RADIATION PHYSICS
MODULE 5
Lecturer: Ms RW Letsoalo
Welhemina.Letsoalo@smu.ac.za
NSB Room 210
X4380
LEARNING OBJECTIVES:
 Describe the origin of nuclear radiation in radioisotopes;
 Discuss the properties of the various radioactive emissions;
 Describe the interaction of radiation with matter;
 Define half-life and solve problems using half-life;
 Describe the various methods of radiation measurement;
 Discuss the effects of radiation on human beings;
 Discuss the uses of radiation in medicine with examples;
 Know the methods of radiation detection;
 List the safety measures against radiation.
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What is an atom?
 All matter is composed of individual entities called elements.
 Each element is distinguishable from the others by the physical and
chemical properties of its basic component – atom.
 atom: “basic unit of matter which consists of a dense, central nucleus
surrounded by a cloud of –ly charged electrons”;
 Atoms are incredibly small, too small even to be seen with a microscope.
 There are 9 sextillion atoms (9,000,000,000,000,000,000,000) in a coin.
 atomic nucleus contains a combination of positively charged particles (+)
protons and electrically neutral particles (0) neutrons;
 Protons + Neutrons are called NUCLEONS.
 electrons (-) of any atom bound to the nucleus by an EM force;
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 Every solid, liquid, gas, and plasma is made up of neutral
or ionized atoms.
 Plasma is another state of matter with different properties other than
that for solid, liquid and gas.
 Over 99.94% of the atom’s mass is in the nucleus.
 # of protons determine chemical properties of an atom;
- A chemical property may only be observed by changing the chemical identity of a
substance. This property measures the potential for undergoing a chemical change. Examples
of chemical properties include reactivity, flammability and oxidation states..
 neutrons cannot alter the number of free protons in the nucleus &
quantity of electrons in the orbit  neutral charge;
• They do not influence the chemical properties of an atom and matter;
• But only the physical properties of atoms by emitting radiation;
• A physical property is an aspect of matter that can be observed or measured
without changing it. Examples of physical properties include color, molecular
weight and volume.
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(A) A neutral atom has no net charge because the numbers of
electrons and protons are balanced.
(B) Removing an electron from a neutral atom produces a net
positive charge; the charged atom is called a positive ion (cation).
(C) The addition of an electron to a neutral atom produces a net
negative charge; therefore the charged atom is called a negative
ion (anion).
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Atomic Structure
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A diagram showing the charges of each part
of the atom.
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• Rutherford-Bohr atom.
• Neil's Bohr improved on Rutherford’s model of the structure of the
atom by explaining electron structure around the nucleus.
• In the Bohr atom, the protons and neutrons are held close
together in the nucleus, with electrons in orbit, in fixed shells,
around the nucleus.
• The energy level of the electrons in the shells (i.e. distance from
the nucleus) increases with the number of protons in the nucleus.
Thus the atoms of each element have their own unique,
characteristic shell structure and energy levels.
•
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The number of electrons in shells around the nucleus equals the number of
protons in the nucleus.
•If given enough energy, a k or l shell electron can move into the m shell
(excitation – raised to the next highest energy state) but after a very
short time there, it will fall back to its original shell while emitting its excess
energy
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• The outermost shell electrons (valence electrons) give the chemical
nature to the atom.
• i.e. a photon of electromagnetic radiation (characteristic radiation). NB.
Pauli’s exclusion principle.
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• The binding energy of an electron is defined as the energy
required to remove it from the atom.
• The binding energy for electrons increases the closer they are to
the nucleus and binding energy increases with the Z value of the
nucleus.
• When an electron is removed completely from the atom, the
process is called ionisation.
• When an electron is raised from a lower energy cell to a higher
energy cell, the process is called excitation.
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•Molecules
• Atoms from the same or different elements can
chemically bond together to form molecules with very
different physical and chemical properties.
• For instance, oxygen is a gas, hydrogen is a gas, but
when they bond together they form liquid water.
• Energy Bands
Energy bands are groups of energy levels which result
from the close proximity of atoms in a solid.
The three most important energy bands are the
CONDUCTION BAND, FORBIDDEN BAND, and
VALENCE BAND.
Each of these bands will be discussed briefly in the
following paragraphs.
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(a) Conduction Band. The upper band in the figure is called the conduction
band because electrons in this band are easily removed by the application
of external electric fields.
 Materials that have a large number of electrons in the conduction band
act as good conductors of electricity.
(b) Forbidden Band. Below the conduction band is the forbidden band or
energy gap.
 Electrons are never found in this band, but may travel back and forth
through it, provided they do not
come to rest in the band.
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• c) Valence Band. The last band or valence band is composed of a
series of energy levels containing valence electrons.
 Electrons in this band are more tightly bound to the individual
atom than the electrons in the conduction band.
 However, the electrons in the valence band can still be moved to
the conduction band with the application of energy, usually
thermal energy.
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_
If an electron
in the valence band is given enough energy (e.g. by heat or by an X-ray) it can
move into the conduction band. The atom left (minus an electron) is called a hole.
+
Electrons flow from minus to plus, and holes "flow" from plus to
minus.
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• In electrical insulators (e.g. rubber, ceramic) the energy gap is
large and electrons cannot get enough energy to move to the
conduction band.
• In semiconductors (e.g. germanium, silicon) the energy gap is
small and at room temperature and there will be a small number
of electrons in the conduction band.
• In good conductors (e.g. gold, copper) the energy gap is zero
and there are many electrons in the conduction band.
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



The nucleus is the central part of an atom.
It is composed of protons and neutrons.
The nucleus contains most of an atom's mass.
It was discovered by Ernest Rutherford in 1911.
 Protons are positively charged particles found in the atomic nucleus.
 Protons were discovered by Ernest Rutherford.
 mass= 1,6726 x 10-27 kg
 Neutrons are uncharged particles found in the atomic nucleus.
 Neutrons were discovered by James Chadwick in 1932.
 mass= 1,6749 x 10-27 kg
 Electrons are negatively charged particles that surround the atom's nucleus.
 Electrons were discovered by J. J. Thomson in 1897
 Mass = 0,00091x10-27 kg

