radiation physics

1. Photon - interaction with
matter
dr rahul ts Jr
dpt of radiotherapy
gmch tsr

2. MATTER
• Matter > elements > atoms
• Atomic structure
– An atom consists of a positively charged nucleus
surrounded by a cloud of negatively charged electrons.
– radius of atom ~10-10 m, radius of nucleus ~10-15 m.
– An atom is specified by the formula A
ZX,
• A is the mass number (number of protons + neutrons),
• Z is the atomic number (number of protons).

3. • Atomic energy levels
– The binding energy of electrons in various orbits
depends on the magnitude of the Coulomb force of
attraction between the positively charged nucleus and
the negatively charged electrons.
• The closer the orbit is to the nucleus, the greater is the binding energy.
– maximum possible number of electrons in any orbit is
given by 2n2
• Nuclear stability
– High n/p ratio gives rise to β- decay and a low n/p ratio
can result in electron capture and β+ decay

4. PHOTONS
• Electromagnetic radiation
– Electromagnetic radiations are characterized by
oscillating electric and magnetic fields, always
perpendicular to each other and to the direction of
their energy propagation.
– Wavelength (λ), frequency (n), and velocity (c) of
electromagnetic waves are related by c = nl.
– If λ is given in meters, the photon energy in
electron volts (eV) is given by E = (1.24 × 10-6)/λ.

5. • When an X-ray or γ ray beam passes through a medium ,
interactions between photons & matter can take place with transfer
of energy to the medium
• The initial step in the energy transfer involves the ejection of
electrons from the atoms of the absorbing medium
Outer electron  ionization,
retunes to normal state + infrared (low energy)
Inner electron  excitation + free outer electron takes its place +
characteristic x-rays
– Characteristic x-rays produces Auger electron

6. • These high speed electrons transfer
their energy by producing IONIZATION and
EXCITATION of the atoms along their path
• If the absorbing medium consists of body tissues sufficient
energy may be deposited with in the cells destroying their
reproductive capacity

7. • Photons are INDIRECTLY ionizing radiations
• Interact with the atoms of a material or absorber
to produce high speed electrons by 3 major
processes
♣ Photoelectric effect
♣ Compton effect
♣ Pair production

8. Types of interaction
1. Coherent scattering
2. Compton effect
3. photoelectric effect
4. pair production
5. photodisintegration.

9. 4 possible types of fate awaits the photon
when it passes through matter
1.May be deflected from its original path &
proceed in a new direction, but with
UNCHANGED energy
2.May be deflected as before, but also LOSE some
energy
3.Disappear altogether
4.May be transmitted unchanged

10. 4 possible types of fate awaits the photon
when it passes through matter
1. May be deflected from its original path &
proceed in a new direction, but with
UNCHANGED energy
coherent
2. May be deflected as before, but also LOSE
some energy
3. Disappear altogether
4. May be transmitted unchanged
scatter
Photoelectric effect
Pair production
incoherent

11. Attenuation processes
Divided into 2 sets
• Photon scattering (elastic scattering,
inelastic scattering)
• Disappearance phenomenon (photo-electric effect,
pair production)

12. ‘Bound’ and ‘free’ electrons
• Strictly speaking there are normally no ‘free’
electrons in matter
• Each electron is bound in the atom by the
electrostatic attraction between itself and the
positive charge on the nucleus
• It can only be ‘free’ if it receives enough energy to
overcome this binding force
• For the outer electrons of any atom, the binding
energy is only a few electron volts , which is small
when compared to the inner electrons and very
small when compared to the energy of X-ray
photons

13. • This leads to the concept that , an
electron may be considered to be ‘free’
when its binding energy is small
compared to the energy of the photons
with which it interacts

14. Elastic scattering
(coherent, classical, unmodified, Thomson, Rayleigh)
• More easily described by considering the
radiation as waves rather than photons
• Interaction is with bound electrons
• Radiation is deflected with out losing any energy
• The electric field of the incident wave
accelerates the particle, causing it to in turn emit
radiation at the same frequency as the incident
wave, and thus, the wave is scattered

15. Elastic scattering

16. • No energy is permanently taken up by the
irradiated material
• The process is of ATTENUATION WITH OUT
ABSORPTION
• Since the process involves bound electrons, it
occurs more in high atomic number materials
and also more with low energy radiations
• The mass attenuation coefficient for elastic
scattering is
α Z²
α 1/ E

17. Elastic scattering…
• Contributes nothing to energy absorption
• Contributes never more than a few percent to the
total attenuation
• This makes it UNIMPORTANT in radiography and
radiotherapy

18. Elastic scattering…
• Low energy photons
• High atomic number material.
• Scattering of photons at small angles
• No energy absorption
• No much clinical significance

19. Compton effect
(inelastic, incoherent )
• Interaction is with free electrons
• In this interaction , 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 Ф

20. Compton effect

21. • The angle through which the photon is scattered,
the energy lost by the photon and the energy
handed on to the electron are all interconnected

