays of the rays

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Radiation in the wavelength range between about 0.1 and 25 A

What’s an X-Ray?What’s an X-Ray?

Visible light and X-ray photons Visible light and X-ray photons are both produced by the are both produced by the movement of electrons in movement of electrons in atomsatoms

Electrons occupy different Electrons occupy different energy levels (orbitals) around energy levels (orbitals) around the atom’s nucleusthe atom’s nucleus

When electrons drop to a When electrons drop to a lower orbital it releases lower orbital it releases energy in the form of a photonenergy in the form of a photon

The energy of the photon The energy of the photon depends on how far the depends on how far the electron dropped between electron dropped between orbitals.orbitals.

Source of X-rays as vacancy filled by Source of X-rays as vacancy filled by cascade of electrons from lower energy cascade of electrons from lower energy

levelslevels

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Radiation PhysicsRadiation Physics

Ionizing radiation is Ionizing radiation is radiation with enough radiation with enough energy to remove electrons energy to remove electrons from their orbitfrom their orbit

This causes the atom to This causes the atom to become charged or ionized.become charged or ionized.

This energy is emitted in the This energy is emitted in the form of waves or particlesform of waves or particles

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Ionizing RadiationIonizing Radiation

KindKind AtomicAtomic

MassMassElectricaElectrical Chargel Charge

Range Range in Airin Air

Range in Range in Body Body TissueTissue

AttenuationAttenuation

AlphaAlpha 44 +2+2 < inch< inch Unable to Unable to penetrate penetrate skinskin

Stopped by a Stopped by a sheet of paper sheet of paper or skinor skin

BetaBeta 1/18401/1840 -1-1 Several Several feetfeet

1/3 inch1/3 inch Stopped by a Stopped by a thin sheet of thin sheet of aluminumaluminum

Gamma Gamma / x-ray/ x-ray

NANA NoneNone Passes Passes throughthrough

Passes Passes throughthrough

Thick lead or Thick lead or steelsteel

NeutroNeutronn

11 NeutralNeutral HundredHundreds of feets of feet

About 10% About 10% goes goes throughthrough

Several feet of Several feet of water or water or plasticplastic

X-Ray and Gamma Ray X-Ray and Gamma Ray PropertiesProperties

Charge: NoneCharge: None

Mass: NoneMass: None

Velocity: 3 x 10Velocity: 3 x 1088 m/s m/s

Origin:Origin:Gamma RaysGamma Rays: Nucleus: Nucleus

X RaysX Rays: Electron Cloud : Electron Cloud & Bremsstrahlung& Bremsstrahlung

What are X-Rays?What are X-Rays?

X-rays are produced when accelerated X-rays are produced when accelerated electrons interact with a target, usually a electrons interact with a target, usually a metal absorber, or with a crystalline metal absorber, or with a crystalline structure. This method of x-ray production is structure. This method of x-ray production is known as known as bremsstrahlungbremsstrahlung. .

The The bremsstrahlungbremsstrahlung produced is produced is proportional to the square of the energy of proportional to the square of the energy of the accelerated electrons used to produce it, the accelerated electrons used to produce it, and is also proportional to the atomic and is also proportional to the atomic number (Z) of the target (absorber).number (Z) of the target (absorber).

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X-raysX-rays

electron

X-ray

target

Electromagnetic radiation Originate in Electromagnetic radiation Originate in energy shells of atom Produced when energy shells of atom Produced when electrons interact with a targetelectrons interact with a target


1) Sources










The Coolidge Tube

Radioisotopes

Secondary Fluorescent Sources.

synchrotron

Sources

X-Ray GenerationX-Ray Generation

X-ray tube X-ray tube

Filament Filament (Tungsten)(Tungsten)

Target metal (Cu, Target metal (Cu, Cr)Cr)

Electrons are Electrons are accelerated by a accelerated by a potential of about potential of about

100 k Volts 100 k VoltsSchematic diagram of the

Coolidge tube.

How X-rays are Produced?How X-rays are Produced?

When fast-moving electrons slam into a metal object, X-rays are produced. The kinetic energy of the electron is transformed into electromagnetic energy.

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The Coolidge Tube

Radioisotopes


synchrotron

Sources

Radioisotopes

A variety of radioactive substances have been employed as sources in X-ray fluorescence and absorption methods.

Generally, the radioisotope is

encapsulated to prevent contamination of the laboratory and shielded to absorb radiation in all but certain directions.

Radioisotopes

The Coolidge Tube

Radioisotopes

Secondary Fluorescent Sources

synchrotron

Sources

In some applications, the fluorescence spectrum of an element that has been excited by radiation from a Coolidge tube serves as a source for absorption or fluorescence studies.

This arrangement has the advantage of eliminating the continuous component emitted by a primary source.

Secondary Fluorescent Sources

The Coolidge Tube

Radioisotopes


synchrotron

Sources

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Continuous X-Ray SpectrumContinuous X-Ray Spectrum

35 keV electrons strike the metal target35 keV electrons strike the metal target They collide with the electrons in the They collide with the electrons in the

metalmetal Rapid deceleration results in emissions Rapid deceleration results in emissions

of protonof proton Photons with a wide range of energies Photons with a wide range of energies

are emitted because the degree of are emitted because the degree of deceleration is differentdeceleration is different

Distribution of continuous radiation from an X-ray tube with a tungsten target. The numbers above the curves indicate the accelerating voltages.

Line spectrum

for a tube with a molybdenum target.

Relationship between X-ray emission frequency and atomic number


1) Sources










Use of a filter to produce monochromatic radiation.


1) Sources










An X-ray monochromator and

detector.

Note that the angle of the detector with respectto the beam is twice that of the crystal face.