Mass neutron >mass proton
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• What then, holds the nucleus together?
• Two positive charges that are close together as they are in a nucleus repel one another
with a very strong electrostatic force.
• The protons have +ve electric charge, therefore all the electric forces within the nucleus
are repulsive.
• The mutual repulsion of the protons tends to push the nucleus apart.
• Neutrons are uncharged and do not participate in any electrical interaction.
• Proton and neutrons both possess mass, but gravitational force is too weak to overcome
the electrical force.
• If these were the only important forces, every nucleus would fly apart, and all atoms
would be reduced to hydrogen with one proton per nucleus.
• The gravitational force of attraction between the nucleons is too weak to counteract the
repulsive electric force, so a different type of a force must hold the nucleus together. This
force is called THE STRONG NUCLEAR FORCE.
• The range of action of the strong nuclear force is extremely short, with the force of
attraction being strong when the two nucleons are as close as 10−15 m and essentially
zero at larger distances.
• This force is 100 times stronger than the electrical force under comparable conditions.
• Thus, two protons repel each other through the electrical interaction if they are some
distance apart, but they attract each other through the strong interaction if they are
close enough together
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The Strong Nuclear Force and the Stability of the Nucleus
The strong nuclear force
• Acts over short distances
• ~ 10-15 m
• can overcome Coulomb repulsion
• acts on protons and neutrons
The protons and neutrons in the nucleus are
clustered together to form an approximately
spherical region as indicated in the figure above.
Experiments shows that the radius r of the nucleus
depends on the atomic mass number A and is given
approximately in meters by
𝒓 ≈ (𝟏. 𝟐 × 𝟏𝟎−𝟏𝟓 m)𝑨𝟏/𝟑
This equation indicates that the radius of E.g.
Aluminium nucleus (A=27) is
𝒓 ≈ (𝟏. 𝟐 × 𝟏𝟎−𝟏𝟓 m)𝟐𝟕𝟏/𝟑 = 𝟑. 𝟔 × 𝟏𝟎−𝟏𝟓m.
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Summary of Nuclear Forces:
 Nuclei give off energy (i.e., radiation) in an attempt to become more stable
 Nuclear instability can be traced to the interaction of i) Coulomb and ii) strong
nuclear force.
Coulomb force
Strong Nuclear force
repulsive
attractive
p+ - p+
p+ - p+, n - n , p+- n
doesn't saturate
short range; falls off quickly
weak (eg. e- to nucleus, ~ few eV to
0.1 MeV)
very strong (several decades of
MeV)
atom is mostly empty space
nucleus is densely packed
 Due to the Coulomb-nuclear force balance, nuclei exhibit a roughly
constant density and radius.
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Nuclei are described using the following nomenclature:
atomic mass
number
atomic
number
A
Z
N


 




Number of protons
and neutrons
Number of
protons
Number of
neutrons
N
N
=A-Z
Number of neutrons
N
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Mass and atomic number
Particle
Relative Mass
Relative Charge
Proton
Neutron
1
1
1
0
Electron
0
-1
MASS NUMBER = number of protons + number
of neutrons
SYMBOL
PROTON NUMBER = number of protons
(obviously)
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• The limited range of action of the strong nuclear force plays an
important role in the stability of the nucleus.
• For a nucleus to be stable, the electrostatic repulsion between the
protons must be balanced by the attraction between the nucleons
due to the strong nuclear force.
• For the element to maintain its stability, the following condition
should hold: i.e. as Z increases in the nucleus, the N has to keep on
increasing even more. See fig 31.2.
• As more and more protons occur in a nucleus,
there comes a point when a balance of repulsive
and attractive forces cannot be achieved by an
increased number of neutrons.
• Eventually, the short-range of action of the strong nuclear force
prevents extra neutrons from balancing the long-range electric
repulsion of extra protons, then it causes the atom to break apart or
rearrange their internal structures as time passes.
• This spontaneous disintegration or rearrangement of internal
structure is called RADIOACTIVITY.
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How many protons, neutrons and electrons?
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Periodic table
The periodic table arranges all the elements in groups
according to their properties.
Vertical columns are
called GROUPS
Mendeleev
Horizontal rows are called PERIODS
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The Periodic Table contd.
Fact 1: Elements in the same group have the same number of
electrons in the outer shell (this correspond to their group
number)
H
He
Li
Be
B
C
N
O
F
Na
M
g
Al
Si
P
S
Cl Ar
K
Ca
Fe
Ni
C
u
Zn
Ag
Pt
E.g. all group 1 metals
have __ electron in
their outer shell
A
u
Ne
Br Kr
I
Xe
H
g
These elements have
__ electrons in their
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outer
shells
These elements
have __ electrons
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in their outer shell
• Atoms have been classified into the following categories:
 Isotopes- have the same nuclei with the same number of protons but
different number of neutrons.
•
(i.e. same Z with different N)
• Eg. Hydrogen, deuterium and tritium.
 Isotones- have the same number of neutrons but different number of
protons.
•
(i.e. having the same N with different Z)
• Eg. Cl-37 has 17 protons & 20 neutrons, K-39 has 19 protons & 20 neutrons.
 Isobars- have the same number of nucleons but different number of
protons.
•
(i.e. having the same A but different Z)
• E.g cl-37 has 17 protons & 20 neutrons, Argon-37 has 18 protons & 19
neutrons.
 Isomers- Have the same number of protons as well as neutrons but differ in
energy and a manner of radioactive decay. (i.e. having the same A same N
but differ in their decay)
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Example of an Isotope:
• Atoms that have the same number of protons but different
numbers of neutrons.
• most common isotope of hydrogen has no neutrons at all;
there is also a hydrogen isotope with one neutron
(deuterium spelled: dee-teriam) and another with two
neutrons (tritium);
Example:
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• specified by the name of the particular element, followed by a
hyphen and Z;
• examples: helium-3, carbon-12, carbon-13, iodine-131 and
uranium-238;
• radioisotopes /radionuclides: unstable; ability to emit
radiation;
• stable isotopes: with a stable ratio of neutrons to protons &
exceedingly long half-lives;
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Energy Atomic mass and energy units
 Masses of atoms and atomic particles are conveniently
given in terms of atomic mass unit (amu).
 Thus, the atom of 𝐶 is arbitrarily assigned the mass equal
to 12 amu.
 1 amu = 1.66 × 10−27 kg
 The mass of an atom expressed in terms of amu is known
as atomic mass or atomic weight/ grams atomic weight.
 According to Avogadro’s law , every gram atomic weight of
a substance contains the same number of atoms.
 This no. is referred to as the Avogadro’s number
12
6
•
𝑁𝐴 = 6.0228 × 1023 atoms.
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• One can calculate other quantities of interest such as the
number of atoms per grams, grams per atoms and electron per
gram.
• Example: Helium (He) 𝐴𝑤 (atomic weight) = 4.0026, Z
(atomic number) = 2
• Therefore:
• 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓
•
• 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓

