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X – Ray methods of Analysis :
• X – Absorption spectroscopy.
• X – Ray Fluorescence Spectroscopy.
• X – Ray Diffractometry.
Production of X - Rays
X-rays were discovered in 1895 by Wilhelm Röntgen
while investigating the glow produced by cathode
rays in a discharge tube.
X-rays are short wavelength (typically l » 1Å)
electromagnetic radiation
The are produced either when accelerated energetic
electrons bombard elemental metallic targets or
energy is released from K shell transitions by
electrons returning to the ground state.
The principle of the X-ray tube is shown in figure,
the design by Coolidge in 1912.
The advances in vacuum technology was made
possible to design a variety of X-Ray tubes.
Less than 1% of the electron energy is converted
into photons the rest is dissipated as heat within the
target material.
If the bombarding electrons have sufficient
energy, they can knock an electron out of an
inner shell of the target metal atoms.
Then electrons from higher states drop down to
fill the vacancy, emitting x-ray photons with
precise energies determined by the electron
energy levels.
These x-rays are called characteristic x-rays.
The short wavelength cut-off of an
X-ray beam is determined by the
tube voltage
hc
Energy E  e V  h 

•
Part of the kinetic energy is emitted in a continuous spectrum of X rays covering a broad
wavelength range with a broad maximum in intensity and falling off to a definite shortwavelength limit.
•
The emitted X - rays have longer wavelengths than the short-wavelength cutoff value of ‘λo’
•
It is independent of the target element and depends only on the voltage across the X-ray tube. At
the cutoff wave length.
•
The relationship between the voltage and ‘λo’ in angstrom units (A ) is given by
o
hc
12,393
o 

eV
V
V is the X-Ray tube voltage in volts, e is the charge of the electron, h is Plank’s constant, and C
is the velocity of light.
This assumes that an electron looses all its kinetic energy in only one collision whereas in
practice an electron will be involved in impacts with many atoms producing lots of low energy
photons, this so called ‘Bremsstrahlung’ - braking radiation produces a continuous spectrum.
Ionisation of the K shell by electrons with just the right amount of energy results in a
characteristic line spectrum, when electrons in the atom return to the ground state with the
emission of an x-ray photon.
(now the rotating anode variety is more widely used in medical applications)
An X-ray tube is a vacuum tube that produces X-rays. They are used in X-ray machines. X-rays are part of
the electromagnetic spectrum, an ionizing radiation with wavelengths shorter than ultraviolet light. X-ray tubes
evolved from experimental Crookes tubes with which X-rays were first discovered in the late 1800s, and the
availability of this controllable source of X-rays created the field of radiography, the imaging of opaque objects
with penetrating radiation. X-ray tubes are also used in CAT scanners, airport luggage scanners, X-ray
crystallography, and for industrial inspection.
99% of incident energy is converted to heat) the dissipation of heat at the focal spot is one of the main
limitations on the power which can be applied. By sweeping the anode past the focal spot the heat load can be
spread over a larger area, greatly increasing the power rating. With the exception of dental X-ray tubes, almost
all medical X-ray tubes are of this type.
Increasing demand for high-performance CT scanning and angiography systems has driven development of
very high performance medical X-ray tubes. Contemporary CT tubes have power ratings of up to 100 kW and
anode heat capacity of 6 MJ, yet retain an effective focal spot area of less than 1 mm2.
X-ray tube is basically a large vacuum tube containing a heated cathode and an anode or target of Copper or
Molybdenum.
Chromium, Iron, Nickel, & Tungsten are also used for special purposes.
Electrons emitted by the cathode are accelerated through a high-voltage field between the target and cathode.
Commonly used potentials are between 50 and 60 kV. Tubes with 100 kV rating also available for increased sensitivity.
Higher voltage increases the intensity of lines emitted by the X – tube target
On impact with the target, the stream of electrons is quickly brought to rest & transfer their kinetic energy to the atoms of
the material that makes up the target.
X-ray tube :
• Crookes tube is the first device designed to generate X-Rays.
• As with any vacuum tube, there is a cathode, which emits
electrons into the vacuum and an anode to collect the electrons,
thus establishing a flow of electrical current,
• A high voltage power source, for example 30 to 150 kilovolts (kV),
is connected across cathode and anode to accelerate the
electrons.
