Radiation Physics

1. Interactions of
Radiation with matter
Dr. Md. Shahed - Ul - Matin
MD (Oncology)
Radiotherapist
Department of Radiotherapy
Dhaka Medical College Hospital

2. What happens when a photon passes
through a medium?
 Energy is transferred to the medium
 Interaction between photons and matter can take place.
 Ejection of electrons from atoms (of absorbing medium)
 These electrons transfers their energy by producing ionization and
excitation of atoms along their path.

3. Some key words-
 Ionization: A process by which a neutral atom acquires a positive and a
negative charge.
 Positive ion: an atom from which an electron has been removed.
 Negative ion: an electron combine with a neutral atom & form a
negative ion.
 Ion pair: Combination of a positively charged ion and a negatively
charged ion(usually a free electron).

5. Excitation:
 Excitation: If the energy is not sufficient to eject an electron from the atom
but is used to raise the electrons to higher-energy levels, the process is termed
excitation.

6. Directly & indirectly ionization radiation:
 Charged particles of sufficient kinetic energy that produce ionization and
excitation by collision with atoms are known as directly ionizing radiation.
Example:
• Electron,
• Proton
• Alpha particles & heavier charged particles.
 Uncharged particles of sufficient kinetic energy that liberate directly
ionizing particles from atoms are known as indirectly ionizing radiation.
Example:
• Photons
• Neutrons.

7. Fluence & Intensity
 Photon Fluence (): Number of photons per
unit area.
 =
𝑑𝑁
𝑑𝑎
(dN= number of photons;
da = sphere of cross-sectional area)
 Energy Fluence (): Quotient between the sum
of energies of all photons and photons that enter a
sphere of cross-sectional area.
 =
𝑑𝐸𝑓𝑙
𝑑𝑎
(dEfl = sum of energies of all photons ,
da = cross-sectional area)

8. Intensity:
 Intensity ()/ Energy fluence
rate ():
Energy fluence per unit time.
 =
𝑑
𝑑𝑡

9. Half value layer
 Half value layer (HVL): is the thickness of an absorber required to
attenuate the primary beam intensity to half of its original value.
 Measured for a
Narrow beam under
Good geometry conditions (Scattered photons are excluded from
measurement).
 Half value layer and attenuation coefficient are related by the following
relationship:
HVL =
0.693


10. Photon beam attenuation
 A narrow monoenergetic beam of
photon is attenuated
exponentially by an absorber.
I (x) = I0e-x
where, I(x) = is the intensity
transmitted by thickness x,
I0 = the intensity incident of the
absorber and
 = the linear attenuation co- efficient.

11. Photon beam attenuation (contd.)
 Attenuation of Poly energetic beam is no longer
exponential.
 The slope of the attenuation curve decreases with
increasing absorber thickness because the absorber
or filter preferentially removes the lower-energy
photons.
 Fig: Aluminum absorber. First HVL = 0.99 mm Al,
second HVL = 1.9 mm Al, and third HVL = 2.0 mm Al.

12. Attenuation coefficient ()
 Attenuation coefficient () :
characterizes a photon beam for its
penetration power in a given medium.
• Depends on:
• Beam energy &
• Material composition (density &
atomic number)
• Related to:
• Primary beam
• Measured
• for a narrow beam
• Scattered photons are excluded
from measurement.

13. Interactions of Radiation with matter
A. Interactions of Photons (x-
rays,  rays):
 5 major types of interactions of
photons with matter:
1. Coherent Scattering
2. Photoelectric effect
3. Compton effect
4. Pair production
5. Photodisintegration
B. Interactions of charged particles
(electron, proton,  particle, nuclei):
1. Ionization
2. Excitation
3. Radiative collision (mainly electrons
interact by bremsstrahlung process)
C. Interactions of Neutrons:
1. Recoiling
2. Nuclear disintegrations

14. Coherent scattering  When an electromagnetic radiation passes
near electron & causes oscillation.
 No energy is changed into electronic
motion.
 No energy is absorbed in medium.
 Scattering of photon at small angles.
 Scattered photon has same wavelength as
incident photon.
 When occurs:
 If photon energy is low.
 High atomic number material.
 Clinical significance: Only academic
interest.

15. Photoelectric effect
 A phenomenon in which a photon is
absorbed by an atom & one of its
orbital electron is ejected.
 Entire energy (h ) of photon is 1st
absorbed by an atom & then all of it is
transferred to atomic electron.
 Ejected electron is called photoelectron
 Kinetic energy of photoelectron is h -
EB, (EB = binding energy of electron)
 Photoelectric interactions can occur in
K, L, M or N shells.