1.
2.
3.
𝑎𝑡𝑜𝑚𝑠
= 𝐴𝑁𝑤𝐴 = 1.505 × 1023
𝑔
𝐺𝑟𝑎𝑚𝑠
= 𝐴𝑁𝑤 = 6.646 × 10−24
𝑎𝑡𝑜𝑚
𝐴
𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
𝑔
= 𝐴𝑁𝑤𝐴 . 𝑍 = 3.009 × 1023
The masses of atomic particles, according to the atomic mass unit:
Electron = 0.000548 𝑎𝑚𝑢
Proton = 1.00727 𝑎𝑚𝑢
Neutron = 1.00866 𝑎𝑚𝑢
𝑚𝑒 ≪ 𝑚𝑝 , 𝑚𝑛
𝑚𝑝 ≅ 𝑚𝑛 ≈ 1 𝑎𝑚𝑢
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The Mass Deficit of the Nucleus
• The mass of the atom is not exactly equal to the sum of the masses of
constituents particles.
• The sum of the individual masses of the separated protons and
neutrons is greater than the mass of the stable nucleus intact.
• The reason for this is that, when the nucleus is formed, a certain mass
is destroyed and converted into energy that keeps the nucleons
together.
• The difference in mass ∆m is called the mass defect of the nucleus.
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The Nuclear Binding Energy
• The nucleons in the stable nucleus are held tightly together by the
strong nuclear force, therefore, some form of energy is required
to separate a stable nucleus into its constituents.
• The more stable the nucleus is, the greater is the amount of
energy needed to break it apart. The required energy is called the
BINDING ENERGY of the nucleus.
• An amount of energy equal to the mass defect must be supplied
to separate the nucleus into individual nucleons.
Binding energy  Mass defect c  m c
2
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2
35
• Usually, binding energies are expressed in energy units of electron
volt (eV)
• Hence, a more convenient unit in atomic and nuclear physics is the
electron volt (eV).
• Electron volt is the kinetic energy acquired by an electron in
passing through a potential difference 1𝑉.
• 1𝑒𝑉 = 1𝑉 × 1.602 × 10−19 C = 1.602 × 10−19 𝐽
• Multiples of this unit are:
• 1 𝑘𝑒𝑉 = 1000 𝑒𝑉
• 1 𝑚𝑖𝑙𝑙𝑖𝑜𝑛 𝑒𝑉 𝑀𝑒𝑉 = 106 𝑒𝑉
• Look at figure 31.3
• Example 2 and 3
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The Mass Deficit of the Nucleus and Nuclear
Binding Energy
Example 3 The Binding Energy of the Helium Nucleus Revisited
The atomic mass of helium is 4.0026u and the atomic mass of hydrogen
is 1.0078u. Using atomic mass units, instead of kilograms, obtain the
binding energy of the helium nucleus.
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The Mass Deficit of the Nucleus and Nuclear
Binding Energy
m  4.0330 u  4.0026 u  0.0304 u
1 u  931.5 MeV
Binding energy  28.3 MeV
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Go to page 956, 9th ed.
for the conversion
factors.
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• In the above graph, the peak for the He isotope indicates that He nucleus
is particularly stable.
• The binding energy per nucleon increases rapidly for nuclei with small
masses and reaches a maximum of approximately 8.7 MeV/nucleon for a
nucleon number A=60.
• For greater nucleon numbers, the binding energy per nucleon decreases
gradually.
• Eventually, the binding energy per nucleon decrease enough so there is
insufficient binding energy to hold the nucleus together.
• Nuclei more massive than the Bismuth -83 nucleus are unstable and hence
radioactive. They will spontaneously undergo a particular decay process
to stabilize.
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• Ionizing radiation is a process in which energy is emitted in the form of
electromagnetic waves/particles carry enough kinetic energy to liberate an
electron from an atom or molecule i.e. ionizing it.
•
•
The best way to think of electromagnetic radiation is as a wave
packet called a photon.
Photons are chargeless bundles of energy that travel in a vacuum
at the speed of light.
• Ionizing radiation is classified into 2 categories:
i. Direct ionizing radiation
ii. Indirect ionizing radiation
• Direct ionizing radiation – a charged particle e.g. electron, proton etc. deposits its
energy into a medium/absorber through direct coulomb interaction (the interaction is then
experienced between the atoms in the absorber and the charged particle).
• Indirect ionizing radiation – a neutral particle (X-ray or γ-ray) deposits its
energy in the absorber/medium following two step process:
• 1st step: the neutral particle releases or produces a charged particle in the
medium/absorber and
• 2nd step: the charged particle released then interacts with the atoms in the
absorber/medium through coulombic interaction.
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Interaction of Radiation with matter
• Three methods of interaction of radiation with matter:
1. PHOTOELECTRIC EFFECT
2. COMPTON SCATTERING
3. PAIR PRODUCTION
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Interaction of Radiation with matter
 radiations from radioactive materials pass through matter 
interact with atoms & molecules and transfer E to them
 effects: ionization and excitation
 ionization: E transferred sufficient to strip an orbital
electron away, creating an ion pair
 excitation: electrons raised to an excited state
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Interaction of Radiation with matter
PHOTOELECTRIC EFFECT:
The photoelectric effect is a phenomenon in which a photon interacts with
an atom and ejects one of the orbital electrons from the atom.
 In this process, the entire energy ( hv ) of the photon is first absorbed by
the atom and then transferred to the atomic electron.
The kinetic energy of the ejected electron (called the photoelectron) is equal
to hv  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
Illustration of the Photoelectric effect
shells.
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PHOTOELECTRIC EFFECT:
 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
monoenergetic electrons produced by the absorption of characteristic x-rays
internally by the atom.
 The Photoelectric Effect is of fundamental importance in diagnostic
radiography since it is the primary method by which contrast is developed
in radiographs.
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Interaction of Radiation with matter
COMPTON SCATTERING:
 In the Compton process, the photon interacts with an atomic
electron as though it were a “free” electron.
 The term “free” here 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 is emitted at an angle θ = called Compton electron
Illustration of the Compton effect
( e ).
 The photon (hv' ), with reduced energy,
 is scattered at an angle ɸ.
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Interaction of Radiation with matter
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
 It then splits and gives up all its energy in the process by creating a
pair consisting of a negative electron (e-) and a positive electron (e+).
 Because the rest mass energy of the electron is equivalent to 0.51 MeV,
 a minimum energy of 1.02 MeV is required to create the pair of
Illustration of the Pair Production
electrons.
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Pair Production
 Thus the threshold energy for the pair production process is 1.02 MeV.
 The photon energy in excess of this threshold is shared between the particles
as kinetic energy.
 The total kinetic energy available for the electron-positron pair is given by
• ( hv  1.02) MeV
 The particles tend to be emitted in the forward direction relative to the
incident photon.
Illustration of the Pair Production
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Radioactive decay
• 3 types of particles emitted by radioactive nuclei, viz
• Alpha particle () 
Helium nucleus
• Beta particle () 
electrons
• Gamma rays () 
electromagnetic radiation
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• An unstable/radioactive nucleus is called the parent and after decay, the new
nucleus is called the daughter.
• In many cases the daughter may also be radioactive and undergo further
decay.
• Radioactive decay is a process involving changes in the nucleus where, in the
majority of cases, protons change into neutrons, or, neutrons to protons.
• A particle may be emitted from or received by the nucleus and gamma (γ ) ray
emitted or characteristic X-rays created.
• The fact that an atom has a radioactive nucleus does not affect its chemical
behaviour.
• For example, Radioactive nuclide follows the same biological pathway as the
stable nuclide.
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• Line of stability
• There are about 270 stable isotopes of the 90 naturally occurring
elements. The ratio of the number of neutrons to the number protons
(N/Z ratio) is an indicator of the stability of a nucleus. On a plot of N
versus Z, the stable isotopes are clustered around an imaginary line
called the line of stability.
•
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• Isotopes that are not close to the line of stability are likely to be unstable
(radioisotopes).
• Isotopes above the line are said to be neutron rich and undergo radioactive
decay where a neutron gives off a β - particle and becomes a proton and
its Z value increases by one.
• Isotopes below the line are said to be proton rich and undergo radioactive
decay where a proton gives off a β + particle (or electron capture) and
becomes a neutron and its Z value
decreases by one.
• In both processes, the new isotopes are
closer to the line of stability.
 More than 800 nuclides are known
(274 are stable)
 “stable” unable to transform into another
configuration without the addition of
outside energy.
 “unstable” = radioactive
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Radioactivity
• The Radioactivity refers to the unstable nucleus ‘s attempt to
achieve stability by losing neutrons  emitting energy in form of
electromagnetic (EM) radiation;
• RADIOACTIVE DECAY: “process by which an unstable
atomic nucleus spontaneously loses energy by emitting
ionizing particles and radiation”.
• this loss of energy results in an atom of one type (parent
nuclide) transforming to an atom of a different type (daughter
nucleus).
• example: carbon-14(parent)  nitrogen-14 (daughter)
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Radioactivity
• SI unit of activity: becquerel (Bq)  one disintegration per
second
• another: curie (Ci)  amount of radium emanation in
equilibrium with 1g of pure radium.
1 Ci = 3.7 x 1010 Bq = 37 GBq
1 Ci = ??? Bq
1 Bq = 2.703 × 𝟏𝟎−𝟏𝟏 Ci
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Radioactive Decay modes
 When an unstable radioactive nucleus disintegrates spontaneously, certain
kinds of particles and or high energy photons are released.
 These particles and photons are collectively called “rays”. Three kinds of
rays are produced by naturally occurring radioactivity: α rays, β rays and γ
rays.
1. Beta Emission: “process whereby a neutron in a radioactive nucleus is
transformed into a proton and an electron”.
n  p       (antineutrino)  energy
beta particle (electron) and antineutrino ejected from the nucleus;
antineutrino: mass-less and no charge;
extra proton exists in the nucleus  new element is
formed with Z one greater than the original element;
3
1
H  23He 
o
1
e or

