Interaction of ionising radiation

1. Interactions of Ionizing
Radiation
Kiran kumar BR

2. Ionization
• Neutral atom acquires a positive or a negative charge is known
as ionization.
This radiation can ionise matter in two ways:
• Directly ionising radiation (charged particles) electrons,
protons, particles and heavy ions, or
• Indirectly ionising radiation (neutral particles) photons (x-rays
and γ-rays) and neutrons.

3. Ionizing Radiation
Directly ionizing radiation
• electrons, protons, and  particles (charged particles)
• sufficient kinetic energy to produce ionization by collision as
they penetrate matter.
• The energy of the incident particle is lost in a large number of
small increments along the ionization track in the medium,
with an occasional interaction in which the ejected electron
receives sufficient energy to produce a secondary track of its
own.
• Indirectly ionizing radiation
• neutrons and photons
• to release directly ionizing particles from matter when they
interact with matter

4. Ionization & Excitation
• The process by which a neutral atom acquires a positive
or a negative charge is known as ionization. Removal of
an orbital electron leaves the atom positively charged,
resulting in an ion pair.
• The stripped electron, in this case, is the negative ion
and the residual atom is the positive ion.
• If the photon 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

5. Photon beam attenuation
• x–the absorber thickness (cm)
• –linear attenuation coefficient (cm-1)
• I–intensity
dxNdN
dxNdN


x
eIxI
dxIdI





0)(

6. • The intensity of a beam of mono energetic photons
attenuated by an attenuator of thickness x is given by
I(x) = I(0) e-μ(hν, Z)x
• where I(0) is the intensity of the un attenuated beam,
and
• μ(hν, Z) is the linear attenuation coefficient which depends
on the energy of the photon hν and the atomic number Z
of the attenuator.

7. Half Value Layer (HVL)
• The Half Value Layer (HVL or x½) is defined as the thickness
of the attenuator that will attenuate the photon beam to
50% of it’s original intensity
• from
I(x) = I(0) e-μ(hν, Z)x
• we have
½ = 1 e-μx
½
-ln 2 = -μx½
x½ = (ln 2)/μ

8. x=HVL I/I0=1/2
A mono-energetic beam

693.0
HVL

9. Linear attenuation coefficient
Linear attenuation coefficient (, cm-1)
• Is a basic quantity used in calculations of the penetration of
materials by quantum particles or other energy beams. It is
a measure of attenuation.
It describes the extent to which the intensity of an energy
beam is reduced as it passes through a specific material.
• It is represented using the symbol µ, and measured in cm-1.
Depend on the energy of the photons
the nature of the material
density of the material

10. Mass attenuation coefficient
Mass attenuation coefficient (/, cm2/g)
• Is a measurement of how strongly a substance absorbs or
scatters the incident radiation.
• Independent of density of material
• Depend on the atomic composition

11. Coefficients
• The absorber thickness can also be expressed in units of
electrons / cm2 and atoms / cm2.
Electronic attenuation coefficient (e, cm2/electron)
Atomic attenuation coefficient (a, cm2/atom)
0
1
N
e


 
0N
Z
a


 
w
A
A
ZN
N

0
Z the atomic number
N0 the number of electrons per
gram
NA Avogradro’s number
AW the atomic weight

12. Energy transfer coefficient (tr)
• When a photon interacts with the electrons in the material,
a part or all of its energy is converted into kinetic energy of
charged particles per unit thickness of absorber is given by
the energy transfer coefficient (µtr).



h
Etr
tr 
trE The average energy transferred into kinetic
energy of charged particles per interaction
Mass energy transfer coefficient - µtr /ρ

13. Energy absorption coefficient (en)
• Energy loss of electrons
Inelastic collisions losses  ionization and excitation
Radiation losses bremsstrahlung
Defined as the product of energy transfer coefficient
and (1-g).
• en= tr(1-g)
• g fraction energy loss to bremsstrahlung
• increases with Z of the absorber the kinetic energies of
the secondary particles

14. Interactions of photons with matter
• Attenuation of a photon beam by an absorbing material is
caused by five major types of interactions.
• Photo disintegration
• Coherent scattering (coh)
• Photoelectric effect ()
• Compton effect (c)
• Pair production ()
• Each of these process can be represented by its own
attenuation coefficient, which varies in its particular way with
the energy of the photon and with atomic number of the
absorbing material.
µ /ρ=σ coh /ρ+τ/ρ+σc /ρ+ Π/ρ

15. Photo Disintegration
• Interaction of a high energy photon with an atomic
nucleus can lead to a nuclear reaction and to the emission
of one or more nucleons.
• In most cases, this process of photodisintegration results
in the emission of neutrons by the nuclei.
nXX A
Z
A
Z
1
0
1
 


16. Coherent scattering
• Electromagnetic waves passing near the electron and
setting into oscillation.
• The oscillating electron reradiates the energy at the same
frequency as the incident electromagnetic wave. These
scattered x –rays have the same wave length as the
incident beam.
• Thus no energy is changed into electronic motion and no
energy is absorbed in the medium. The only effect is
scattering of the photon at small angles.
• The coherent scattering is probable in high atomic number
materials and with photons of low energy.

