1. X - Rays
Course: B.Tech
Subject: Engineering Physics
Unit: IV
Chapter: 2
1

2. Objectives
• Introduction and production of X-Rays
• Properties of X-Rays
• Diffraction of X-Rays
• The Bragg’s X-Ray spectrometer
• Continuous spectra
• Characteristics Radiation
• Moseley’s law
• Absorption of X-Ray
• Compton effect
• Applications of X-Rays

3. Introduction of X-Rays
• Rontgen discovered X-rays in 1985 during some
experiments with a discharge tube.
• He noticed that a screen coated with barium
platinocyanide present at a distance from the
discharge tube. Rontgen called these invisible
radiations “X-rays”.
Finally, he concluded that X-rays are produced
due to the bombardment of cathode rays on the walls
of the discharge tube.
• X-rays are highly penetrating and it can pass through
many solids.
• X-rays occur beyond the UV region in the
electromagnetic spectrum.

4. Production or Generation of X-
rays
X-rays are produced by an X-ray tube. The
schematic of the modern type of X-ray tube is
shown in above figure.
2

5. It is an evacuated glass bulb enclosing two
electrodes, a cathode and an anode.
The cathode consists of a tungsten filament
which emits electrons when it heated. The
electrons are focused into a narrow beam with
the help of a metal cup S.
The anode consists of a target material, made of
tungsten or molybdenum, which is embedded in
a copper bar.
Water circulating through a jacket
surrounding the anode and cools the anode.
Further large cooling fins conduct the heat away
to the atmosphere.

6. • The face of the target is kept at an angle
relative to the oncoming electron beam. A very
high potential difference of the order of 50 kV is
applied across the electrodes.
The electrons emitted by the cathode are
accelerated by the anode and acquire high
energies of order of 105 eV. When the target
suddenly stops these electrons, X-rays are
emitted.
• The magnetic field associated with the
electron beam undergoes a change when the
electrons are stopped and electromagnetic waves

7. • The grater of the speed of the electron beam, the
shorter will be the wavelength of the radiated X-
rays. Only about 0.2 % of the electron beam energy
is converted in to X-rays and the rest of the energy
transforms into heat. It is for the reason that the
anode is intensively cooled during the operation of
X-ray tube.
• The intensity of the electron beam depends on
the number of electron leaving the cathode. The
hardness of the X-rays emitted depends on the
energy of the electron beam striking the target. It
can be adjusted by varying the potential difference
applied between the cathode and anode. Therefore,
the larger potential difference, the more penetrating
or harder X-rays.

8.  Properties of X-Rays…
They have relatively high penetrating power.
They are classified into Hard X-rays & Soft X-
rays.
The X-rays which have high energy and
short wavelength is known as Hard X-rays.
The X-rays which have low energy and
longer wavelength is known as Soft X-rays.
X-rays causes the phenomenon of
flouroscence.

9.  Properties of X-Rays…
They are absorbed by the materials through
which they traverse.
X-rays travel in straight line. Their speed in
vacuum is equal to speed of light .
X-rays can affect a photographic film.
X-rays are undeflected by electric field or
magnetic field.

10.  Diffraction of X-Rays – Bragg’s
law
Consider a crystal as made out of
parallel planes of ions, spaced a distance d
apart. The conditions for a sharp peak in the
intensity of the scattered radiation are:
1. That the X-rays should be secularly
reflected by the ions in any one plane.
2. That the reflected rays from successive
planes should interfere constructively.
• Path difference between two rays reflected
from adjoining planes: 2dsinθ,

11. • For the rays to interfere constructively,
this path difference must be an integral
number of wavelength λ,
nλ =2dsinθ -------
(1)
Bragg angle is just the half of the total angle by
which the incident beam is deflected.
3

12. The Bragg’s X-Ray spectrometer
• An X-ray diffraction experiment requires,
• X-ray source
• The sample
• The detector
• Depending on method there can be variations
in these requirements. The X-ray radiation
may either monochromatic or may have
variable wave length.
• Structures of polycrystalline sample and single
crystals can be studied. The detectors used in
these experiments are photographic film.