32
15
Modern Physics By RW Letsoalo 2015
32
P 16
S 
57

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Radioactive Decay modes

 
decay emission  daughter nucleus in excited state;
promptly decays to stable nuclear arrangement by emitting a
-ray


A
Z


,  decay

X  Y 
 Y
A *
Z 1
A
Z 1
example: Xe-133 decays into three diff. excited states of Cs133 giving of -rays of varying energies;
Modern Physics By RW Letsoalo 2015
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Radioactive Decay modes
Look at
figure
31.7
β-decay occurs when a neutron in an unstable parent nucleus decays
into a proton and an electron.
A
Z
P 
Example 6
pg 967
A
Z 1
D 
0
1
e
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Radioactive Decay modes
2. Positron Decay (𝜷+ ): proton on nucleus transformed into a
neutron and +ly charged electron (positron);
A
z
P
D  e
A
Z 1
0
1



& neutrino are ejected from the nucleus
p   n  e    energy
18
9

F 18
O



8
The emitted positron does not exist within the nucleus but, rather,
is created when a nuclear proton is transformed into a neutron.
after ejection, it loses kinetic E in collisions with atoms;
combines with an electron in an annihilation reaction  masses
converted into E in form of two 0.511MeV annihilation photons;
Modern Physics By RW Letsoalo 2015
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62
Radioactive Decay modes
3. Alpha Emission: nucleus ejects α-particle (2p and 2n);
Experimental evidence shows that α rays consists of a positively
charged particles, each one being the 24 He , the nucleus of helium.
common in elements with more than 83 protons;
A
Z

A 4
X

Z  2Y
A
Z
P 
A 4
Z 2
D 
4
2
He
ejection of an α-particle results in the atom changing into a new
element with an atomic number less than before the emission;
4
U  234
Th

90
2 He
238
92
239
94
4
Pu  235
U

92
2 He
Look at example 4 pg 965
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64
Radioactive Decay modes
Isomeric Transition: occurs in an atom where the nucleus is in
an excited metastable state, following emission of  or 
particle;
excess energy in the nucleus is released by emission of a ray, with the nucleus returning to ground state;
similar to gamma emission, but involves excited metastable
(isomeric) states.
Modern Physics By RW Letsoalo 2015
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Radioactive Decay modes
4. Internal Conversion: is a radioactive decay process where an excited
nucleus interacts with an electron in one of the lower atomic orbitals,
causing the electron to be emitted/ejected from the atom.
 This process does not take place in a nucleus but in an atom itself.
The excited nucleus transfers the energy to an orbital electron, which is then
ejected from the atom (monoenergetic electron).
 EIC electron = Etrans – BEatomic electron
IC and gamma decay are competing processes
Internal conversion coefficient (α)
α= Fraction of decays occurring by gamma emission/Fraction
of decays occurring by IC
Modern Physics By RW Letsoalo 2015
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5. Electron Capture: orbital electron is captured by the nucleus and combines with a proton to
form a neutron;
p   e   n    energy
neutrino emitted from nucleus with some of transition E
X 
K
40
o
EC
A
A
19
1
Z
Z 1
remaining E in form of char. X-rays and Auger electrons
Y
EC
40
e 
18 Ar
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Radioactive Decay modes
6. Gamma-ray Emission (γ) :
• nucleus in an excited state may emit one or more photons (=
packets of EM radiation) of discrete energies;
• production of α and β particles  release of energy emitted
in the form of waves of energy known as gamma (γ) rays;
• emission of γ-rays does not alter # of protons or neutrons,
but has an effect of moving the nucleus from a higher
(unstable *) to a lower (stable) energy state; 𝑜𝑜 𝛾 ;
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Radioactive Decay modes
 γ-ray emission follows β-, α- and other nuclear decay
processes
A
z
P  P  
A
Z
Bi 
214
83
214
84
Po 
214
84
*
Po
*
e
1
Po  
214
84
 γ-decay does not cause a transformation of one element into another.
Look at example 7 pg 968
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Summary: Radioactive Decay Modes


Beta Emission:  , 
excess neutron converted into a proton and electron
proton stays in the nucleus and electron emitted (beta
particle)
1 extra proton gained by daughter element and 1 less neutron

n  p     (neutrino )  energy

32
15

P S  e
32
16

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Summary: Radioactive Decay Modes
daughter in an excited state  ground state by emitting radiation

A
A * 
A
Z
Z 1
Z 1
X  Y 
 Y
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Summary: Radioactive Decay Modes
Alpha Emission:
unstable nucleus emits a helium nucleus (2p and 2n)/particle.
elements with > 83 protons
A
Z

