xray diffraction instrumentation

Instrumentation of x-ray Diffraction
Submitted to: Dr Rakesh Kumar Marwaha
Submitted by: Bindu
M Pharmacy (DRA)

Instrumentation

Radiation source

Collimator

Monochromator

Sample handling

Detector

X ray diffraction device

Radiation Source

X ray tube

Radioisotopes

Secondary fluorescent

X ray Tube

X rays are generally produced in XRT.

Features & Functioning of XRT

Composed of evacuated tube possessing
cathode (tungsten filament) at one end &
anode(metal target) at another end.

Passage of current through tube causes
tungsten filament to glow & emits electron.

Among the two electrodes large voltage
difference is applied, causing electrons to move
at high velocity from filament and strike to
anode.


Due to high velocity impact of electrons on to
the target, inner shell electrons of metal gets
dislodge, which causes the outer shell electrons
to jump to a lower energy shell to replace the
dislodge electrons.

These electronic transitions results in the
generation of X-rays. The produced X-rays are
allowed to move through a window of X-ray
tube.

Collimator

A device that is used for narrowing of a beam of
particles or waves.

Key Concepts

Collimator makes, random directional X-rays to
be narrow and parallel.

These are usually made up of tungsten,
stainless steel and ceramics.

Pitch ranges from 400microns to 6 microns.

Monochromators
A device that is used for removal of unwanted
radiation.
1. Filters Monochromators
2. Crystal Monochromators

xray diffraction instrumentation

Filters

Made up of specialized material that absorb
unwanted radiation and passes the desired
radiation.

e.g X-rays produced from molybdenum are
monochromatized by zirconium filters.

These zirconium filters absorb the molybdenum
and allows to pass the k(beta) lines.

For target element like copper, iron,
cobalt,nickel the filters like Ni, Mn, & Co are
commonly used.

Crystal Monochromators

These are the device that partially polarize a
non polarized X-ray beam on the basis of
Bragg's diffraction.

For broad band use: pyrolytic graphite material

For narrow band use: Si, Ge or quartz
Types:
1. Flat Crystal
2. Curved Crystal

Detectors
X-rays can be detected using two types of
detectors:
1. Photographic Detectors
2. Counter Detectors
a) Geiger Muller Tube Counter
b) Proportional Counter
c) Scintillator Counter
d) Semiconductor Detector

Photographic detectors

A plane or cylindrical film is used to determine
the position & intensity of X-rays.

Cylindrical films are developed by exposing the
detectors to X-rays

The extent of blackening of developed film is
expressed as density. Density is the direct
measurement of X-ray energy which causes
blackening of photographic film.
D = log.I/I

Counter Detectors
1. Geiger muller Tube Counter

Geiger Muller Tube Counter

It is composed of glass tube(19 mm dia). The
tube is comprised of a half metal cylinder of
about 4 inches length, made up of copper.

Along the axis of cylinder a thin metal wire of
tungsten is tied. The cylinder & wire are
connected to an electrical voltage source.

The tube is filled with gas,usually Argon at a low
pressure.a voltage is set up between the
cathode and anode .

Working

When X-rays enters the Geiger tube, a collision
occurs between the gas molecule and X-rays.
Thereby electrons are ejected out of atoms of
neutral molecules of argon gas.

This causes production of positive molecular
ions and free electrons. These electrons being
negatively charged, moves towards anode and
positively charged argon ions moves toward
cathode.

A potential gradient is applied to accelerate
electrons. This causes electrons to pick much
energy to eject more electrons out of atom.


This in turn picks up further energy and liberates
even more electrons. Such a progressive process
is called avalanche.

Positive ions hit the cathodic half cylinder with
enough energy to eject further more electrons.
Therefore avalanche of electrons incline on wire
which is detected as a pulse of electric current.

The electric pulse so generated indicates passing
of a charged particle through the tube. This pulse
can be read or measured through a meter.

Merits:
1.significant signals are obtained for a given
X-ray intensity
2. Economical
3. Requires less Maintenance
Demerits:
1. Used for measuring low rate X-rays.
2. Low efficiency below 1angstrom
3. Unable to measure energy of ionizing
radiation

Proportional Counter

Similar in construction to Geiger-Muller Counter

Heavy gas like P-10 or xenon or krypton is used.

Proportional counter is a combination of two
ionization regions
a) Ion drift region: region that exist in outer volume
of the chamber.
b) Avalanche region: region that exist in the
immediate vicinity of the anode.

A specialized circuit is connected to tube so that
X-rays of particular energy can be measured.


Therefore the output of proportional counter
apparatus depends upon incident X-ray
intensity.

Merits: count high rates without error
good sensitivity and efficiency

Demerits: expensive
complex circuit.

Scintillator Counter
Scintillator: a material that exhibits the property of
luminescence on excitement by ionizing
radiation.

A scintillator detector is a combination of scintilla
tor with an electric light sensor such as PMT or
photodiode.

PMT absorbs the light emitted by the scintillator
and further re-emits in the form of electrons.

The subsequent multiplication of those electrons
result in an electrical pulse which on analysis
provides information about incident radiation
particles.


There are different scintillation materials available
such as sodium iodide, anthracene and
naphthalene etc.

A scintillator counter possesses many advantages
such as resolution of gamma rays for Co & Cs,
linearity, density, speed, transparency and also
manufacturing cost.

Semiconductor Detector

These detectors assists in promotion of X-rays
through generated electrons into conduction
bands. Therefore the current so generated is a
direct measurement ofX-rays intensity.

These semiconductor detectors commonly
makes use of Indium antimonide or Lead
telluride or Mercury cadmium telluride.

In these detectors the thin layer of p-type of
semiconductor is kept over the n-type surface to
make a diffused p-n junction.


The p-type surface is exposed to radiation which
generates hole and electron pairs.

These holes and electron pairs are separated by
internal field existing on p-n junction.

As a result voltage is generated. This assembly
of semiconductor is arranged into a vacuum
bottle cooling unit.

The cooling is done through liquid nitrogen or
Joule Thomson coolers of compressed nitrogen
gas or liquid helium.

References

N K Fuloria, S Fuloria, “ Spectroscopy
Fundamentals and Data Interpretation”,
Studium Press Pvt. Ltd, X-Ray Spectroscopy,
Interpretation, page no: 457-465.

http://tocs.ulb.tu-darmstadt.de/95887040.pdf

https://www.xos.com/XRD

xray diffraction instrumentation

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