It gives some easy and detailed information about the basics of a radiological physics and will explain about the interactions of Electron in the target atoms.

1. Interaction of Radiation with Matter
Aswin.E.R
M.sc., Radiology And Imaging Technology
Kovai Medical Centre and Hospital Institute of Medical Sciences

2. RADIATION
• Radiation is small pockets of energy, travels as waves through
space or matter.
• It transfers energy from one point to another point

3. Electro Magnetic Radiation
• Radio waves
• Microwaves
• Infrared
• Visible light
• Ultra violet
• X-rays
• Gamma rays
• Cosmic rays

4. Electro Magnetic Radiation
• No mass
• Unaffected by electrical or magnetic fields
• Constant speed (2.998 x 108 m/sec)
• Absorption (removal of radiation)
• Scattering (Change in trajectory)
Medicine :
• X-rays
• Gamma rays
• Visible light
• Radiofrequency

5. IONIZING RADIATION
• Ionization is process of removal of electron from neutral atom
• UV, x-rays, and gamma rays
has sufficient energy to do
ionization
• Produce ionized atoms
and molecules

6. NON IONIZING RADIATION
• Visible light,
• Infrared,
• Radio Waves,
• TV broadcasts

7. Basics things
that Included in
this Topic
• Ionization
• Specific Ionization
• Binding Energy
• Excitation
• Attenuation
• Linear Attenuation Co- efficient
• Half value Layer
• Tenth value layer
• K Edge Absorption

8. Ionization
• The process of removal of orbital
electrons from the neutral atom is known
as ionization.
• An atom is normally electrically neutral if
one or more orbital electrons are removed
from the atom ,the remainder of the atom
is left with positively charged and is known
as positive ion.
• Ionisation is the process by which
an atom or a molecule acquires a negative
or positive charge by gaining or
losing electrons, often in conjunction with
other chemical changes. The resulting
electrically charged atom or molecule is
called an ion.

9. Specific
Ionization
• It is the number of ion pairs
produced per unit track
length. High LET radiation
(like alpha & beta particles)
ionizes water into H and OH
radicals over a very short
track.

10. Binding Energy
Amount of energy required to separate a particle from a system of
particles or to disperse all the particles of the system.

11. Excitation
In an atom if energy is supplied the electrons can be
moved from the inner orbits to the outer orbits
Now the atoms will have the more energy than its
normal state. It is said to be in a excited state and the
process is known as Excitation

12. Attenuation

13. Half Value Layer
• Thickness of material that attenuates radiation beam by 50 %
is the half-value layer or half value thickness (HVL or HVT).
• Linear attenuation coefficient & Half value layer (HVL)
 = 0.693 / HVL

14. Tenth value Layer
• The thickness of the material that attenuates an x ray beam by 90% is
the tenth value layer

15. Linear Attenuation Coefficient
• Reduction in the radiation intensity per unit path length, unit
cm1.
• Atomic number (Z), density (), Energy, thickness (x)

16. Mass Attenuation Coefficient
• Linear attenuation coefficient /density
• Unit is cm2/gm
• Independent of density

17. TYPES OF INTERACTION
• Coherent scattering
• Photoelectric absorption
• Compton scattering
• Pair production
• Photodisintegration

18. Coherent or Rayleigh Scattering
• X-ray (<10 keV) interacts with matter and
makes the atom excited
• Atom releases excess energy as a scattered
x-ray
• Wavelength equal to that of the incident x-
ray
• Direction of the scattered x-ray is different
from that of the incident X-ray (forward)

19. Cont.,
• Scattered X-rays in forward direction
• Classical or Thomson scattering
• No transfer of energy, no ionization
• Only change of direction, with out a change in energy
• No contribution to image formation,
• Important in low energy diagnostic x-rays, e.g. mammography
(15 to 30 keV)
• < 5% of interactions in soft tissue above at 70 keV; and 12% at
~30 keV (Image noise)