X

U
238
92
A 4
Z 2
Y
Th He
234
90
4
2
daughter nucleus releases its excitation energy by -emission.
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Summary: Radioactive Decay Modes
Isomeric Transition
decay of an excited nucleus to a lower-E level (ground state)
by emitting -ray.
Internal Conversion
 -ray emitted by nucleus as it goes from excited state to
ground state interacts with orbital electron of same atom.
electron ejected instead of -ray (=conv. electron)
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Summary: Radioactive Decay Modes
Electron Capture
nucleus abs. an electron from the innermost orbit
electrons combines with a proton to form a neutron
p  e  n   energy

A
Z

X 
EC
A
Z 1
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76
Summary: Radioactive Decay Modes
Positron Emission
proton transformed into a neutron and positron   which is
immediately ejected from nucleus.
 
p   n  e   energy
loses its kinetic E in collisions with atoms  combines with
an electron  masses converted into E  as two 0.511MeV
annihilation photons.
18
9
F  O   
18
8

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Summary: Radioactive Decay Modes
Gamma-ray Emission
γ-ray emission follows β-, α- and other nuclear decay
processes
emission of γ-rays does not alter # of protons or neutrons, but
has an effect of moving the nucleus from a higher (unstable)
to a lower (stable) energy state.
*
1
Bi 214
Po

e
84
214
83
Po 
214
84
*
Po  
214
84
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31.5. The neutrino
•Read from pg 970 - 971
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Technetium and its production
Not for exam
purposes.
 Technetium is the chemical element with atomic number 43 and the
symbol Tc.
 It is the lowest atomic number element without any stable isotopes;
every form of it is radioactive, meaning it gives off atomic particles.
 Nearly all technetium is produced synthetically/artificial, and only
minute amounts are found in nature.
 Technetium-99m is a metastable nuclear isomer of technetium-99,
symbolized as 99mTc,
 that is used in tens of millions of medical diagnostic procedures
annually, making it the most commonly used medical radioisotope.
Modern Physics By RW Letsoalo 2015
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Not for exam purposes.
 Technetium-99m when used as a radioactive tracer can be detected in
the body by medical equipment (gamma cameras).
 It is well suited to the role because it emits readily detectable
140 keV gamma rays and its half-life for gamma emission is 6.0058
hours.
 The "short" physical half-life of the isotope and its biological halflife of 1 day (in terms of human activity and metabolism) allows for
scanning procedures which collect data rapidly but keep total patient
radiation exposure low.
 The same characteristics make the isotope suitable only for
diagnostic but never therapeutic use.

99mTc
is a very versatile (having many functions) radioisotope. It is easy to
produce in a technetium-99m generator, by decay of 99Mo.
99Mo → 99mTc + e− + ν
•
 The molybdenum isotope has a half-life of approximately 66 hours (2.75
days), so the generator has a useful life of about two weeks.
Modern Physics By RW Letsoalo 2015
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Not for exam purposes.
 Molybdenum-99 spontaneously decays to excited states of
through beta decay.
99Tc
 Over 87% of the decays lead to the 142 keV excited state of Tc99m. A β− electron and a ν antineutrino are emitted in the process
 (99Mo → 99mTc + β− + ν)
 The β− electrons are easily shielded for transport, and 99mTc
generators are only minor radiation hazards,
 mostly due to secondary X-rays produced by the electrons (also
known as Bremsstrahlung).
Modern Physics By RW Letsoalo 2015
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Not for exam purposes.
 Most commercial 99mTc generators use column chromatography, in
which 99Mo in the form of molybdate, MoO42- is adsorbed onto acid
alumina (Al2O3).
 When the 99Mo decays it forms pertechnetate TcO4-, which because
of its single charge is less tightly bound to the alumina.
 Pulling normal saline solution through the column of immobilized
99Mo elutes the soluble 99mTc, resulting in a saline solution
containing the 99mTc as the dissolved sodium salt of the
pertechnetate.
 The pertechnetate is treated with a reducing agent such as Sn2+ and a
ligand.
 Different ligands form coordination complexes which give the
technetium enhanced affinity for particular sites in the human body.
 99mTc decays by gamma emission, with a half-life: 6.01 hours. The
short half-life ensures that the body-concentration of the
radioisotope falls effectively to zero in a few days.
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Not for exam purposes.
• Use of Tc-99m tracer in nuclear medicine
 Functional brain imaging - 99mTc-HMPAO (hexamethylpropylene amine
oxime, exametazime
 Cardiac ventriculography - 99mTc, is injected, and the heart is imaged to
evaluate the flow through it, to evaluate coronary artery disease, valvular
heart disease, congenital heart diseases, cardiomyopathy, and other cardiac
disorders
 Sentinel-node identification - to identify the predominant lymph nodes
draining a cancer, such as breast cancer or malignant melanoma
 Blood pool labeling - 99mTc is combined with a tin (Sn) compound, it binds
to red blood cells and can therefore be used to map circulatory system
disorders. It is commonly used to detect gastrointestinal bleeding sites.
 Pyrophosphate for heart damage - A pyrophosphate ion with 99mTc
adheres to calcium deposits in damaged heart muscle, making it useful to
gauge damage after a heart attack.
 Sulfure colloid for spleen scan; Bone scan; Myocardial perfusion
imaging - used for the diagnosis of ischemic heart disease, MPI is one of
several types of cardiac stress test.
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• http://www.necsa.co.za/Necsa/
SAFARI-1 you tube video
Not for exam
purposes.
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Not for exam
purposes.
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Radioactive Decay and Activity
 radionuclides are unstable & decay by particle emission, EC
or -ray emission;
 radioactive decay (RD) characterized by disappearance
of a constant fraction of the activity present in the sample
during a given time interval;
 every radioisotope has a unique rate of nuclear decay;
 decay of radionuclides is a random process  which atom
from a group of atoms will decay at a specific time;
 average of radionuclides disintegrating during a period of
time  disintegration rate of that particular radionuclide;
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Radioactive Decay and Activity
The question of which radioactive nucleus in a group of nuclei
disintegrates at a given instant is decided like the winning
numbers in a state lottery: individual disintegration occur
randomly.
 Naturally-Occurring-Radioactive-Materials (NORM) results
from activities such as burning coal, making and using
fertilizers, oil and gas production.
Radon in homes is one occurrence of NORM which may need
to be controlled, by ventilation.
http://world-nuclear.org/info/Safety-and-Security
• /Radiation-and-Health/Naturally-Occurring-Radioactive• Materials-NORM/#.Uhx50r78IeE
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Radioactive Decay and Activity
 A banana contains naturally occurring radioactive potassium.
 The major natural source of radioactivity in plant tissue is potassium.
 The activity of natural potassium is about 31 Bq/g – meaning that, in
one gram of the element, about 31 atoms will decay per second.
 Plants naturally contain other radioactive isotopes, such as carbon-14
(14C), but their contribution to the total activity is much smaller,
 Since a typical banana contains about half a gram of potassium, it
will have an activity of roughly 15 Bq.
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Radioactive Decay and Activity
As time passes, the number N of parent nuclei decreases, as Fig
31.14 shows:
To help describe the graph, it is useful to define the half-life
(𝑇1/2 ) of a radioactive isotope as the time required for one-half
of the nuclei present to disintegrate. Example: check pg 971,
table 31.2
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Radioactive Decay and Activity
The half-life of a radioactive
decay is the time in which ½
of the radioactive nuclei
disintegrates/decays.
N  Noe
T1 2 
Modern Physics By RW Letsoalo 2015
 t
ln 2

