This document discusses the interaction of ionizing radiation with matter. It describes three main types of ionizing radiation: photons, charged particles, and neutral particles. For photons, the key interaction mechanisms are coherent scattering, the photoelectric effect, the Compton effect, and pair production. Charged particles like electrons and protons interact through soft collisions, hard collisions, and delta ray production. Neutral particles like neutrons interact primarily through scattering, absorption, and fission. The document provides details on the energy transfers and secondary emissions that result from each interaction mechanism.

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Interaction of Ionizing Radiation
with Matter
Dr. Amal Yousif Al-Yasiri
University of Baghdad- College of Dentistry
Types of ionizing radiation
• 1- Photons include (γ-rays and X-Rays)
• 2- Charged particles include ( α -particles, β -
particles, electrons, positrons and protons
• 3- Neutral particles include neutrons

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Interaction of photons
• Photons such as gamma-rays and X-rays are part
of electromagnetic radiation. They are considered
ionizing radiation due to their ability to ionize the
material interact with ( i.e. make ion pairs)
• However other photons such as visible light,
infrared, microwaves, and radio-waves are part of
electromagnetic radiation but they do not have
the ability to ionize the material interact with (i.e.
they do not have the ability to make ion pairs)
• In this lecture, Photons word represents Gamma
rays and X-rays

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Interaction of photons
• If a photon enters a thin layer of matter, it is
possible that it will penetrate through without
interaction, or it may interact and transfer
energy to the matter in several ways:
– Coherent scattering
– Photoelectric effect
– Compton effect
– Pair production
Coherent scattering
• If the photon energy is low enough, in this
case, the incident photon’s electrical field
accelerates one or more orbital electrons and
causes them to radiate.
• There are two types of coherent scattering:
1-Thomson scattering, in which a single orbital
electron is involved
2- Rayleigh scattering, in which the orbital
electrons act as a single group

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Coherent scattering
• Probability of coherent scattering increases
with Z and decreases with photon energy (<
10 keV)
Photoelectric absorption
• In photoelectric absorption, the total energy of the photon
is transferred to an orbital electron, usually close to the
nucleus, and the photon disappears.
• The electron is then ejected from the atom with an energy
equal to the energy of the photon minus the binding
energy of the electron (hυ-Eb).
• The incoming photon must have an energy > Eb
• This interaction dominates for low photon energies,
absorbing media with high Z, e.g., lead is an excellent
absorber of low energy photons

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Photoelectric absorption
• After ejection of the electron, the neutral atom
becomes a positively charged ion with a vacancy
in an inner shell that must be filled.
• Atom returns to a stable condition by filling the
vacancy with a nearby, less tightly bound electron
farther out from the nucleus, and characteristic
X-rays radiation or Auger electrons are emitted.
• Auger electrons are not ejected by characteristic
x-rays! They are simply the result of an atom
attempting to reach a lower energy state.
Photoelectric absorption

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Compton effect
• The Compton effect is the interaction of a photon with
a loosely bound orbital electron in which part of the
incident photon's energy is transferred as kinetic
energy to the electron and the remaining energy is
carried away by the photon.
• The energy of the incident photon (Eo) is equal to the
sum of the the energy of the scattered photon (Ese)
and the kinetic energy of the ejected electron (Ee-)
• The binding energy of the electron that was ejected is
very small and can be ignored.
• The probability of interaction is independent of Z
explicitly but depends on Avagadro’s number(Z/A)
Compton effect

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Pair production
• The incident photon interacts with a nucleus and the photon is
completely converted into an electron and positron.
• Pair production is only possible for photons above 1.022 MeV.
• Total kinetic energy of final particles is Ephoton - 1.022 MeV.
• Created positron interacts with a nearby electron, converting both
particles to two 0.511 MeV annihilation photons.
• Pair production becomes more likely with increasing atomic
number and increasing photon energy.
Pair production

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Charged particles
• Electron - elementary subatomic particle carrying negative
charge
• Positron - antiparticle of the electron, carrying positive charge
• Alpha particle - two protons and two neutrons, identical to a
helium nucleus, carrying two positive charges.
• Proton - subatomic particle carrying a positive charge, identical
to hydrogen nucleus.
• Heavy charged particle - atomic ions, nuclei stripped of their
electrons. (Heavy refers to their mass relative to the electron).
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Some differences between charged particles
and photons
• An individual photon may pass through a slab of matter with no interactions at all,
or it lose its energy in one or a few “catastrophic” events.
• But for charged particles:
1-The probability of a charged particle passing through a layer of mater without
any interaction is zero.
2- 1 MeV electron would typically undergo >105 interactions before losing all of its
kinetic energy.
• Charged particles (e-, e+, p+, α+2) lose their energy very differently from uncharged
particles.
• A charged particle loses its kinetic energy gradually in a friction-like process, known
as the “continuous slowing down approximation” (CSDA).