20. Compton Scattering
• In the Compton effect, the incident X-ray
interacts with an outer shell electron and
ejects it from the atom, thereby ionizing the
atom
• The X-ray continues in a different direction
with less energy
• Both scattered X-ray and Compton electron
have sufficient energy to do further
ionization / excitation
• The energy of the scattered X-ray is equal to
the difference between the energy of the
incident X-rays and the energy of the
ejected electron


 e
sc E
E
E0

21. Direct Hit
• If the photon makes a direct hit on the electron, electron will
travel straight forward (=0),and the scattered photon will be
scattered back
• 180 deg deflection: more energy is transferred to electron
• X-rays scattered back in the direction of
the incident X-ray beam are called
Backscatter radiation

22. Grace Hit
• If the photon makes a grazing hit with the electron, the electron
will emerge at =90, and the photon will straight forward (=0)
• 0 degree deflection, photon receive max energy

23. Cont.,
• Probability of Compton interaction decreases with as the X-ray energy
increases (1/E)
• Probability of Compton effect does not depend on the atomic number
• It occur in all energies in tissue, important in X-ray imaging
• Predominant interaction in the diagnostic energy range with soft tissue
(100 keV-10 MeV)
• Scattered X-rays provide no useful information, reduces image contrast
• Create radiation hazards in radiography and fluoroscopy
• In fluoroscopy large amount is scattered from the patient.
• Contribute to occupational radiation exposure

24. FEATURES OF COMPTON SCATTERING
Most likely to occur Outer shell electrons
Loosely bound electrons
As the X-Ray
energy increases
Increased penetration in
tissue, with out interaction
Higher than photoelectric
As atomic number
increases
No effect
As mass density
increases
Proportional increase

25. Photo Electric Effect
• In the photoelectric effect (PE), a photon of energy E collides with an
atom and ejects one of bound electrons from the orbit.
• The ejected electron is called photoelectron and it has kinetic energy
equal to E-orbital binding energy.
• In this process, all the incident photon energy is transferred to the
electron
• . The incident photon must have energy equal or greater than the
orbital binding energy of the electron, to perform photoelectric
effect.

26. • After the photoelectric effect, the atom is said to be ionized and there is
vacancy in the shell.
• This vacancy is filled by a electron of lower binding energy from higher
orbit.
• This will create a cascade of electron transition event from outer orbit to
inner orbit.
• The difference in binding energy is released as characteristic x-rays or
Auger electrons.
• The photoelectric effect involves tightly bound electrons.
• Tightly bound electrons are mostly available in the K shell and hence most
photoelectric interactions occur at the K shell.

27. • The probability of photoelectric cross section
per unit mass is proportional to Z3/E3, where Z
is the atomic number and E is the incident
photon energy.
• As the x-ray photon energy increases, the
subject contrast decreases.
• As the atomic number increases the subject
contrast increases, that is why barium (Z= 56)
and iodine (Z= 53) are used as contrast agents.

28. K edge Absorption
• The absorption of photon increases markedly as the incident photon
energy is increased from below to above the binding energy of the K-
shell. This is known as K-edge absorption.

29. Pair Production
• When a photon having an energy > 1.02 MeV, passes near the nucleus of an
atom, is subjected to strong nuclear field.
• The photon may suddenly disappear and become a positron and electron pair.
For each particle 0.511 MeV energy is required and the excess energy >1.02
MeV, would be shared between the positron and electron as kinetic energy.
• Actually the interaction is between a photon and the nucleus. This process is an
example for the conversion of energy into mass as predicted by Einstein.
• The threshold energy for the pair production process is 1.02 MeV. The
probability of pair production increases with energy for a given material and
also increases with atomic number (Z2).
• It is very important for photons having energy > 5MeV.

30. Cont.,
• The electron loses energy by excitation and ionization
and filling vacancy in the orbital shells.
• The positron travels in the medium and loses its energy
by ionization, excitation and bremsstrahlung process.
• Finally the positron combines with a free electron and
produces two photons of each energy 0.511 MeV, that
are ejected in opposite directions.
• The above process is called the positron annihilation.
This is an example of conversion of mass into energy and
the basis for positron emission tomography.

31. PHOTODISINTEGRATION
• Photon interacts directly with nucleus.
• Nucleus is raised to an excited state and instantly emits a
nucleon or nuclear fragment
• Photon energy >10 MeV
• No role in diagnostic radiology