91
Radioactive Decay and Activity
 The activity of a radioactive sample is the number of
disintegrations/decay per second.
 Each time a disintegration occurs, the number N of radioactive
nuclei decreases.
 As a result, the activity can be obtained by dividing ∆N, the
change in the number of nuclei, by ∆t, the time interval during
which the change takes place.
 The average activity over the time interval ∆t is the magnitude of
N
N
A
or
t
t
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Radioactive Decay and Activity
 Since the decay of any individual nucleus is completely
random, the number of disintegrations per second that occurs
in a sample is proportional to the number of radioactive nuclei
present so that N   N
t
 Where λ is a decay constant: probability of disintegration per
unit time.
 Although radioactive decay involves discrete events of nuclear
disintegration, the number of events is so large that it can be
treated like a continuum and the methods of calculus employed
to predict the behavior.
 The result from the decay probability can be put in the
differential form: dN
dt
  N
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Radioactive Decay equations
 d N 
 N
dt
of the original amount
Rearranging and the integrating,
 d N 
 dt
N 
t
d N 

  dt
N 0 N 
0
Nt
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Formula A
N
 ln
  t
N0
N
 t

e
N0
 t
 N  N0e
Formula B
Where:
N/A = Current amount of radioactivity
𝑵𝒐 /A0= Original amount of radioactivity
e = base natural log (appr. 2.718)
λ= the decay constant = 0.693/t1/2 (where
t1/2= half-life)
t = the amount of time elapsed from A0 to
A/ 𝑵𝒐 to N
The above formulas gives the number N of radioactive nuclides and
A the activity present at time t.
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Half-Life
• half-life of a radionuclide: “the time required for the
radionuclide to decay to 50% of its initial activity”.
T1 2 
ln 2

• λ= decay constant
• mean life: av. life of a group of radionuclides, denoted by .
 
1

• in one mean life, activity of a radionuclide is reduced to 37%
of its initial value.
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Half-Life
1 N N e
0
2 0
 T 1
2
1  e  T 12
2
ln  1 2   T12
ln(1)  ln(2)  T12
 ln 2
 T1 2

T1 2 
ln 2

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Half-Life
• Example 8: the radioactive decay of Radon gas Pg 967
• Example 9: the activity of Radon Pg 968.
• Example 10: the activity per gram Pg 970.
• Example 11: the ice man Pg 970 – 972.
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Modern Physics By RW Letsoalo 2015
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Modern Physics By RW Letsoalo 2015
100
Half-Life
Radioisotope
Half-life
Radioisotope
Half-life
calcium-47
4.5 days
krypton-79
1.5 days
carbon-14
5730 years
mercury-197
2.7 days
chromium-51
27.8 days
molybdenum-99
2.78 days
cobalt-57
270 days
phosphorus-32
14.3 days
gold-198
2.7 days
sodium-24
15.0 hours
iodine-125
60 days
strontium-87
2.8 hours
iodine-131
8.1 days
technetium-99m
6.02 hours
iron-59
45.1 days
yttrium-90
64.2 hours
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Half-Life
Calculation: At 11:00 a.m., the Tc-99m radioactivity was
measured as 9mCi on a certain day. What was the activity at
8:00 a.m. and 4:00 p.m. on the same day? HL (Tc-99m) = 6 hr.
ANSWER:
The time from 8:00 a.m. to 11:00 a.m. is 3 hours.
At  9mCi(11a.m.)
A0  ?(initially )
At  A0 e  t
  0.1155hr 1
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102
Half-Life
9  A0 e
0.11553
 A0  9  e 0.3465
 A0  12.7269 mCi
• The time from 11:00 a.m. to 4:00 p.m. is 5 hours.
A0  9mCi(initially )
At  ?
At  A0 e
 t
At  9  e
0.11555
 At  9  e
 0.5775
 At  5.05mCi
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103
If the radioactivity of Hg-197 (HL= 65hr) is 100mCi on
Wednesday noon, what is its activity at 8 a.m. the Tuesday
before and at noon the Friday after?
• ANSWER: Hg-197 with T1  65hrs
2
T1 2 
ln 2