• The X-ray spectrum depends on the anode material and the
accelerating voltage .
• Electrons from the cathode collide with the anode material.
• Usually tungsten, molybdenum or copper, are the anode materials.
• About 1% of the energy generated is emitted/radiated, usually
perpendicular to the path of the electron beam, as X-rays.
• The rest of the energy is released as heat. Over time, the anode
metal will be deposited from the target onto the interior surface of
the tube, including the glass surface.
• This will slowly darken the tube and was thought to degrade the
quality of the X-ray beam,
• The range of photonic energies emitted by the system can be
adjusted by changing the applied voltage, and installing aluminum
filters of varying thicknesses.
• Aluminum filters are installed in the path of the X-ray beam to
remove "soft" (non-penetrating) radiation.
• The Crookes tube was improved by William Coolidge in 1913.
The Coolidge tube, also called hot cathode tube, is the most widely
used. It works with a very good quality vacuum (about 10-4 Pa, or
10-6 Torr).
Crookes X-ray tube from early
1900s. The cathode is on the
right, the anode is in the center
with attached heat sink at left.
The electrode at the 10 o'clock
position is the anticathode. The
device at top is a 'softener' used
to regulate the gas pressure.
Coolidge tube
• In the Coolidge tube, the electrons are produced
by thermionic effect from a tungsten filament heated by an
electric current.
• The filament is the cathode of the tube.
• The high voltage potential is between the cathode and the
anode, the electrons are thusaccelerated, and then hit the
anode.
• There are two designs: end-window tubes and sidewindow tubes.
• In the end-window tubes, the filament is around the anode,
the electrons have a curved path.
• In the side-window tube, An Electrostatic Lens to focus the
beam onto a very small spot on the anode
• The anode is specially designed to dissipate the heat and
wear resulting from this intense focused barrage of
electrons the area heated is cooled by circulating coolant.
• The anode is precisely angled at 1-20 degrees off
perpendicular to the electron current so as to allow escape
of some of the X-ray photons which are emitted essentially
perpendicular to the direction of the electron current.
• The anode is usually made out of tungsten or
molybdenum.
• The tube has a window designed for escape of the
generated X-ray photons.
• The power of a Coolidge tube usually ranges from 1 to
4 kW.
Coolidge side-window
tube (scheme)
K: filament
A: anode
Win and Wout: water inlet
and outlet of the cooling
device (C)
Rotating anode tube
• The rotating anode tube is an improvement of the Coolidge tube in X-Ray
production.
• X-ray production using this method is very efficient
• The anode consists of a disc with an annular target close to the edge.
• The anode disc is supported on a long stem which is supported by bearings
within the tube.
• The anode can then be rotated by electromagnetic induction from a series of
stator windings outside the evacuated tube.
• Because the entire anode assembly has to be contained within the evacuated
tube, heat removal is a serious problem, further exacerbated by the higher power
rating available.
•
Direct cooling by conduction or convection, as in the Coolidge tube, is difficult.
• In most tubes, the anode is suspended on ball bearings with silver powder
lubrication which provide almost negligible cooling by conduction.
• A recent development has been liquid gallium lubricated fluid dynamic
bearings which can withstand very high temperatures without contaminating the
tube vacuum.
• The large bearing contact surface and metal lubricant provide an effective
method for conduction of heat from the anode.
• The anode must be constructed of high temperature materials.
• The focal spot temperature can reach 2500°C during an exposure, and the anode
assembly can reach 1000°C following a series of large exposures. Typical
materials are a tungsten-rhenium target on a molybdenum core, backed with
graphite.
• The rhenium makes the tungsten more ductile and resistant to wear from impact
of the electron beams.
• The molybdenum conducts heat from the target.
• The graphite provides thermal storage for the anode, and minimizes the rotating
mass of the anode.
Simplified rotating anode
tube schematic
A: Anode
C: cathode
T: Anode target
W: X-ray window
In an X-ray diffraction measurement, a crystal is mounted on a goniometer and
gradually rotated while being bombarded with X-rays, producing a diffraction
pattern of regularly spaced spots known as reflections. The two-dimensional
images taken at different rotations are converted into a three-dimensional model
of the density of electrons within the crystal using the mathematical method
of Fourier transforms, combined with chemical data known for the sample. Poor
resolution (fuzziness) or even errors may result if the crystals are too small, or
not uniform enough in their internal makeup.