For absorption analysis, the source is an X-ray tube andthe sample is located in the beam as shown.

For emission work, the sample becomes a fluorescent source of X-rays as shown in the insert.

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1) Sources










X-R.y Detectors and Signal Processors

a) photographic emulsions b) Gas-Filled Detectors c) Scintillation Counters d) Semiconductor Detectors


Gas-Filled Detectors

The Geiger Tube

Proportional Counters

Ionization Chambers

Gas amplification for various types of gas-filled detectors.

The luminescence produced when radiation strikes a phosphor represents one of the oldest methods of detecting radioactivity and X-rays, and one of thenewest as well.

In its earliest application, the technique involved the manual counting of flashes that resulted when individual photons or radiochemical particles struck a zinc sulfide screen.

The tedium of counting individual flashes by eye led Geiger to the development of gas-filled qetectors, which were not only more convenient and reliable, but more responsive to radiation as well.

The advent of the photomultiplier tube and better phosphors, however, has reversed this trend, and scintillation counting has again become one of the important methods for radiation detection.

Scintillation counters

Vertical cross section of a lithium drifted silicon detector for X-rays and radioactive radiation.


1) Sources










Signal Processes and Readout DeVices

a) Energy Dispersive Systems

b) Counters and Scalers

X-rayphotons

Schematic diagram of a signal heigh~ analyzer. Lower plot shows heIght of transmitted signals upon exit from various electronic components.

Signal Processes and Readout DeVices

a) Energy Dispersive Systems

b) Counters and Scalers


What is it?What is it? How does it work?How does it work? PropertiesProperties AdvantagesAdvantages DisadvantagesDisadvantages

X-RAYFLUORESCENCE METHODS

Although it is feasible to excite an X-ray emission spectrum by incorporating the sample into the target area of an X-ray tube, the inconvenience of this technique discourages its application to many types of materials.

Instead, excitation is more commonly brought about by irradiation of the sample with a beam of X-rays from a Coolidge tube or a radioactive source.

Under these circumstances, the elements in the sample are excited by absorption of the primary beam and emit their own characteristic fluorescent X-rays. This procedure is thus properly called an X-ray fluorescence or emission method. X-Ray fluorescence is perhaps the most widely used of all analytical methods for the qualitative identification of elements having atomic numbers greater than oxygen (>8);

in addition, it is often employed for semiquantitative or quantitative determination of these elements.

Instruments

Various combinations of the instrument components discussed in the previous section lead to several recognizable types of X-ray fluorescence instruments.

The three basic types are :

wavelength dispersive energy dispersive And nondispersive

wavelength dispersive Instruments

Wavelength dispersive instruments always employ tubes as a source because of the large energy losses suffered when an X-ray beam is collimated and dispersed into ita component wavelengths.

Radioactive sources produce X-ray photons at a rate less than 10-4 that of a Coolidge tube;

the added attenuation by a monochromator would then result in a beam that was difficult or impossible to detect and measure accurately.

Wavelength dispersive instruments

Wavelength dispersive instruments are of two 'types :

single-channel or sequential

and

multichannel or simultaneous.

Multichannel instruments

Multichannel instruments are widely used for the determination of several components in materials of industry such as :

steel other alloys, cement, ores, and petroleum products.

Both multichannel and single-channel instruments are equipped to handle samples in the form of

metals, powdered solids, evaporated films, pure liquids, or solutions.

Energy dispersive Instruments

Energy dispersive X-ray fluorescence spectrometer.

Excitation by X-rays from a Coolidge tube

Energy dispersive X-ray fluorescence spectrometer.

Excitation by X-rays from a radioactive substance.

Nondispersive Instruments

Cutaway view ofa commercial nondispersive X-ray fluorescence instrument.

(Reprinted from Amer. LDb. 6(9~ 62 (1974).

Copyright 1974 by International Scientific Communications, Inc.)

Some Quantitative Applications of X-Ray Fluorescence.

X-ray fluorescence spectrometry is perhaps the most powerful tool available to the chemist for the rapid quantitative determination of all but the lightest elements in complex samples.

For example, Baird and Henke have demonstrated that nine elements can be determined in samples of granitic rocks in an elapsed time, including sample preparation, of about 12 min.

The precision of the method is better than wet chemical analyses and averages 0.08% relative.

It is noteworthy that one of the elements analyzed is oxygen, which ordinarily can be determined by difference only.

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AdvantagesAdvantages

X-ray spectra is simple and regularX-ray spectra is simple and regular Matrix effect in X-ray emission are Matrix effect in X-ray emission are

systematic, predictable and readily systematic, predictable and readily evaluatedevaluated

X-ray fluorescence spectroscopy is X-ray fluorescence spectroscopy is non-destructive.non-destructive.

X-Ray Fluorescence X-Ray Fluorescence SpectrometrySpectrometry DisadvantagesDisadvantages

Small surface layer contributes to the Small surface layer contributes to the observed X-ray line intensityobserved X-ray line intensity

Not all of the elements in a sample Not all of the elements in a sample can be measured using the same X-can be measured using the same X-ray tube, crystals, and detectorray tube, crystals, and detector

Reference :1) Handbook of Practical X-Ray Fluorescence Analysis B. Beckhoff B. Kanngieكer N. Langhoff R.Wedell H.Wolff

2) X-Ray Analysis Ron Jenkins

3) DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

4) Fundamentals of Analytical Chemistry Douglas A. Skoog & Donald M. West & F.

James Holler & Stanley R. Crouch

5) Principles of instrumental analysis Skoog

X-Ray Fluorescence Spectrometry X-Ray History Who Discovered X-rays? What’s an X-Ray? How X-rays...

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