ln 2
T1 2
ln 2

65hr
 
  0.0107hr 1
Modern Physics By RW Letsoalo 2015
104
Wednesday 12:00 p.m.
day, t = 28 hours
Tuesday 8:00 a.m. previous
At  A0 e
 t
100  A0  e
 0.010728
 A0  100  e
0.2996
 A0  135mCi
Modern Physics By RW Letsoalo 2015
105
Wednesday 12:00 p.m.
Friday 12:00 a.m.
t = 48 hours
At  A0 e
 t
At  A0  e
 0.010748
 At  100  e
 0.5136
 At  60mCi
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Radioactive Dating
• One important application of radioactivity is the determination
of the age of archeological (the scientific study of material
remains (as fossil relics, artifacts, and monuments) of past
human life and activities.)or geological samples.
• If an object contains radioactive nuclei when it is formed,
then the decay of these nuclei marks the passage of the time like
a clock, half of the nuclei disintegrating during each half-life.
• If the half-life is known, a measurement of the number of
nuclei present today relative to the number present initially
can give the age of the sample.
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Biological Effects of Ionizing Radiation
 The human body is made up of many organs, and each organ
of the body is made up of specialized cells.
 Ionizing radiation can potentially affect the normal operation
of these cells.
 EFFECTS OF RADIATION ON CELLS
 Biological effect begins with the ionization of atoms.
 The mechanism by which radiation causes damage to human
tissue, or any other material, is by ionization of atoms in the
material.
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Biological Effects of Ionizing Radiation
 We consider the chromosomes to be the most critical part of the
cell since they contain the genetic information and instructions
required for the cell to perform its function
 And to make copies of itself for reproduction purposes.
 Also, there are very effective repair mechanisms at work
constantly which repair cellular damage - including
chromosome damage.
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Biological Effects of Ionizing Radiation
 Ionizing radiation absorbed by human tissue has enough energy
to remove electrons from the atoms that make up molecules of
the tissue.
 When the electron that was shared by the two atoms to form a
molecular bond is dislodged by ionizing radiation, the bond is
broken and thus, the molecule falls apart.
 This is a basic model for understanding radiation damage.
 When ionizing radiation interacts with cells, it may or may not
strike a critical part of the cell.
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110
Biological Effects of Ionizing Radiation
• The following are possible effects of radiation on cells:
1) Cells are undamaged by the dose
 Ionization may form chemically active substances which in
some cases alter the structure of the cells.
 These alterations may be the same as those changes that occur
naturally in the cell and may have no negative effect.
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Biological Effects of Ionizing Radiation
2) Cells are damaged, repair the damage and operate normally
 Some ionizing events produce substances not normally found in
the cell.
 These can lead to a breakdown of the cell structure and its
components.
 Cells can repair the damage if it is limited. Even damage to the
chromosomes is usually repaired.
 Many thousands of chromosome aberrations (changes) occur
constantly in our bodies.
 We have effective mechanisms to repair these changes.
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Biological Effects of Ionizing Radiation
3) Cells are damaged, repair the damage and operate
abnormally
 If a damaged cell needs to perform a function before it has
had time to repair itself, it will either be unable to perform the
repair function or perform the function incorrectly or
incompletely.
 The result may be the cells that cannot perform their normal
functions or that now are damaging to other cells.
 These altered cells may be unable to reproduce themselves or
may reproduce at an uncontrolled rate.
 Such cells can be the underlying causes of cancers.
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Biological Effects of Ionizing Radiation
4) Cells die as a result of the damage
 If a cell is extensively damaged by radiation, or damaged in
such a way that reproduction is affected, the cell may die.
 Radiation damage to cells may depend on how sensitive the
cells are to radiation.
 All cells are not equally sensitive to radiation damage.
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Biological Effects of Ionizing Radiation
 working with radioactive materials necessary to develop
specific units  express what needs to be known about
radioactive material involved.
 3 fundamental concepts NB when discussing radiation
and its effects on physical objects:
actual radioactivity involved;
amount of E the radiation imparts on other objects;
question of biological effects on humans
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Radiation Units
• 3 NB units:
Becquerel (Bq) (French nuclear physicist Atoine Henri Becquerel)
gray (Gy) (British radiobiologist Louis Harold Gray)
sievert (Sv) (Swedish radiologist Rolf Maximilian Sievert)
 detection of radiation is very important in medical work,
w.r.t. protecting personnel and patients from exposure to
radiation & for diagnostic and therapeutic purposes.
 radiation levels should be kept as minimal as possible.
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Biological Effects of Ionizing Radiation
 becquerel (Bq): # of nuclear disintegrations per second from
a radioactive source.
 base unit: 1/s
 does not measure E or differentiate betw. ionizing and nonionizing radiation
 how much of any radioactive decay is present, ind. on what
type of material or what type of decay is present
 older non-SI unit : curie (Ci)
1 Ci = 3.7 x 1010 Bq
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Biological Effects of Ionizing Radiation
 measuring how much Energy is actually imparted by the
radiation  good indication of how damage can be inflicted.
 gray (Gy): express the E absorbed from a dose of radiation
or relate the amount of radiation absorbed by tissue
 “the amount of absorbed energy per unit mass of material”.
1 Gy = 1J/kg
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Biological Effects of Ionizing Radiation
 non-SI unit: rad
 1 rad = 0.01 Gy (J/kg)
 non-SI unit  units outside the SI. Certain units are not part of the International System of Units,
that is, they are outside the SI, but are important and widely used .
 gray and rad describe the irradiation of tissue regardless of
whether the tissue is plant or animal.
 damage to tissue by absorbing a given amount of energy
varies according to the type of radiation imparted.
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Biological Effects of Ionizing Radiation
 Ionizing radiation consists of photons and/or moving particles
that have sufficient energy to knock an electron out of an atom or
molecule, thus forming an ion.
 Exposure is a measure of the ionizing radiation produced in air
by X-rays or γ-rays.
 The Roentgen (R) is the special unit of exposure.
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Biological Effects of Ionizing Radiation
 Specifically, the Roentgen is defined as 2.58 x 10-4 Coulombs of charge
produced by X- or gamma rays per kilogram of air.
 Exposure is the sum of the electrical charges on all ions of one
sign produced in air when all electrons liberated by photons in a
volume element of air are completely stopped in air, divided by the
mass of the air in the volume element.
 In passing through the air, the beam produces positive ions whose total
charge is q.
 Exposure is the charge per unit mass of the air.
1

q
Exposure (in roentgens)  
4 
 2.58  10  m
coulombs per ki log rams may also be used .
• Look at example 1 pg 987
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Biological Effects of Ionizing Radiation
 An absorbed dose applies to the energy deposited by any kind of
radiation in any kind of material.
 The special unit of absorbed dose, the rad, is equivalent to the
absorption of 100 ergs of energy per gram of material.
 For biological purposes, the absorbed dose is a more suitable quantity
because it is the energy absorbed from the radiation per unit mass of
the absorbing material:
Energy absorbed
Absorbed dose 
Mass of absorbing material
gray
1 Gy  1J kg
1 rad  0.01 gray
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Biological Effects of Ionizing Radiation
 relative biological effectiveness (RBE) relates radiation
damage to human tissue.
 sievert (Sv): relates specifically to the biological effect of
radiation absorbed by humans.
 attempts to express an equivalence of absorbed dose taking
into account biological harm.
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Biological Effects of Ionizing Radiation
 To compare the damage produced by different types of radiation,
the relative biological effectiveness (RBE) is used.
Dose of 200 - keV X - rays that
RBE 
produces a certain biological effect
Dose of radiation that produces
the same biological effect
Check table 32.1
Look at example 2 pg 989
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Biological Effects of Ionizing Radiation
 The product of the absorbed dose in rads (not in grays) and the
RBE is the biologically equivalent dose:
Bio log ically equivalent dose  Absorbed dose  RBE
(in rems)
(in rads )
 non-SI unit for biologically equivalent dose : rem (roentgen equivalent,
man)
• 1 Sv = 100 rem
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Biological Effects of Ionizing Radiation
 amount of radiation energy by living cells has a potential
harmful effects to irradiated organisms
 effect of radiation on humans varies according to the type of
radiation emitted and tissue irradiated.
 from acute sickness (e.g. vomiting, nausea) and death within
days/weeks to induction of cancer years later
 radiation-induced
hereditary/genetic
biological
effects:
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somatic
and
126
Biological Effects of Ionizing Radiation
SOMATIC EFFECTS:
 Cause damage to ordinary body cells resulting in injuries
which affect only the irradiated organism
 Results in depletion of mitotic, dividing cells or interference
of cell division processes
 There are two categories:
Prompt somatic effects are those that occur soon after an acute dose
(typically 10 rad or greater to the whole body in a short period of time).
Delayed somatic effects are those that may occur years after radiation
doses are received.
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Biological Effects of Ionizing Radiation
• HEREDITARY/ GENETIC EFFECTS:
 Appear in the future generations of the exposed person as a result of
radiation damage to the reproductive cells.
 Genetic effects are abnormalities that may occur in the future generations of
exposed individuals.
 genetic damage results in embryonic or foetal death or child suffers from
severe abnormalities
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Biological Effects of Ionizing Radiation
Time
Symptoms
0-48 hours
loss of appetite, nausea, vomiting,
fatigue
2 days – 3 weeks
recovery from these symptoms
patient appears quite well
3 weeks - 8weeks
haemorrhages, diarrhoea, loss of
hair, fever and death may occur
6 weeks – several months
surviving patients show general
improvement and severe
symptoms disappear
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Biological Effects of Ionizing Radiation
Effects
Latent (Hidden) Period
Cataract Formation (eye)
5-10 years
Leukaemia
8-10 years
Lung Tumours
10-20 years
Bone Tumours
15 years
Thyroid Tumours
15-30 years
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Biological Effects of Ionizing Radiation
• Radiation effects can be categorized by when they appear.
1. Deterministic effects: (Prompt) effects, including radiation
sickness and radiation burns, seen immediately after large
doses of radiation delivered over short periods of time.
2. Stochastic effects: (Delayed) effects such as cataract
formation and cancer induction that may appear months or
years after a radiation exposure
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Biological Effects of Ionizing Radiation
The following table provides an estimate of life expectancy lost due to several causes:
Health Risk
Estimated Life
Expectancy Lost
Smoking 20 cigarettes a 6 years
day
Overweight by 15%
2 years
Alcohol (US average)
1 year
All accidents
207 days
All natural hazards
7 days
Occupational dose of
300 mrem/year
15 days
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Not for exam
purposes.
132
Biological Effects of Ionizing Radiation
• α – particles:
 relative large size and mass restrict ability to penetrate matter.

