Basic Physics of Ionizing Radiation

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Basic Physics of Ionizing Radiation
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What is Radiation?

• Radiation is energy travelling through space.
• Sunshine is one of the most familiar forms of
  radiation, without it we would not exist.
• But too much of it can be dangerous, therefore we
  limit our exposure (Radiation Protection)
• There are many forms of radiation from particles
  to waves (commonly termed Electro-Magnetic
  Radiation).

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What is Radiation?

• Radiation can be thought of as the transmission
  of energy through space.
• Two major forms of radiation
• Electromagnetic (EM) radiation
• Particulate radiation
• Both forms can interact with matter, and transfer
  their energy to the matter.

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Electromagnetic Radiation

• Electromagnetic radiation has no mass, and moves
  through space at the speed of light (3.0 x108
  meters per second).
• Electromagnetic radiation can be described by two
  models
• Wave Model
• Photon Model

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EM Radiation Wave Model

• EM radiation is a pair of perpendicular,
  time-varying electric and magnetic fields
  traveling through space with the velocity of
  light (c).
• The distance between maxima of the EM fields is
  the wavelength (?).
• The frequency (?) of the wave is given by

? c / ?
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Wavelength

• Distance between two peaks or troughs in a wave.
• Wavelength (l) Colour
• Wavelength (l) Energy (E)
• (As l increases, E decreases)

l
Ultraviolet 390-425nm 425-445nm 445-500nm 500-575n
m 575-585nm 585-620nm 620-740nm Infrared
l
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Scientific Notation
Value Symbol Value (Scientific Notation)
1,000,000,000 Giga (G) 1 x 109
1,000,000 Mega (M) 1 x 106
1,000 Kilo (k) 1 x 103
1 1 x 100
0.001 Milli (m) 1 x 10-3
0.000001 Micro (m) 1 x 10-6
0.000000001 Nano (n) 1 x 10-9
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The de Broglie Wave Hypothesis

• h Planks Constant 6.6 x 10-34 J/s
• p momentum mass x velocity
• l wavelength

Value Bullet Electron
Mass 0.03kg 9.1 x 10-31 kg
Velocity 330 m/s 1 x 108 m/s
Wavelength (l) 6.6 x 10-35 m 7 x 10-12 m
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EM Radiation Photon Model
E h c / l
Electromagnetic radiation can also be described
as discrete packets of energy called photons.
The energy (E) is related to the wavelength (l)
in the wave model through Plancks Constant (h)
and the speed of light (c).
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Ionizing EM Radiation

• EM radiation with wavelengths shorter than 100
  nanometers can remove electrons from the outer
  atomic shells.
• This process produces ions.
• Ions can interact with living tissue to produce
  biological damage.
• A major source of ionizing radiation is nuclear
  transformation.

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Nuclear Transformation
Stable
Radioactive
Ionizing Radiation a, ß, or ?
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Gamma Rays
Z, M
Z, M
g
Gamma rays are electromagnetic radiation
resulting from nuclear transformation.
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Production of X-Rays
Electron or beta
X-Ray
Target Nucleus (Heavy metal)
X-rays are produced when a charged particles
(electrons or betas) are decelerated by a strong
electrostatic field, such as that found near the
nuclei of heavy metals (tungsten, lead).
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Types of Ionizing Radiation
Alpha Particles Stopped by a sheet of paper
Radiation Source
Beta Particles Stopped by a layer of clothing or
less than an inch of a substance (e.g. plastic)
Gamma Rays Stopped by inches to feet of
concrete or less than an inch of lead
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Particulate Radiation

• Charged particles are emitted from the atomic
  nucleus at high energy in some nuclear
  transformations. These include alpha and beta
  particles.
• Uncharged particles (neutrons) are produced by
  fission or other nuclear reactions.
• Both types of particles produce ionization.

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Alpha Particles
Z - 2, M - 4
42a
Z, M
Alpha Particle (Helium Nucleus)
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Beta Particles
00n
Antineutrino
Z1, M
Z, M
0-1b-
Beta Particle
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Four Primary Types of Ionizing RadiationAlpha
Particles
Alpha Particles 2 neutrons and 2 protons They
travel short distances, have large mass Only a
hazard when inhaled
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Four Primary Types of Ionizing RadiationBeta
Particles

Beta Particles Electrons or positrons having
small mass and variable energy produced inside
the nucleus. Electrons form when a neutron
transforms into a proton and an electron or
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Gamma Rays
Gamma Rays (or photons) Result when the nucleus
releases Energy, usually after an alpha, beta
or positron transition
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X-Rays
X-Rays Occur whenever an inner shell orbital
electron is removed and rearrangement of the
atomic electrons results with the release of the
elements characteristic X-Ray energy
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Neutrons
Neutrons Have the same mass as protons but are
uncharged They behave like bowling balls
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Four Primary Types of Ionizing Radiation

• Alpha particles
• Beta particles
• Gamma rays (or photons)
• X-Rays (or photons)
• Neutrons

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Radioactive Atom
X-ray
gamma ray
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Radioactive Atom
X-ray
gamma ray
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Direct Ionization Caused By

• Protons
• Alpha Particles
• Beta Particles
• Positron Particles

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Indirect Ionization Caused By

• Neutrons
• Gamma Rays
• X-Rays

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Specific forms of ionizing radiation
Directly ionizing
Particulate radiation
consisting of atomic or subatomic particles
(electrons, protons, etc.) which carry energy in
the form of kinetic energy of mass in motion.
Indirectly ionizing
Electromagnetic radiation
in which energy is carried by oscillating
electrical and magnetic fields traveling through
space at the speed of light.
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Interaction of Charged Particles with Matter
Ionization
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Interaction of x or ? rays (photons) with matter
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Concept of Physical Half-life

• Radioactive nuclei undergo disintegration at a
  rate that is proportional to the number of
  untransformed nuclei present.
• The physical half-life is the time required for
  one-half of the remaining nuclei to transform.
• The half-life is characteristic of the
  radionuclide.

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• Each second
• a fraction of the parent atoms decay
• each atom which decays throws out a radioactive
  particle
• each atom which decays produces a new daughter
  atom.

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• After a time that we call the elements half life
• half the parent atoms have decayed
• the radioactivity count rate has dropped by one
  half

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Half original decay rate
Half life about 45s
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• After another half life
• a further half of the parent atoms have decayed
• the radioactivity count rate has dropped by a
  further half

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Half original decay rate
A further half of original decay rate so now
only one quarter of original
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Half Lives can have many different
values e.g. Uranium 238 5000 million
years Cobalt 60 5 years Iodine 131 8
days Barium 143 12 seconds Polonium 213 4
millionths of a second
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Half-Life

• The Half-Life of a radioactive isotope is the
  time taken for its activity to drop to half its
  initial value.
• 100 atoms of Tc-99m (used in Nuclear Medicine
  Departments).
• The half-life of Tc-99m is 6 hours.
• How many atoms have decayed in 12 hours?

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• Example 1
• A radioiodine compound has 5 microcuries of
  radioactivity on a given date. How much
  radioactivity remains after 40 days may be found

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• Example 2
• A 5 microcurie dose of radioiodine is
  administrated to a patient for diagnostic
  purpose. If the physical half-life of the isotope
  is 8 days and the biological half-life is 2 days,
  what activity will remain after 8 days