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Charged particle tracks
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Electron-electron scattering
• When kinetic energy is conserved in an
interaction, the interaction is said to be elastic.
• If Eb cannot be ignored, kinetic energy is not
conserved, and the interaction is said to
inelastic.
• Two types of electron interactions
1- Soft collisions
2-Hard collisions

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Soft collisions
• The type of electron interaction
depends largely on the distance
(termed, impact distance, b)
between the electron and an atom
relative to the atomic radius (a).
• When b>>a, soft collisions occur in
which the influence of the particle's
coulomb force field affects the atom
as a whole.
• The net effect is the transfer of a
very small amount of energy (a few
eV) to an atom of the absorbing
medium.
b
a
e-
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Hard collisions
• When b ~ a, it becomes more
likely that a hard collision will
occur in which the incident
particle will interact primarily
with a single atomic electron,
ejecting it from the atom
(ionization) with considerable
kinetic energy
The ejected electron is called a
delta () ray.
• When b<<a Coulombic forces
between the electron and
nuclear lead to scattering of the
electron without much loss of
energy.
ba
e-

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Delta rays
• Delta ray is a secondary electron
with high enough energy that it
creates ionization tracks of its
own.
• Sometimes their energies are so
high that an individual delta ray
creates its own delta rays.
Delta ray of a
Delta ray
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Stopping power
• Electrons and charged particles gradually lose energy to matter through thousands or
millions of collisions (depending on energy).
• A particle’s stopping power in a medium is the average rate of energy loss of per unit
pathlength (MeV/cm), and is represented by
where the minus sign refers to the fact that from the particle’s perspective, energy is
lost per unit distance traveled. Thus, including a minus sign makes S(E) positive.

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Linear energy transfer
• The linear energy transfer (LET) is the average rate of energy
transferred to a material by a charged particle passing
through.
• LET is similar to stopping power, but from the material’s
perspective.
• LET and stopping power are very important concepts for
radiation dosimetry and radiobiology.
• They form the physical basis for radiation therapy from a
biological perspective.
• They are also the keys to our ability to measure radiation
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Radiative energy loss
• An electron traveling through a medium may be scattered at
reduced energy during interaction with a nucleus in the
medium.
• This “radiative” energy loss appears as electromagnetic
radiation (bremsstrahlung photons) during the interaction.
• The probability of bremsstrahlung production varies with Z2 of
the medium.
– High Z media are much more effective in producing
bremsstrahlung.
– The amount of bremsstrahlung produced increases dramatically
with the Z of the medium, but the relative shape of the spectrum
remains constant.

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Radiative energy loss
• A bremsstrahlung photon may possess an
energy up to the entire kinetic energy of the
incident electron.
Electron beam characteristics
• Continuous slowing due to very
many interactions leads to a
predictable stopping depth.
• The electron beam practical
range (Rp) is approximately
Rp =E/2
where E is the beam energy in
MeV
• At depths beyond Rp, a small
level of energy deposition is
due to brehmssralung

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Protons
• Protons are ~ 2000 times more massive than electrons.
• Stopping power caused by ionization interactions is proportional to
square of particle charge and inversely proportional to square of
velocity
• Thus, as particle slows down, its rate of energy loss also increases 
more ionization
• Dose in water increases at first very slowly with depth, and then
increases very sharply near end of particle’s range, and this gives rise
to the Bragg Peak
• Bragg Peak: Concentrated energy deposition within a very narrow
region at the end of the particle’s range.
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Protons

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Neutron interactions
• Neutron is an uncharged particle, it's mass close to that of
proton
• Neutrons are stable inside a nucleus. This structural
stability is lost when neutrons are in a free, independent
state.
• As the neutron is a little heavier than the proton, Einstein’s
famous mass-energy relation equates this extra mass with
an extra energy. This energy is just enough for the neutron
to transform into a proton by emitting an electron and an
antineutrino
• This transformation can also take place within a nucleus
when there are too many neutrons present: the resulting
electron emission is what is referred to as beta decay.
Neutron interactions
• Neutron interactions depends on energies:
from > 100 MeV to < 1 eV
• Neutrons are uncharged particles:
• ⇒ No interaction with atomic electrons of the
material
• ⇒ interaction with the nuclei of these atoms

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Neutron interactions
• Neutron interactions
1. Scattering
(n,n) Elastic scattering
(n,n’) Inelastic scattering
2. Absorption
3. Fission
Elastic scattering
• Elastic Scattering (mostly for En < 10 MeV)
• billiard ball type of collision;
• The neutron collides with a nucleus and scatters in a
different direction
• The energy the neutron loses is gained by the target
nucleus which moves away at an increased speed.
• If the target nucleus is massive, neutron scatters with the
same speed or little energy loss:
• If the target nucleus is light, neutron loses much energy ⇒
very effective slowing down process
• Elastic scattering is not effective in slowing down neutrons
with very high energy (above 150 MeV)

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Elastic scattering
Inelastic Scattering
• Inelastic Scattering (mostly for En ≥ 10 MeV,
heavy material)
• The neutron strikes a nucleus and form a
compound nucleus
• The nucleus is unstable: emission of n and γ
•

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Inelastic Scattering
Absorption
• In absorption reaction: The neutron is captured by a nucleus,
then this compound unstable nucleus emits a particle or
gamma rays to reach the stable state
• When the compound nucleus emits only a gamma photon, in
this case, the reaction is called Radiative Capture, This
reaction is the most important one for neutrons with very low
energy
• (n,γ) Radiative Capture
• (n,e) Absorption
• (n,α) Absorption
• (n, P) Absorption