X-RAYS
SDM COLLEGE
UJIRE
SNKAKATHKAR
DEPT OF PHYSICS
WHAT ARE X-RAYS ?
• X-rays are electromagnetic
radiations having
wavelength ranging from 0.01 to
10 nanometers
• Frequency between (3×10 Hz to
16
19
3×10 Hz)
• energies between 100eV to
100 KeV.
•
DISCOVERY
• IN 1895 Röntgen discovered X
rays
• In 1901 Röntgen was awarded the
very first Nobel Prize in Physics.
• He did not even want the rays to be
named after him
Production
• X-rays are produced when fast moving
electrons strike a metal target.
• The electrons are liberated from the
heated filament and accelerated by a
high voltage towards the metal target.
• The X-rays are produced when the
electrons collide with the atoms and
nuclei of the metal target.
4
* KAKATHKAR
• When the electrons are suddenly
decelerated on impact, some of the
kinetic energy is converted into EM
energy, as X-rays.
• Less than 1 % of the energy
supplied is converted into X-
radiation during this process. The
rest is converted into the internal
energy of the target.
COOLIDGE X-RAY TUBE
LIVE PRODUCTION OF X RAYS
Evacuated
glass tube
Target
Filamen
t
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ELECTROMAGNETIC SPECTRUM
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Properties of X-rays
• X-rays travel in straight lines.
• X-rays cannot be deflected by electric field or
magnetic field.
• X-rays have a high penetrating power.
• Photographic film is blackened by X-rays.
• Fluorescent materials glow when X-rays are
directed at them.
• Photoelectric emission can be produced by X-rays.
• Ionization of a gas results when an X-ray beam is
passed through it.
* KAKATHKAR 9
SOFT and HARD X rays
SOFT X RAYS HARD XRAYS
• Have longer wavelength • Have short wavelength
• less penetrating • Are more penetrating
• Are produced by low voltage • Are produced by high voltage
• Are produced by slow moving • Are produced by fast moving
electrons
electrons
• (energy above 5–10 keV, below
• soft X-rays are easily absorbed 0.2–0.1 nm wavelength)
in air; • Due to their penetrating ability, hard
• Used in Soft x-ray microscopy. X-rays are widely used to image the
inside of objects, e.g., in medical
AnX-
radiography and airport security
ray microscope uses em radiatio
• wavelengths of hard X-rays are
n in the soft X-ray band to similar to the size of atoms they are
produce images of very small also useful for determining crystal
objects structure
X-ray Spectum
• Graph obtained by plotting intensity of x rays verses
wave length for a given accelerating potential is x ray
spectrum
• Using a crystal as a wavelength selector, the intensity
of different wavelengths of X-rays can be measured.
* KAKATHKAR 11
X RAY SPECTRA
• Close analysis of X ray spectra
reveals that for a particular
accelerating Potential,X rays of
different wavelengths or frequencies
are liberated with varying intensities.
• This is due to different velocities of
accelerated electrons towards the
cathode having different energies.
each electron gives rise to its own X
ray
X RAY SPECTRA
FEATURES OF X RAY SPECTRA
• Consider the graph for 15 KV potential
• A continuous X-radiation in which the
intensity varies smoothly with wavelength
• The intensity reaches a maximum value as
the wavelength increases, then the intensity
falls at greater wavelengths
• This is called continuous X ray spectrum
FEATURES OF X RAY SPECTRA
• Minimum wavelength which depends on the
tube voltage. The higher the voltage the
smaller the value of the minimum
wavelength.
• Sharp peaks of intensity occur at
wavelengths which are unaffected by change
of tube voltage
• This is called characteristic X ray spectrum
EXPRESSION FOR MINIMUM WAVELENGTH
• When an electron hits the target its entire kinetic
energy is converted into a photon.
• Energy of an electron which is accelerated by a PD
of V volts is, K E=eV.
• Hence hf = eV and the maximum frequency
fmaxλmin=C
fmax=
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ORIGIN OF CHARACTERISTIC X RAYS
• When high energetic
electron falls on the
anode it will penetrate
deep into the surface of
the target and knock out
tightly bound electron
• Now another electron
from higher orbits jumps
to occupy the vacancy
emitting extra energy as
characteristic x radiation
CHARACTERISTIC X-RAY SPECTRA
• Different target materials
give different wavelengths for
the peaks in the X-ray
spectra.
• The peaks for any target
element define its
characteristic X-ray spectrum.
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Origin of characteristic X rays
Moseley’s law
• Moseley did a systematic study of characteristic x
rays using 38 different materials
• Characteristic x ray spectral lines are of two groups
• K series containing K alpha,K beta lines of shorter
wavelength
• L series containing of L alpha,L beta of longer
wavelength
• Unlike optical spectra these contain only simple few
lines
Mo Target impacted by electrons accelerated by a 35
kV potential
Kα
Characteristic
White Kβ radiation →
Intensi
radiati due to energy
on transitions
ty
in the atom
0.2 0.6 1.0 1.4
Wavelength
* (λ) KAKATHKAR 22
CHARACTRISTIC X RAYS FOR DIIFERENT TARGET METALS
Target Metal λ Of Kα radiation (Å)
Mo 0.71
Cu 1.54
Co 1.79
Fe 1.94
Cr 2.29
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Statement of Moseley law
• The square root of frequency of emitted x-
rays during the characteristic x-ray emission
is directly proportional to the atomic
number of the target.