22. By applying the laws of conservation of energy
and momentum, following relationships can be
derived
E = hvo α ( 1- cos Ф)
1+ α (1-cos Ф)
hv’ = hvo 1
1 + α (1-cos Ф)
hvo = energy of incident photon
hv’ = energy of scattered photon
E = energy of electron
α = hvo/μoc² where μoc² is the rest mass
energy of electron
( 0.511 Mev)

23. • If the angle Ф, through which the photon is scattered is small , a
very small share of the energy is given to the electron, and the
photon loses very little energy
If Ф= 0˚, then E = 0, hv΄ = hvo
• In a head on collision , in which the photon is turned back along its
original track (180˚) ,the maximum energy is transferred to the recoil
electron
Emax = hvo 2α
1+ 2α
and the scattered photon will be left with minimum energy
hv΄min = hvo 1
1+2α
• Most collisions will lie somewhere between these 2 extremes

24. Dependence on energy and atomic number
• As the energy increases the relative importance of
scattering as an attenuation process increases ,
but the absolute amount of scattering steadily
decreases with increase in energy
• Independent of atomic number Z, depends only on
the number of electrons per gram
• With the exception of hydrogen, most materials
have approximately the same number of
electrons/gram
• Compton mass attenuation coefficient ( σ/ρ ) is
nearly the same for all materials

25. Direction of scattering and recoil electrons
• Although any photon can be scattered in any
direction , the general pattern of scattered
radiation in space changes with photon energy
• For low energy photons there is roughly an equal
chance of being scattered in any direction

26. As the photon energy increases the scattered photon is more and
more likely to be travelling in forward direction

27. • Compton interaction probability in water increases
with photon energy from 10 to 150 keV. It then
decreases with further increase in energy.
• Maximum energy of a photon scattered at 90
degrees is 0.511 MeV, and at 180 degrees it is
0.255 MeV

28. Disappearance phenomena
• Photoelectric effect
• Pair production

29. Photoelectric effect
• Photon interacts with an atom and ejects one of
the orbital electrons from the atom
• Entire energy of the photon is absorbed by the
electron
• The kinetic energy of the ejected electron
(photoelectron) is equal to hv – EB(binding energy)

30. Photoelectric effect

31. • After the electron is 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
• There is also possibility of emission of Auger electrons
which are mono energetic electrons produced by the
absorption of characteristic X-rays internally by the atom

32. Dependence on energy and atomic number
τ/ρ α Z³
E³
τ/ρ = photoelectric mass attenuation
coefficient
Photoelectric effect in water (or soft
tissue) is predominant for photon
energies of 10 to 25 keV

33. • The relationship with atomic number forms the
basis of many applications in diagnostic radiology
• The difference in Z of various tissues such as
bone, muscle, fat amplifies differences in X-ray
absorption, provided the primary mode of
interaction is photoelectric
Basis of: •Diagnostic x-rays
•Use of contrast, eg. barium
•Use of lead as radiation protector.

34. Pair production
• If the energy of the photon is greater than 1.02
Mev
• Photon strongly interacts with the
electromagnetic field of 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+)
• As the rest mass energy of electron is equal to
0.51 Mev , a minimum energy of 1.02 Mev is
required to create the pair of electrons
• Thus the threshold energy for pair production is
1.02 Mev

35. Pair production

36. The photon energy in excess of 1.02 Mev is shared between the particles as
kinetic energy
The particles tend to be emitted in the forward direction relative to the
incident photon

37. The positron created as a result of pair production process lose its energy
as it traverses the matter
Near the end of its range the slowly moving positron combines with one of the
free electrons in its vicinity to give rise to 2 annihilation photons each having
0.51 Mev energy
2 photons are ejected in opposite directions

38. Dependence on energy and atomic number
• Since the pair production is caused by the nuclear
field , the chance of its occurrence increases with
the magnitude of that field , and hence with the
nuclear charge , or the atomic number of the
irradiated material
• In marked contrast with other attenuation
processes described , pair production increases
with energy
• Mass attenuation coefficient for pair production
(П/ρ) α Z
α E

39. Photodisintegration
High energy photon + atomic nucleus
Nuclear reaction
Emission of nucleons.
•10 -15 Mev
•Neutron is ejected commonly
•Neutron + KE
•Very rare

40. Relative importance of various types of interactions
• The total mass attenuation coefficient μ/ρ is the sum of
4 individual coefficients
• μ/ρ = σ /ρ + σ/ρ + τ/ρ + П/ρ
Coherent Compton PE effect pair production
• Coherent scattering is only important for very low
energy (< 10 kev) and at therapeutic energies it is often
omitted from the sum

41. The μ/ρ decreases rapidly with energy until the photon energy far
exceeds the electron binding energies and the Compton effect
becomes the predominant mode of interaction
In the Compton range of energies μ/ρ of lead & water do not differ
greatly since this type of interaction is independent of atomic number
The coefficient however decreases with energy until pair production
becomes important . The dominance of pair production occurs at
energies much greater than the threshold energy of 1.02 Mev

44. Thank you…