16. Photoelectric effect Photoelectric interactions
Electron ejects from the atom
Vacancy is created in the shell
Atom is in excited state
Vacancy can be filled by an outer shell
electron with the emission of characteristic
x-ray (fluorescent x-ray).
Alternatively, this transition can give rise to
ejection of another electron ( Auger
electron)
For soft tissue (Z ~7.64) w ~ 0;
for tungsten (Z = 74) w ~ 0.93
Large Z values: Fluorescent radiation is common
Small z values: Auger electron is favored.

17. Photoelectric effect (contd.)
1. proportional to the Z3 of the attenuating medium and
2. inversely proportional to the cube of the energy of the incident photon
(E), and
3. proportional to the physical density of the attenuating medium (p)
 Thus, the overall the probability of photoelectric absorption can be
summarized as follows:
Photoelectric absorption ~ p
𝑍3
𝐸3

18. Photoelectric effect (contd..)
 Therefore, if Z doubles, photoelectric absorption will increase by a factor
of 8 (2³ = 8), and if E doubles photoelectric absorption will reduce by a
factor of 8. Small changes in Z and E can therefore significantly affect
photoelectric absorption.
 For a low-energy photon, the photoelectron is emitted at 90 degrees.
 As the photon energy increases, the photoelectrons are emitted in a more
forward direction

19. Photoelectric attenuation coefficient (/):
 The graph for lead has discontinuities at about 15
and 88 keV. These are called absorption edges
and correspond to the binding energies of L and K
shells
 A photon with energy less than 15 keV does not
have enough energy to eject an L electron.
 Below 15 keV, the interaction is limited to the
M- or higher-shell electrons.
 If the photon energy is increased, the probability
of photoelectric attenuation decreases
approximately as 1/E3 until the next discontinuity,
the K absorption edge.

20. Clinical importance
 To improve contrast, decrease your x-ray
energy (kV).
 Z3 dependence is important in radiology,
Contrast material, such as BaSo4 ( Z = 56),
Hypaque (iodine=53)
 In therapeutic radiotherapy,
the low-energy beams (superficial &
orthovoltage machines) cause unnecessary
high absorption of x-ray energy in bone
due to Z3 dependence. 60 Kev 120 Kev

21. Compton effect
 Collision between the photon & a free
electron. Free electron means binding
energy of electron is much smaller than
the energy of the bombarding photon.
 The electron receives some energy &
emitted at angle of .
 The photon with reduced energy is
scattered at angle of .

22. Compton effect (contd.)
 Not dependent on atomic number Z
 Depends only on the number of
electrons per gram.
 The energy of incident photon must
be large compared with electron
binding energy.
 Compton effect also decreases with
increasing photon energy.

23. Considering Compton effect, the attenuation per g/cm2 for bone is nearly the same as that
for soft tissue.
1 cm of bone will attenuate more than 1 cm of soft tissue, because bone has a higher
electron density, (number of electrons per cubic centimeter)

24. Pair Production
 High energy photon interacts with electromagnetic field of a nucleus.
 Photon gives all its energy to create a pair of negative electron (e-) and a
positive electron (e+).
 Because the rest mass energy of electron is .51 MeV, a minimum energy of
1.02 MeV is required to create the pair of electrons.
 Threshold energy (minimum energy) for pair production is 1.02 MeV.

25. Pair Production (contd.)
 The excess photon energy above 1.02 Mev
is shared between the particles as kinetic
energy.
 The total kinetic energy available for the
electron–positron pair is given by (h – 1.02)
MeV.
 Usually, each particle acquire half of the
available energy, but any distribution is
possible.
 Here energy is converted into mass as
predicted by Einstein’s equation E = mc2
(e-) (kinetic energy)
Photon 0.51 + 1.0 MeV
3.02 Mev
0.51 + 1.0 MeV
(e+) (kinetic energy)

26. Annihilation Radiation
 The reverse process, namely the conversion of mass into energy, takes
place when a positron combines with an electron to produce two photons,
called the annihilation radiation.
 The positron loses its energy as it traverses the matter by (just like
electron) –
 Ionization,
 Excitation
 Bremsstrahlung

27. Annihilation Radiation
 The slowly moving positron combines with one
of the free electrons to give rise to two
annihilation photons each having 0.511 MeV
(same as rest energy of electron).
 2 photons are ejected in opposite direction as
momentum is conserved in this process.