17. Coherent scattering

18. Photoelectric effect
• A photon interacts with an atom and ejects one of the
orbital electrons from the atom.
• In this process, the entire energy hνof the photon is first
absorbed by the atom and then transferred to the atomic
electron.
• The kinetic energy of the ejected electron ( photo electron )
is equal to hν – EB, where EB is the binding energy of the
electron. Interactions of this type can take place with
electrons in the K, L, M or N shells.

19. • 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.
• There is also the possibility of emission of Auger electrons,
which are mono energetic electrons produced by the
absorption of characteristic x rays internally by the atom.

21. • The probability of photoelectric absorption depends on
the photon energy as illustrated in Figure
• where the mass photoelectric attenuation coefficient (t/r)
is plotted as a function of photon energy
• relationship between t/r and photon energy.
• That photoelectric attenuation depends strongly on the
atomic number of the absorbing material. The following
approximate relationship

23. Compton effect
• The photon interacts with an atomic electron as
though it were a “free” electron.
• The term free means that the binding energy of the
electron is much less than the energy of the
bombarding photon.
• In this interaction, the electron receives some energy
from the photon and it is emitted at an angle θ.
• The photon, with reduced energy, is scattered at an
angle Φ.
• It can be analyzed in terms of a collision between two
particles, a photon and an electron.

25. Compton effect
h0 

h’
Free electron
E
)cos1(1
)cos1(
0





 hE
)cos1(1
1
' 0



 hh
= h0/m0c2 = h0/0.511
m0c2 is the rest energy of electron

26. Special cases of Compton effect
• Direct Hit ( θ= 0 & Φ = 180 deg )
electron will travel forward and the scattered
photon will travel back forward
• Grazing Hit ( θ= 90 & Φ= 0 deg )
electron will be emitted at right angle and the
scattered photon will go in the forward direction.
• 90 Degree Photon Scatter
photon is scattered at right angles to its original
direction (Φ= 90 deg ).

27. Dependence of Compton effect on energy
• As the photon energy increase,
the photoelectric effect decreases
rapidly and Compton effect
becomes more and more
important.
• The Compton effect also
decreases with
increasing photon energy.

28. 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.
• In this process, 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 consists of a
negative electron (e-) and a positive electron (e+).
• Because the rest mass energy of the electron is equivalent
to .51 MeV, a minimum energy of 1.02 MeV is required to
create a pair of electrons. Thus the threshold energy for
the pair production process is 1.02 MeV.

29. • The photon interacts with the electromagnetic field of an
atomic nucleus.
• The threshold energy is 1.02 MeV.
• The total kinetic energy for the electron-positron pair is
(h-1.02) MeV

30. Annihilation Radiation
• Positron losses its energy as it traverses the matter by the
same type of interactions as an electron does
(Ionization, Excitation and Bremsstrahlung ).
• Near the end of its range, the slowly moving positron
combines with one of the free electrons in its vicinity to
give rise to two annihilation photons, each having 0.51
MeV energy.
• Because momentum is conserved in the process, the two
photons are ejected in opposite directions.

32. Interactions of charged particles
• Charged particle interactions are mediated by Coulomb
force between the electric field of the travelling particle
and electric field of the orbital electrons and nuclei of the
atoms of the material.
• Collisions between the particle and the atomic
electrons result in ionization and excitation.
• Collisions between the particle and the nucleus result
in radiative loss of energy or bremsstrahlung.

33. • Scattering without sufficient energy loss.
• Nuclear reactions ( Heavy charged particles )
• Stopping power (S) = kinetic energy loss / unit path length
• Mass stopping power (S/, MeV cm2/g)

34. Heavy charged particles ( Protons )
• The particle slows down
• energy loss 
• ionization or absorbed
dose 
• Dose deposited in water
increases at first very slowly
with depth and then very
sharply near the end of the
range, before dropping to an
almost zero value.
• This peaking of dose near the
end of the particle range is
called the Bragg peak.
2
2
(velocity)
charge)particle(the
S

35. Electrons
• Interactions of electrons when passing through the matter
are quite similar to particles.
• However, because of their relatively small mass, the
electrons suffer multiple scattering and changes in
direction of motion, during the slowing down process
smears out the Bragg peak.
• Electron may interact with electromagnetic field of the
nucleus and be decelerated so rapidly that a part of energy
is lost as Bremsstrahlung.
• The rate of energy loss as a result of Bremsstrahlung
increases with increase in the energy of the electron and
the atomic number of the medium.

36. Ionization
Excitation
Bremsstrahlung

37. Interactions of neutrons
• Recoiling protons from hydrogen and recoiling heavy
nuclei from other elements.
• Energy is redistributed after the collision between the
colliding particles. Energy transfer is very efficient if the
colliding particles having same mass.
• Neutron losses very little energy when colliding with a
heavier nucleus. The most efficient absorbers of a neutron
beam are the hydrogenous materials.
• Nuclear disintegrations produce by the neutrons results in
the emission of heavy charged particles, neutrons,and 
rays and give rise to about 30% of the tissue dose.

38. Thank you…