13.  The schematic diagram of Bragg’s X-ray
spectrometer is given in above.
4

14. • X-ray from an X-ray tube is collimated by passing
team through slits S1 and S2. This beam is then
allowed to fall on a single crystal mounted on a table
which can be rotated about an axis perpendicular to
the plane of incident of X-rays. The crystal behaves
as a reflected grating and reflects X-rays. By
rotating the table, the glancing angle θ at which the
X-ray is incident on the crystal can be changed. The
angle for which the intensity of the reflected beam is
maximum gives the value of θ. The experiment is
repeated for each plane of the crystal. For first order
reflection n = 1 so that, λ = 2d sinθ; for n = 2, 2λ = 2d
sinθ; ……., and so on.
• A photographic plate or an ionization chamber is
used to detect the rays reflected by the crystal.

15.  Continuous or Bremsstrahlung X-ray
• "Bremsstrahlung" means "braking radiation"
and is retained from the original German to
describe the radiation which is emitted when
electrons are decelerated or "braked" when
they are fired at a metal target.
• Accelerated charges give off electromagnetic
radiation, and when the energy of the
bombarding electrons is high enough, that
radiation is in the x-ray region of
the electromagnetic spectrum.

16. Continuous X-rays…
5

17. Continuous X-rays…
• It is characterized by a continuous distribution
of radiation which becomes more intense and
shifts toward higher frequencies when the
energy of the bombarding electrons is
increased.
• The curves above are who bombarded tungsten
targets with electrons of four different energies.
• The continuous distribution of x-rays which
forms the base for the two sharp peaks at left is
called "Bremsstrahlung" radiation.

18.  Characteristic X-rays
• Characteristic X-rays are emitted from heavy
elements when their electrons make transitions
between the lower atomic energy levels.
6

19. Characteristic X-rays…
• Characteristic X-rays emission which shown as
two sharp peaks in the illustration at left occur
when vacancies are produced in the n = 1 or K-
shell of the atom and electrons drop down from
above to fill the gap.
• The X-rays produced by transitions from the n =
2 to n = 1 levels are called Kα X-rays, and those
for the n = 3->1 transition are called Kβ X-rays.
• Transitions to the n=2 or L-shell are designated
as L - shall X-rays (n= 3->2 is Lα, n = 4->2 is Lβ,
etc.

20. Uses of Characteristic X-rays..
• Characteristic X-rays are used for the
investigation of crystal structure by X-ray
diffraction.
• Crystal lattice dimensions may be
determined with the use of Bragg's law in
a Bragg spectrometer.

21.  Moseley’s law and its importance.
• The English physicist Henry Moseley (1887-
1915) found, by bombarding high speed
electrons on a metallic anode, that the
frequencies of the emitted X-ray spectra were
characteristic of the material of the anode.
• The spectra were called characteristic X-rays.
• He interpreted the results with the aid of the
Bohr theory, and found that the wavelengths λ of
the X-rays were related to the electric charge Z
of the nucleus. According to him, there was the
following relation between the two values
(Moseley’s law; 1912).

22. 1/λ = c(Z - s)2
Where,
c and s are constants applicable to all elements
Z is an integer.
When elements are arranged in line
according to their position in the Periodic
Table , the Z value of each element increases
one by one.
Moseley correctly interpreted that the Z
values corresponded to the charge possessed
by the nuclei. Z is none other than the atomic
number.

23. It was found that the characteristic X-ray of an
unknown element was 0.14299 x 10-9 m. The
wavelength of the same series of the characteristic X-ray
of a known element Ir (Z = 77) is 0.13485
x 10-9 m. Assuming s = 7.4, estimate the atomic number
of the unknown element.

24. Importance of Moseley’s law
• Atomic no. is more important than Atomic
weight as it is equals to charge of nucleus.
• Difference between Ni, Co, Te & I etc., is
explained when periodic table was constructed
with atomic no.
• Moseley predicted the existence of elements
with atomic no. 43, 61, 72 & 75. Thus, X-ray
spectrum analysis new elements can be
discovered.