X-rays primarily interact with electrons in atoms.
When x-ray photons collide with electrons, some photons from the incident beam will be deflected away
from the direction where they originally travel, much like billiard balls bouncing off one anther.
If the wavelength of these scattered x-rays did not change (meaning that x-ray photons did not lose any
energy), the process is called elastic scattering (Thompson Scattering) in that only momentum has been
transferred in the scattering process.
These are the x-rays that we measure in diffraction experiments, as the scattered x-rays carry information
about the electron distribution in materials.
On the other hand, in the inelastic scattering process (Compton Scattering), x-rays transfer some of their
energy to the electrons and the scattered x-rays will have different wavelength than the incident x-rays.
Diffracted waves from different atoms can interfere with each other and the resultant intensity distribution
is strongly modulated by this interaction.
If the atoms are arranged in a periodic fashion, as in crystals, the diffracted waves will consist of sharp
interference maxima (peaks) with the same symmetry as in the distribution of atoms.
Measuring the diffraction pattern therefore allows us to deduce the distribution of atoms in a material.
The peaks in a x-ray diffraction pattern are directly related to the atomic distances.
Let us consider an incident x-ray beam interacting with the atoms arranged in a periodic manner as
shown in 2 dimensions in the above illustrations.
The atoms, represented as green spheres in the graph, can be viewed as forming different sets of planes
in the crystal (colored lines in graph on left).
For a given set of lattice planes with an inter-plane distance of d, the condition for a diffraction (peak) to
occur can be simply written as
Scheme of X-ray spectrometer ( Block diagram )
1 high voltage unit with stabilizer, 2 current stabilizer,
3 X-ray tube, 4 slit, 5 collimator, 6 specimen, 7 detector,
8 amplifier, 9 scaler, 10 rate meter 11 printer 12 recorder
The scheme of a goniometer (S source,
Cl & C2 coIlimtros, D detector)
Collimator: It is a device that filters a stream of rays so that only those traveling parallel to a specified
direction are allowed through. Collimators are used in neutron, X-ray, and gamma-ray optics because it is
not yet possible to focus radiation with such short wavelengths into an image through the use of lenses
as is routine with electromagnetic radiation at optical or near-optical wavelengths.
Source of radiation
Source of radiation
Stack of parallel slits or bunch of
fine tubes acts as collimator
Un-collimated radiation
Collimated radiation
Various elemental effects
Results Using a Mo X-Ray generator:
Zirconium - Absorbs Bremsstrahlung & K-Beta.
Iron - Absorbs the entire spectra.
Molybdenum - Absorbs Bremsstrahlung - Leaving K-Beta & K-Alpha.
Aluminium - 'Pinches' Bremsstrahlung* & Removes 3rd Generation peaks.
Silver - Same as Aluminium, But to greater extent.
Indium - Same as Iron, But to lesser extent.
Copper - Same as Aluminium, Leaving only 1st Generation Peaks.
- Bremmstrahlung pinching is due to the atomic mass. The denser the atom, the
higher the X-Ray Absorption. Only the higher energy X-Rays pass through the
filter, appearing as if the bremmstrahlung continuum had been pinched.
- In this case, Mo appears to leave K-Alpha and K-Beta alone while absorbing
the Bremsstrahlung. This is due to Mo absorbing all of the spectra's energy, but
in doing so produces the same characteristic peaks as generated by the target.
X-Ray Diffraction
Tubes with copper targets, which produce their strongest characteristic radiation (K 1) at a wavelength of
about 1.5 angstroms, are commonly used for geological applications.
If an incident X-ray beam encounters a crystal lattice, general scattering occurs.
Although most scattering interferes with itself and is eliminated (destructive interference),
Diffraction occurs when scattering in a certain direction is in phase with scattered rays from other atomic
planes.
Under this condition the reflections combine to form new enhanced wave fronts that mutually reinforce each
other (constructive interference).
The relation by which diffraction occurs is known as the Bragg law or equation.
Because each crystalline material has a characteristic atomic structure, it will diffract X-rays in a unique
characteristic pattern.
An X-ray diffraction
pattern of a
crystallized enzyme.
The pattern of spots
(called reflections)
can be used to
determine the
structure of the
enzyme.
