X rays Production & Propagation

1. X - Rays

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.
• Their wavelengths range from 0.01 to 10 Å.

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.

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 in the form of
X-rays are generated.

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.
On passing through a gas X-rays ionize the gas.

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.

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.

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-rays
• "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…

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.

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.

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.

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.

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….