25.  Absorption of X-Ray
When the X-rays hit a sample, the
oscillating electric field of the electromagnetic
radiation interacts with the electrons bound in
an atom.
7

26. A narrow parallel monochromatic x-ray beam
of intensity I0 passing through a sample of thickness
x will get a reduced intensity I according to the
expression:
ln (I0 /I) = μ x ----
--- (1)
Where μ is the linear absorption coefficient,
which depends on the types of atoms and the density
ρ of the material.
At certain energies where the absorption
increases drastically and gives rise to an absorption
edge. Each such edge occurs when the energy of the
incident photons is just sufficient to cause excitation
of a core electron of the absorbing atom to a
continuum state, i.e. to produce a photoelectron.

27. The absorption edges are labeled in the
order of increasing energy, K, LI, LII, LIII, MI,….,
corresponding to the excitation of an electron
from the 1s(2S½), 2s(2S½), 2p(2P½), 2p(2P3/2),
3s(2S½), … orbitals (states), respectively.
Thus, the energies of
the absorbed radiation
at these edges
correspond to the
binding energies of
electrons in the K, L,
M, etc.., shells of the
absorbing elements.8

28. Compton effect
A phenomenon called Compton scattering,
first observed in 1924 by Compton, and
provides additional direct confirmation of the
quantum nature of electromagnetic radiation.
When X-rays impinges on matter, some of the
radiation is scattered, just as the visible light
falling on a rough surface undergoes diffuse
reflection.

29. • Observation shows that some of the scattered
radiation has smaller frequency and longer
wavelength than the incident radiation, and
that the change in wavelength depends on the
angle through which the radiation is
scattered.
• Specifically, if the scattered radiation emerges
at an angle φ with the respect to the incident
direction, and if f and i are the wavelength
of the incident and scattered radiation,
respectively, it is found that,

30. Where, m0 is the electron mass.
9

31. 10

32. • In figure, the electron is initially at rest
with incident photon of wavelength  and
momentum p; scattered photon with longer
wavelength f and momentum p and
recoiling electron with momentum P. The
direction of the scattered photon makes an
angle φ with that of the incident photon, and
the angle between p and p is also φ.
called Compton wavelength.
nm
mc
h
c 00243.0

33. • Compton scattering cannot be
understood on the basis classical
electromagnetic theory.
• On the basis of classical principles, the
scattering mechanism is induced by motion
of electrons in the material, caused by the
incident radiation.

34.  Applications of X-Rays…
X-rays are used in industrial, medical, pure
science research and X-ray crystallography etc…
• X-rays are used to detect defects in radio valves.
• X-rays are used to detect cracks in structures.
• X-rays are used to analyses the structures of
alloys and other composite bodies by diffraction
of X-rays.
• They are also used to study are structure of
materials like rubber, cellulose, plastic, fibres
etc…
• X-rays can destroy abnormal internal tissues.

35.  Applications of X-Rays…
• X-rays are used in analysis of crystal structure
and structure of complex organic molecule.
• They are also used in determining the atomic
number and identification of various chemical
elements.
• X-rays are used to detect fractures and
formation of stones in human body.
• They are also being used for tumor treatment
and for this purpose hard X-rays are used.
• X-rays are also used in X-ray crystallography for
Laue method, Rotating crystal method, Powder
method, etc….

36. Image references links
1. http://alphaoneimaging.com/wp-content/uploads/2013/03/x-ray.jpg
2. https://encrypted-
tbn2.gstatic.com/images?q=tbn:ANd9GcTINZXxYufauFlzdPYTHCq6JR3CDPH
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3. https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcQXoHJnzqe1dRx96r9vnPZ6tgu5CObfV
_CgMChog2K9_L3R352PibF8MlOr
4. https://encrypted-
tbn3.gstatic.com/images?q=tbn:ANd9GcQEGAb7sSHYT6vGOwZhb10hzb5vJU
ZYIm-esldrsOKnKXWQJKJRlmN18G8
5. http://postimg.org/image/jd2eb1od9
6. http://postimg.org/image/i88c5o1wd
7. http://postimg.org/image/9ehfokexp
8. http://postimg.org/image/vnvcv4ae5
9. http://postimg.org/image/45meqowil
10. http://postimg.org/image/l7f8ssbdp