28. Pair Production & Annihilation Radiation

29. Pair production (contd.)
 The attenuation coefficient for pair production () varies with Z2 per atom, Z per
electron, and approximately Z per gram.
 Pair production probability increases slowly with photon energy beyond the 1.02
MeV threshold. It increases approximately 6% at 4 MeV and 20% at 7 MeV
(approximately average energies of 12 – 21 MV X-ray beams, respectively)
 This annihilation radiation is detected in PET and what is used to form images of
tracer concentration in the body.

32. Photodisintegration
 Interaction of a high energy photon with atomic nucleus can lead to a
nuclear reaction.
 Important only at high photon energies.
 In most cases, results in emission of one or more neutron.
 Responsible for neutron contamination of therapy beams of energy
greater than 10 MV.
 Eg, Nucleus of
63
29
Cu bombarded with photon beam:
Photon
63
29
Cu
62
29
Cu +
1
0
n

33. Interaction of charged particles (Directly
ionizing radiation):
 Heavy charged particles:
 Proton
 Meson
  particles
 Atomic nuclei
 Light charged particles (tiny
mass):
 Electron
 Positron

34. Interaction of Heavy charged particles :
 Charged particles interact primarily by ionization and excitation.
Radiative collisions (bremsstrahlung) are possible but more likely for
electrons than heavier charged particles.
 All charged particles show Bragg peak near the end of their range (except
electron).
 The rate of energy loss per unit path length or stopping power caused
by ionization interactions for charged particles is proportional to -
Stopping power ∞
(𝑃𝑎𝑟𝑡𝑖𝑐𝑙𝑒 𝑐ℎ𝑎𝑟𝑔𝑒)2
𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 2

35. Interaction of Electron with matter
 Predominantly by ionization and excitation.
 Occasionally the stripped electron receives sufficient energy to produce an
ionization track of its own. This ejected electron is called a secondary electron, or a
 ray.
 If the energy transferred to the orbital electron is not sufficient to overcome the
binding energy, it is displaced from its stable position and then returns to it; this
effect is called excitation.
 Due to relatively small mass, the electrons suffer greater multiple scattering and
changes in the direction of motion so, The Bragg peak is not observed in electron
beams

37. Interaction of neutrons:
Recoiling & Nuclear disintegration
Recoiling protons from
hydrogen:
 The energy transfer is very
efficient, if the colliding particles
have the same mass (neutron
colliding with a hydrogen nucleus)
Recoiling heavy nuclei from other
elements
 Neutron loses very small energy
when colliding with a heavier
nucleus.
Most efficient absorbers of a neutron beam are the hydrogenous materials such as
paraffin wax or polyethylene.
Lead which is very good absorber for X-rays, a poor shielding material against
neutron.

38. Interaction of neutrons:
 Dose deposited in tissue from a
high-energy neutron beam is
predominantly contributed by recoil
protons.
 Because of higher hydrogen
content, dose absorption in fat is
20% higher than muscle in
neutron interactions.
 Nuclear Disintegration:
Emission of heavy charged
particles, neutrons and  rays and
give rise to 30% of tissue dose.
 Nuclear dosimetry is much
complicated due to such diverse
secondary radiation production.

39. Neutron shield:
 Shielding of neutrons involves
three steps:
1. Slow the neutrons
2. Absorb the neutrons
3. Absorb the gamma rays
1.A Neutrons are slowed to thermal
energies with hydrogenous material:
water, paraffin, plastic.
1.B To slow down very fast neutrons,
iron or lead might be used in front of
the hydrogenous material.
2. Hydrogenous materials are also very
effective at absorbing neutrons
3. Concrete, especially with barium
mixed in, can slow and absorb the
neutrons, and shield the gamma rays.

40. Summery
1. Coherent Scattering – Academic interest
2. Photoelectric effect – Diagnostic X-ray, Linac, 60Co
3. Compton effect – Linac, 60Co
4. Pair production – Linac, 60Co, used in PET- CT scan
5. Photodisintegration

41. Summary
Interaction with Uses
Coherent Scattering Electron Academic interest
Photoelectric effect Electron Diagnostic X-ray, Linac, 60Co
Compton effect Electron Linac, 60Co, Diagnostic X-ray,
Pair production Nucleus Linac, 60Co, PET-CT
Photodisintegration Nucleus Neutron contamination
Ionization Electron Electron therapy, Proton
therapy
Excitation Electron -
Recoiling Nucleus Neutron treatment
Nuclear disintegration Nucleus Neutron treatment

42. Thank you
very much