Download What is “Radiation”?

Basic Physics of Ionizing
Radiation
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 ElectroMagnetic Radiation).
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.
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
EM Radiation: Wave Model
• EM radiation is a pair of perpendicular, timevarying electric and magnetic fields traveling
through space with the velocity of light (c).
• The distance between maxima of the EM fields
is the wavelength (λ).
• The frequency (ν) of the wave is given by:
ν=c/λ
Wavelength
• Distance between two peaks or
troughs in a wave.
l
l
• Wavelength (l) = Colour
• Wavelength (l) = Energy (E)
– (As l increases, E decreases)
Ultraviolet
390-425nm
425-445nm
445-500nm
500-575nm
575-585nm
585-620nm
620-740nm
Infrared
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 x 100
1
0.001
Milli (m)
1 x 10-3
0.000001
Micro (m)
1 x 10-6
0.000000001
Nano (n)
1 x 10-9
The de Broglie Wave
Hypothesis
h = Plank’s 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
6.6 x 10-35 m
7 x 10-12 m
Wavelength
(l)
EM Radiation: Photon Model
E=hc/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
Planck’s Constant (h) and the speed of light (c).
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.
Nuclear Transformation
- Δm
Radioactive
Stable
Ionizing Radiation: α, β, or γ
Gamma Rays
Z, M
Z, M
g
Gamma rays are electromagnetic radiation
resulting from nuclear transformation.
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).
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
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.
Alpha Particles
Z - 2, M - 4
4
++
a
2
Z, M
Alpha Particle
(Helium Nucleus)
Beta Particles
0
0n
Antineutrino
Z+1, M
Z, M
0
b
-1
Beta Particle
Four Primary Types of
Ionizing Radiation:
Alpha Particles
Alpha Particles: 2 neutrons and 2 protons
They travel short distances, have large mass
Only a hazard when inhaled
Four Primary Types of
Ionizing Radiation:
Beta 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:
Gamma Rays
Gamma Rays (or photons): Result when the nucleus releases
Energy, usually after an alpha, beta or positron transition
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
Neutrons
Neutrons: Have the same mass as protons but are uncharged
They behave like bowling balls
Four Primary Types of
Ionizing Radiation
•
•
•
•
•
Alpha particles
Beta particles
Gamma rays (or photons)
X-Rays (or photons)
Neutrons
Radioactive Atom
Ionizing Radiation
alpha particle
X-ray
beta particle
gamma ray
Radioactive Atom
Ionizing Radiation
alpha particle
X-ray
beta particle
gamma ray
Direct Ionization Caused By:
• Protons
• Alpha Particles
• Beta Particles
• Positron Particles
Indirect Ionization Caused By:
• Neutrons
• Gamma Rays
• X-Rays
Specific forms of ionizing radiation
Directly ionizing
Particulate radiation
Indirectly ionizing
consisting of atomic or subatomic
particles (electrons, protons, etc.)
which carry energy in the form of
kinetic energy of mass in motion.
Electromagnetic radiation
in which energy is carried by
oscillating electrical and magnetic
fields traveling through space at
the speed of light.
Interaction of Charged Particles with Matter:
Ionization
Interaction of x or g rays (photons) with matter
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.
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.
After a time that we call the element’s half life
half the parent atoms have decayed
the radioactivity count rate has dropped by one
half
count rat e/ decays per s
600
500
400
Half original decay rate
300
Half life = about 45s
200
100
0
0
50
100
150
200
t ime / s
250
300
350
400
After another half life
a further half of the parent atoms have decayed
the radioactivity count rate has dropped by a further half
count rat e/ decays per s
600
500
400
Half original decay rate
300
A further half of original decay rate
200
– so now only one quarter of original
100
0
0
50
100
150
200
t ime / s
250
300
350
400
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
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?
• Example 1 :
• A radioiodine compound has 5
microcuries of radioactivity on a given
date. How much radioactivity remains
after 40 days may be found
• Example 2 :
• A 5 microcurie dose of radioiodine is
administrated
to
a
patient
for
diagnostic purpose. If the physical halflife of the isotope is 8 days and the
biological half-life is 2 days, what
activity will remain after 8 days