Importance of the law
• Based on the study of x rays ,Mosley proved that
atomic number is the characteristic property of an
element but not at weight
• Therefor It is the atomic number but not the
atomic weight which determines the arrangement
of elements in the periodic table
• This helped to perfect the periodic table by
discovery of new elements Hafnium,Illinium etc
• It helped to determine the atomic number of rare
earth elements and fixing their position in the
periodic table
Uses of X-rays
• In medicine
To diagnose illness and for
treatment.
• In industry
To locate cracks in metals.
• X-ray crystallography
To explore the structure of
materials.
* KAKATHKAR 26
X RAY DIFFRACTION
INTRODUCTION
• Diffraction is a wave
phenomenon in which
the apparent bending
and spreading of waves
when they meet an
obstruction.
• Diffraction occurs with
electromagnetic waves,
such as light and radio
waves, and also in
sound waves and water
waves.
Crystalline solids
• In crystals constituent
atoms are arranged
periodically and are
confined to different
planes
• When waves are incident
on these planes ,they are
diffracted by the
obstacles and give rise to
diffraction patterns
X RAY DIFFRACTION
• For electromagnetic radiation to be
diffracted the spacing in the grating should
be of the same order as the wavelength of
radiation
• In crystals the typical inter-atomic spacing is
2-3 Å so it can act as grating for x rays
• Hence, X-rays can be used for the study of
crystal structures
Crystal structure by X ray
diffraction
• When X ray beam is
incident on a
crystalline material,
the scattered beams
may add together in a
few directions and
reinforce each other
to give diffracted
beams resulting in
bright and dark bands
X RAY CRYSTALLOGRAPHY
• X-ray crystallography is a
technique in
crystallography in which
the pattern produced by
the diffraction of x-rays
through the closely
spaced lattice of atoms in
a crystal is recorded and
then analyzed to reveal
the nature of that lattice.
BRAGG’S LAW
• Bragg law identifies the angles of the
incident radiation relative to the lattice
planes for which diffraction peaks occurs.
• Bragg derived the condition for constructive
interference of the X-rays scattered from a
set of parallel lattice planes.
BRAGG’S LAW
• Consider a crystal with inter-atomic distance
dhkl ,let an x ray beam is incident on atomic
planes at an angle θ.let λ be the wave length
of x rays.
• The two rays reflected from atomic planes
interfere each other to give interference
pattern.
• For constructive interference path diff
should be equal to nλ
Bragg’s Law
Rays 1 and 2
interfere
2 1 1 2 constructively if
Total Path Difference
is integral multiple of
O
θ θ the
Total p.d. = AB +λBC
wavelength,
A C ∆OAB and ∆OCB are
dhkl equivalent.
θ θ ∴AB=BC=dhkl sinθ
B Total path diff is
▪ 2 dhkl sinθ
▪ or For constructive interference: nλ = 2d Sinθ 35
BRAGG’S SPECTROMETER
• used to determine
the wavelength of X
– rays or crystal
structure
• It is similar in
construction to an
ordinary optical
spectrometer
BRAGG’S SPECTROMETER
• X–rays from an X-ray tube are
made to pass through two
fine slits S1 andS2 which
collimate it into a fine pencil.
• This fine X-ray beam is then
made to fall upon the
crystal ‘C’ (sodium chloride)
mounted on the
spectrometer table.
BRAGG’S SPECTROMETER
• This table is capable
of rotation about a
vertical axis and its
rotation can be read on a
circular graduated scale S.
• The reflected beam after
passing through the slits
S3 and S4 enters the
ionization chamber.
BRAGG’S SPECTROMETER
• The X-rays entering the
ionization chamber
ionize the gas which
causes a current to
flow between the
electrodes and the
current can be
measured by
galvanometer G.
BRAGG’S SPECTROMETER
• The ionization current is a
measure of the intensity of X-
rays reflected by the crystal.
• The ionization current is
measured for different values
of glancing angle θ. A
graph is drawn between the
glancing angle θ and
ionization current
BRAGG’S SPECTROMETER
• Here the peaks refer to
different orders of
diffraction maxima,
n=1,2,3 etc
• If d is the spacing
between grating
elements for nth order
Braggs equation is
2d Sinθ= nλ
Measurement of wave length or d
• For the measurement of wavelength of
characteristic spectra the metal understudy
is used as the target if d is known
wavelength can be measured for n=1
• Experiment is repeated for other orders and
average is determined.
• If wave length is known d can be estimated
