by DR K Syed Akram Consultant Radiation Oncologist Yashoda Hospital

1. Production of X-rays and
Interaction of X-rays with
Matter
DR. K SYED AKRAM
26-06-2012

2. Production of X-rays

3. • X-rays were discovered by Prof. Wilhelm Conrad
Rontgen in 1895 while studying cathode rays
(stream of electrons) in a gas discharge tube.
• He observed that another type of radiation was
produced (presumably by the interaction of
electrons with the glass walls of the tube) that
could be detected outside the tube.
• He named the new radiation as x-rays.

4. Production of X-Rays
• The X-ray tube
• Basic X-ray circuit
• Voltage rectification
• Physics of X-ray production
• X-ray energy spectra

5. The X-Ray Tube
• The device that produces the X Ray
beam.
• X ray tube consists of a glass envelope
that has been evacuated to high vacuum.
• At one end is a cathode (negative
electrode) and at the other end is an
anode (positive electrode), both
hermetically sealed in the tube .

6. Cathode
• Cathode consists of a wire filament, a circuit to
provide filament current, and a negatively
charged focusing cup.
• Wire filament : usually made of tungsten filament
that when heated emits electrons, a
phenomenon known as thermionic emission.
• The function of the cathode cup is to direct the
electrons toward the anode so that they strike
the target in a well-defined area, the focal spot.

7. Anode
• consists of a thick copper rod at the end of which
is placed a small piece of tungsten target.
• choice of tungsten as the target based on the
criteria that the target must have high atomic
number and high melting point.
thus it withstands intense heat produced in the
target by the electronic bombardment.
• Efficient removal of heat from the target is by
conduction of heat through a thick copper anode
to the outside of the tube where it is cooled by
oil, water, or air.

9. Basic X-Ray Circuit
• High-voltage circuit : provide the accelerating
potential for the electrons.
• Low-voltage circuit: supply heating current to the
filament.
• Voltage applied between the cathode and the anode
is high enough to accelerate all the electrons across
to the target.
• The filament temperature or filament current controls
the tube current (the current in the circuit due to the
flow of electrons across the tube) and hence the x-
ray intensity.

10. Physics of X-Ray Production
• The electrons that travel from the cathode to the
anode are called projectile electrons.
• When they strike the heavy metal atoms of the
anode they interact with the atoms and transfer
their kinetic energy to the target.
• The electrons interact with either the nucleus or the
orbital electrons of the target atoms.Thus produces
1) Bremsstrahlung x-rays
or
2) Characteristic x-rays.

11. Bremsstrahlung produced by a high-
energy electron deflected in the electric
field of an atomic nucleus
Bremsstrahlung x-rays

13. Bremsstrahlung x-rays
• German pronunciation:from bremsen "to
brake" and Strahlung "radiation", i.e.
"braking radiation" or "deceleration
radiation“.
• The electron, while passing near a
nucleus, may be deflected from its path by
the action of Coulomb forces of attraction
and lose energy as bremsstrahlung
radiation.
• A part or all of its energy is dissociated
from it and propagates in space as
electromagnetic radiation.

15. Characteristic x-rays
• When the projectile electron interacts with
an inner shell electron of the target atom.
• The interaction Ionize the target atom by
removing an inner shell electron.
• A outer shell electron falls down to replace
the lost electron.

16. Characteristic x-rays
• The transition of orbital electrons from the
outer shell electron to fill the vacancy in
the inner shell is accompanied by the
emission of an x-ray photon known as
Characteristic x-rays .

17. Characteristic x-rays
• An electron, with kinetic energy E0, may interact with the
atoms of the target by ejecting an orbital electron, such
as K, L, or M electron, leaving the atom ionized.
• The original electron will recede from the collision with
energy E0 - ΔE, where ΔE is the energy given to the
orbital electron.
• A part of ΔE is spent in overcoming the binding energy
of the electron and the rest is carried by the ejected
electron.
• When a vacancy is created in an orbit, an outer orbital
electron will fall down to fill that vacancy.
• In doing so , the energy is radiated in the form of
electromagnetic radiation. This is called characteristic
radiation.

19. Interaction of X-rays with
Matter

20. Interaction of X-rays with
Matter
• An x-ray beam emitted from a target or a γ-ray
beam emitted from a radioactive source consists
of a large number of photons, usually with a
variety of energies
• When an x- or γ-ray beam passes through a
medium, interaction between photons and matter
can take place with the result that energy is
transferred to the medium.
• The probability for each interaction depends on
the
- The energy of the photon and
- The atomic number of the medium.

21. Types of x-ray Interactions
1.Coherent scattering
2.Photoelectric Absorption
3.Compton effect
4.Pair production
5.Photo disntegration

22. Coherent scattering
• coherent scattering, also known as
classical scattering or Rayleigh scattering.
• Incident photon undergoes a change in
direction without a change in wavelength.

24. Coherent scattering
• occurs when a low-energy incident photon
passes near an outer electron of an atom
(which has a low binding energy).
• The incident photon interacts with the electron
in the outer-shell by causing it to vibrate
momentarily at the same frequency as the
incoming photon.
• The incident photon then ceases to exist. The
vibration causes the electron to radiate energy
in the form of another x-ray photon with the
same frequency and energy as in the
incident photon.