The Gemini A Ultra is mounted
with a 135 mm diagonal Atlas
CCD detector (2:1
demagnification and 2k x 2k
Kodak 4-port CCD chip) and comounted Enhance (Mo) and
Enhance Ultra (Cu) X-ray sources.
The Enhance Ultra providing a
rotating anode alternative high
brilliance copper X-ray source.
The basic geometry of an X-ray diffractometer involves a source of monochromatic radiation and an X-ray
detector situated on the circumference of a graduated circle centered on the powder specimen. Divergent
slits, located between the X-ray source and the specimen, and divergent slits, located between the
specimen and the detector, limit scattered (non-diffracted) radiation, reduce background noise, and collimate
the radiation. The detector and specimen holder are mechanically coupled with a goniometer so that a
rotation of the detector through 2x degrees occurs in conjunction with the rotation of the specimen through x
degrees, a fixed 2:1 ratio.
A curved-crystal monochromator containing a graphite crystal is normally used to ensure that the detected
radiation is monochromatic. When positioned properly just in front of the detector, only the K radiation is
directed into the detector, and the Kß radiation, because it is diffracted at a slightly different angle, is
directed away. The signals from the detector are filtered by pulse-height analysis, scaled to measurable
proportions, and sent to a linear ratemeter for conversion into a continuous current. Common output devices
include strip-chart recorders, printers, and computer monit
X – Ray Diffractometer
X – Ray absorptiometer :
• Techniques employing the mass absorption coefficient or the critical absorption edges are used
for measurement and analysis.
• It has been known that the mass absorption coefficient, neglecting scattering, is proportional to
the 4th power of the atomic number Z, and cubit power of the wavelength. Thus
• where K is a constant. Hence, if absorption can be measured it is only simple to trace out the
constituent by simple relations as in the above equation.
• The measurement method uses the comparison principle.
• White or monochromatic X-ray is allowed to pass through two channels alternately, consisting
of a reference and the test.
• The output falls on a fluorescent screen followed by a light funnel and a photo- multiplier or a
scintillation detector.
• By driving in or out a variable thickness wedge attenuator in the reference channel, the
photomultiplier output is held constant so that the output of the comparator or better at the
servo-output. i.e.. coupled to the wedge movement.
• The measurement may be made continuously on-line and is most suitable for binary mixtures.
A typical example is analysing lead in petrol, i.e., a heavier element in lighter ones.
• The general arrangement of a nondispersive X-ray
absorptiometer is shown in Figure
• A tungsten target X-ray tube is operated at 15—45
kV.
• In the X-ray beam is a synchronous motor-driven
chopper that alternately interrupts half of the X-ray
beam.
• A variable-thickness aluminum attenuator (in the
shape of a wedge) is placed between the chopper
and the reference sample compartment.
• Duplicate reference and sample cells up to 65 cm
long can be accommodated and those for liquids
and gases arranged for continuous-flow process
stream analyses if desired.
• Both halves of the X-ray beam fall on a common
phosphor-coated photomultiplier tube, which is
protected from visible light by a thin metallic filter.
• In operation, a reference sample is placed in the
appropriate cell and the specimen to be analyzed
in the sample tube.
• The attenuator is adjusted until the absorption in
the two X-ray beams is brought into balance.
• The change in thickness of aluminum required for
different samples is a function of the difference in
composition.
• Prior calibration enables a determination in terms of the solute in an unknown.
• Liquids are simplest to handle.
• With solids the thickness of solid specimens and the density of samples of powders must be
uniform to a precision greater than that expected in the results.
• Polychromatic absorptiometry can be used to determine chlorine in hydrogen.
• Sulfur in crude oil can be distinguished from the carbon-hydrogen residuum.
• Other examples are barium fluoride in carbon brushes, barium or lead in special glasses, and
chloride in plastics and hydrocarbons.
• In fact, the method is applicable to any sample that contains one element that is markedly heavier
than the others and when the matrix is essentially invariant in concentration.
X – Ray Fluorescence Spectrometer :
The commonly used techniques in this type are
(i)The wavelength dispersive type,
(ii) The energy dispersive type and
(iii) The non-dispersive type.
Single channel wave-length dispersive type :
• In single channel type X-ray fluorescence spectrometry. a
pair of X-ray sources is used with W- target for shorter
wavelength and another with Cr for longer wavelength.
