3. Theory
When a monochromatic beam of X-rays is allowed to fall
on the powder of a crystal, then following possibilities may
happen:-
There will be some particles out of the random
orientation of small crystal in the fine powder, which lie
within a given set of lattice planes for reflection to occur
4. While another fraction of the grains will have another set
of planes in the correct position for the reflections to
occur and so on.
Also, reflections are also possible in the different order
for each set.
7. Production of X Ray
X-rays are produced inside the x-ray tube when high energy projectile
electrons from the filament interact with the atoms of the anode.
Conditions necessary:
Source of electrons
Target (anode)
High potential difference
Cooling facility
9. Continue….
There is a cathode which is a filament of tungsten metal heated by a battery
to emit the thermionic electrons.
This beam of electrons moves towards anode and attain the kinetic energy
and 99% of energy is converted into heat via collision and remaining 0.5- 1%
is converted to X-rays via strong coulomb interactions (Bremsstrahlung
process).
10. Continue…
Generally the target gets very hot in use.
This problem has been solved to some extent by cooling the tube with water.
12. In order to get a narrow, focused beam of x-rays, the x- rays generated by the
target material are allowed to pass through a closely packed metal plates
separated by a small gap.
The collimator absorbs all the x-rays except the narrow beam that passes
between the gap.
14. Continue….
A monochromator is an optical device that transmits a mechanically
selectable narrow band of wavelengths of light or other radiation chosen from
a wider range of wavelengths available at the input.
The name is from the Greek roots mono-, "single", and the chroma, Latin
"color" , and suffix at or, denoting an agent.
There are two types;
Filter Monochromator
Crystal Monochromator
15. Filter Monochromator
A filter is a window of material that absorbs undesirable radiation but allows
the radiation of required wavelength to pass.
An interesting example is use of zirconium filter which is used for
molybdenum radiation.
16. Continue…
When X-rays emitted from molybdenum are allowed to pass through a
Zirconium filter, the Zirconium strongly absorbs the radiation of molybdenum
at short wavelengths but weakly absorbs the K alpha lines of molybdenum.
Thus, zirconium allows the K beta lines to pass.
17. Crystal monochromator
A crystal monochromator is made up of a suitable crystalline material
positioned in the X-ray beam so that the angle of reflecting planes satisfied
Bragg's equation for the required wavelength.
The beam is split up by the crystalline material into the component
wavelengths in the same way as a prism splits up the white light into rainbow.
Such a crystalline substance is called an analyzing crystal.
19. Detectors
A. Photographic methods:
In order to record the position and intensity of X-ray beam a plane or cylindrical film is
used. The film after exposing to X-rays is developed.
The blackening of the developed film is expressed in terms of density units D given By,
D = log I/I
Where as,
I and I refer to the incident and transmittance intensities of X-rays.
The quantity D is related to the total X-ray energy that causes the blackening of
photographic film.
20. B. Counter method :
The Geiger tube is filled with an inert gas like argon and the central wire
anode is maintained at a positive potential of 800 to 2500V.
When an X-ray is entering the Geiger tube, this ray undergoes collision with
the filling gas, resulting in the production of an ion pair: the electron
produced moves towards the central anode while the positive ions move
towards outer electrode.
22. Crystal
What is crystal ?
A crystal is a solid whose atoms are arranged in a highly ordered repeating
pattern.
These patterns are called as crystal system. WH 9 12 ជ 13 14
Recommended X-RAY CRYSTALLOCRAPHY X-ray Crystallography.
23. Types of crystal
1. Covalent crystals
Covalent crystals are crystals whose atoms are connected with covalent
bonds
Extremely strong and very hard to break
Have high melting points
Example-Diamonds
24. 2. Ionic Crystals
Ionic crystals are crystals whose atoms are held together with ionic bond.
These crystals are typically solid with a high melting point.
Example- Table salts
3. Metallic Crystals
Metallic crystals are crystals made of metal elements.
They are extremely good conductors of heat and electricity
Example-Gold nuggets
25. 4. Molecular Crystals
molecular crystals are crystals formed from weak bonds called hydrogen
bonds.
Because of this, molecular crystals typically have lower melting points
than other crystals.
Example- dry ice, frozen carbon dioxide etc.
26. Applications :
X-ray powder diffraction is most widely used for the identification
of unknown crystalline materials (e.g. minerals, inorganic compounds).
Characterization of crystalline materials
Identification of fine-grained minerals such as clays and mixed layer
clays that are difficult to determine optically
Determination of unit cell dimensions
To identify crystalline phases and orientation
27. Applications :
To determine structural properties
To measure thickness of thin films and multi-layers
To determine atomic arrangement
Measurement of sample purity
Particle size determination
Determination of Cis- Trans isomerism
It is used to assess the weathering and degradation of natural, synthetic and minerals.