25. Coherent scattering
• These scattered x-rays have the same
wavelength as the incident beam. Thus, no
energy is changed into electronic motion and
no energy is absorbed in the medium.
• The only effect is the scattering of the photon
at small angles.
• The coherent scattering is probable in high-
atomic-number materials and with photons of
low energy.

26. Photoelectric Absorption
• Photoelectric absorption or photoelectric
effect is a phenomenon in which a photon
interacts with a tightly bound orbital
electron of an atom and ejects orbital
electrons from the atom and disappears.

28. Photoelectric Absorption
• In this process, the entire energy (hγ) of the
photon is first absorbed by the atom and then
transferred to the atomic electron which is
ejected with kinetic energy (EK).
• The kinetic energy of the ejected electron (called
the photoelectron) is given as
EK = hγ – EB
EK is the kinetic energy of photoelectron.
hγ is the energy of the incident photon.
EB is the binding energy of the electron.

29. Photoelectric Absorption
• After the electron has been ejected from the atom, a
vacancy is created in the shell, thus leaving the
atom in an excited state .
• The vacancy can be filled by an outer orbital
electron with the emission of characteristic x-rays .
• Interactions of this type can take place with
electrons in the K, L, M, or N shells .
• There is also the possibility of emission of Auger
electrons, which are monoenergetic electrons
produced by the absorption of characteristic x-rays
internally by the atom.

31. Photoelectric Absorption
• The probability of a photoelectric
interaction is a function of the photon
energy and the atomic number of the
target atom.
• A photoelectric interaction can not occur
unless the incident x-ray has energy equal
to or greater than the electron binding
energy.

32. Photoelectric Absorption
• The probability of photoelectric interaction
is inversely proportional to the third power
of the photon energy.
• The probability of photoelectric interaction
is directly proportional to the third power of
the atomic number of the absorbing
material.
• Probability of photoelectric absorption per
unit mass is approximately ∝ Z3
/E3.

33. Photoelectric Absorption
• Low atomic number target atoms such as
soft tissue have low binding energies.
• Therefore the photoelectric electron is
released with kinetic energy nearly equal
to the incident x-ray.
• Higher atomic number target atoms will
have higher binding energies.
• Therefore the kinetic energy of the
photoelectron will be proportionally lower.

34. Effective Atomic Numbers
• Human Tissue
– Muscle
– Fat
– Bone
– Lung
• Other Material
– Air
– Concrete
– Lead
• Effective Atomic #
– 7.4
– 6.3
– 13.8
– 7.4
– 7.6
– 17
– 82

35. Compton Effect
• In the Compton process, the photon interacts
with an atomic electron as though it were a
“free” electron.
• “free”electron - binding energy of the electron
much less than the energy of the bombarding
photon.
• the electron receives some energy from the
photon and is emitted at an angle θ.
• The photon, with reduced energy is scattered
at an angle φ.

37. Compton Effect
• Compton interaction involves essentially free
electrons in the absorbing material, it is
independent of atomic number Z.
• depends only on the number of electrons per
gram.
• Most materials except hydrogen have the
same number of electrons per gram.

39. Compton Effect
• . In the energy range of 100 Kev to 10 Mev,
compton interaction is more predominant than
the photoelectric or pair production processes.
• As the photon's energy increases, a greater
proportion of the energy is transferred to the
electron, so the scattered photon necessarily
retains a smaller proportion of the incident
energy.

40. Pair production
• If the energy of the photon is greater than 1.02
MeV, the photon may interact with matter
through the mechanism of pair production.
• The photon interacts strongly with the
electromagnetic field of an atomic nucleus and
gives up all its energy in the process of creating
a pair consisting of a negative electron (e-
) and
a positive electron (e+
).

42. Pair production
• The photon energy in excess of this
threshold is shared between the particles
as kinetic energy.
• Rest mass energy of the electron is
equivalent to 0.51 MeV, a minimum
energy of 1.02 MeV is required to create
the pair of electrons.
• The threshold energy for the pair
production process is 1.02 MeV.

43. Pair production
• Probability for this process increases with
the square of the atomic number (z²) of
the medium.
• This process increases with increase in
energy of the photon.

44. Annihilation reaction
• Positron combines with one of the free
electrons in its vicinity to give rise to two
annihilation photons, each having 0.51
MeV energy.
• As momentum is conserved in the
process, the two photons are ejected in
opposite directions .

46. Photodisntegration
• occur when a high energy photon is absorbed
by the nucleus of an atom,resulting in an
emission of a neutron ((x, n) reaction) or
proton ((x, p) reaction) and a transformation of
the nucleus into a radioactive reaction
product.

48. Clinical Application
• PHOTOELECTRIC ABSORPTION-
Diagnostic radiation(Radiography).
• COMPTON EFFECT- Therapeutic
radiation(Radiotherapy).
• PAIR PRODUCTION – Nuclear medicine.

49. thank you