• Crystals also are changed and a He- atmosphere or
vacuum for longer wavelength is maintained.
• Multi channel types are large systems requiring as many
crystals and detectors as the channel numbers.
• Each detector has its own amplifier, pulse height selector,
scaler, counter integrator etc.
• The best way to tackle the operation of such an instrument
is to use a computer/micro-processor which also helps in
its control, data processing and display or results.
Wave length dispersive type of
single channel fluorescence
spectrometer
Energy dispersive type Fluorescence diffractometer :
• The energy dispersive type does not require
collimators and crystal diifractors.
•
•
They also contain no moving components in the
excitation and detection sides.
Here source is a polychromatic one and the
detector is the semiconductor type.
•
Detector is closely mounted and hence energy
reaching the detector is very high.
•
The instrument has low resolution at longer wavelengths, longer thaii 1 m.
• In its multi channel variety all the lines are measured simultaneously and the advantage of FT
• spectrometry can be taken for improved signal to noise ratio.
• Wave length detected angle indicating the element and the peak the content of the element.
• In the non-dispersive type a sample containing a certain element is irradiated with X-ray of
suitable
• wavelength which produces a fluorescent line of the element.
• This radiation is then passed through a pair of filters with absorption edges just above and below
the fluorescent wavelength.
• The difference in the detected signals in the two cases is in proportion to the amount of element
present in the sample.
• Commercial routine instruments in this category have an accuracy of about 1%.
Fluorescent spectrometer for liquid samples :
• A schematic set-up for fluorescent spectrometer
for liquid samples is shown in the figure.
• The samples are kept in beakers on a rotating
table, the samples are also rotated for uniform
exposure.
• Radiation from an X-ray generator strikes a
specimen to produce characteristic fluorescent
lines.
• The fluorescent radiation is analysed by a rotating
crystal in an X-ray goniometer.
• In the goniometer, the primary slit, the analyzing
crystal and the secondary slit are on the focal
circle for satisfying the Bragg’s conditions.
• This method can be used for analysis of solid, liquid and gas samples as well but is commonly
used for heavier elements and can measure down to a value of 1 ppm.
• Interference occurs due to absorption, particularly if the fluorescent radiations of the detectants
are absorbed by another constituent in the sample. For example, Pb absorbs fluorescent
radiation of chlorine as also the main radiation.
Radiation Detectors :
• Detection actually means measurement of the radiation with its energy content
and other related properties.
• The detection system would, therefore, consist of a detector and a measuring
circuit.
• The detectors are to be chosen according to the requirement.
• Photographic film is used for general diffraction studies.
• The GM counter and proportional; counter which are gas ionization types, the
scintillation counters and the semiconductor detectors are being increasingly
used for detection purposes.
• The semiconductor type is used for soft or low energy radiation detection.
• The choice is to a large extent dependent on the X-ray energy to be detected.
• The interaction of radiation in different types of detectors
is of different nature. There are, two different methods of
measurement after detection of the radiation.
• One is the counter type in which the detector output, in
the form of a series of pulses, is counted by a counter
over a certain interval of time.
• Each pulse represents an interaction of radiation with
the detector element.
• The other is mean level detection, where the pulses that
occur are so high per unit of time that resolution to
individual ones is not possible and an average effect,
generally in the form of current output, is measured.
Set up of gas-tilled detector chamber
• Ionizatio chambers including proportional and Geiizer Muller counters employ gas filled
chambers with varying applied voltages to the electrodes.
• A central electrode separated from the chamber by insulator is kept at a potential E. with respect
to the chamber wall and this is maintained through the shunt combination of reistsnace R and
capacitance C.
• The RC product is chosen to be much larger compared to the time required by any ion in the
chamber to be collected by appropriate electrode.
• For a charge q across C, the voltage output is
• When radiation falls on the chamber, let -ion pairs be produced and let them drift towards the
oppositely charged electrodes.
• Then the pulse voltage magnitude, being the measure of the ions collected by the electrode at
various supply voltages.
Radiation Detectors :
If the pulse voltages are plotted for
various applied voltages, it is as shown
in the figure.
V
The curve is divisible in to several
regions with respect to applied voltages.
IV
I
II
III
Pulse amplitude as a function of applied voltage for
the ionization type of detectors.