cannot penetrate the dead outer layer of skin  unable to irradiate vital organs.
 inhalation/ ingestion can lead to serious tissue damage at the site of radioactive
irradiation.
• β-particles:
 smaller size enables them to penetrate a few mm of skin which appears similar to a
burn.
 cause tissue damage inside the body when inhaled or ingested.
• γ-rays:
 have no mass and travel at the speed of light
 exposure to γ-rays is far more harmful to the body, since they can penetrate the entire
body.
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Biological Effects of Ionizing Radiation
 most sensitive tissues: bone marrow, gonads (organs that
produces testes/ovaries), lymph tissue & lens of the eye.
 rapidly dividing cells (foetus) very susceptible to radiation
 radiation exposure during pregnancy should be avoided.
 low doses of radiation alter structure of chemicals involved
in cell division.
 alteration of hereditary info. in sex cells can result in future
generations with mutations.
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Biological Effects of Ionizing Radiation
 damage to somatic cells can result in regions of abnormal
cells which may lead to cancer.
 with incr. doses of radiation, cell death occurs.
 further incr. in the amount of radiation absorbed results in
rapid death.
 radiation damage to body tissue is similar regardless of
whether the tissue has been exposed to one large radiation
dose or a series of smaller doses which equal the large dose
 effects of radiation exposure are cumulative.
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Biological Effects of Ionizing Radiation
 radiation workers: badges to record the accumulated amount
of radiation  checked @ regular intervals to determine if the
quantity of accumulated radiation is above the recommended
safety levels.
Type of Exposure
Maximum Permissible Dose(mSv)
Radiation workers: whole body,
gonads or lenses of the eyes
50
Skin of the whole body
Hands and feet
General Population: whole body
Gonads
300
750
5
1.7
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Radiation badge/ Thermoluminescent (TLD )
badge
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Uses of Radiation in Medicine
 medical uses include diagnosis of disease, therapy and
research.
• DIAGNOSTIC USE
 radioisotopes employed for diagnostic purposes are selected based on
the following criteria:
- ability to emit γ-radiation;
- short half-life;
- ability to be eliminated from the body shortly after completion of
diagnostic test;
- smallest possible detectable dose.
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Uses of Radiation in Medicine
 routine X-rays to complex CT scans employed in Radiology,
 and injections of radioactive material for nuclear imaging.
 radiation treatment and diagnosis should weigh the risk of
the radiation with the benefit of the treatment.
• THERAPEUTIC USE
 main objective: selective destruction of abnormal cells (= uncontrolled rapidly
dividing cancer cells) which requires highly-localized intense dose of radiation.

failure to localize the radiation will result in damage of normal surrounding cells.
 specific radioisotopes selectively taken up by particular tumours can be administered
orally.
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Uses of Radiation in Medicine
 accumulation of radioisotopes by the much localized tumour
cells results in a radioisotope conc. sufficient to destroy
these cells.
 for example: iodine-131 is selectively concentrated by the
thyroid gland
 the administration of iodine-131 orally at a higher level than
that used for diagnosis, results in the tumour being irradiated
with a dose that causes death of tumour cells.
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Uses of Radiation in Medicine
 Radiation may also be used in radiation therapy - uses high-energy radiation
to shrink tumors and kill cancer cells (1). X-rays, gamma rays, and charged
particles are types of radiation used for cancer treatment.
 The radiation may be delivered by a machine outside the body (external-beam
radiation therapy (EBRT)), e.g. Intensity-modulated radiation therapy
(IMRT), Image-guided radiation therapy (IGRT), Stereotactic radiosurgery,
Proton therapy,
 Or it may come from radioactive material placed in the body near cancer cells
(internal radiation therapy, also called Brachytherapy).
 Radiation therapy is sometimes given with curative intent.
 Radiation therapy may also be given with palliative intent.
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Uses of Radiation in Medicine
Not for exam
purposes.
 Brachytherapy:
 radioisotopes can also be implanted in a sealed metal
container beside the tumour.
 cobalt-60 and radium-226 are encased in metal cases made
of aluminium, gold, nickel or platinum in shapes of needles,
beads or wires.
 γ-radiation passes through the tissue and irradiates the
nearby tumour cells.
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Uses of Radiation in Medicine
Not for exam
purposes.
RADIOISOTOPE
SYMBOL
USE
chromium-51
51Cr
red blood cell analysis
iodine-131
131I
thyroid function
technetium-99m
99mTc
iron-59
59Fe
evaluation of body – iron
concentration
phosphorus-32
32P
detection of cancer cells
scans of the brain, lung perfusion,
bone scans, renal function
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Uses of Radiation in Medicine
Not for exam
purposes.
 the long half-lives of cobalt-60 and radium-226 require these
radioisotopes to be removed when the tumour has been
destroyed to prevent unnecessary tissue damage.
 Y-90 has a shorter half-life (64.2 hours) than cobalt-60 and
radium-226 and is a beta emitter.
 the low penetration of beta rays results in only the immediate
area of the implant being irradiated and yttrium can be
implanted directly into the tumour.
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Uses of Radiation in Medicine
Not for exam
purposes.
 radioisotopes can be applied from a fine beam that emanates
from a radioactive source and is usually either one of the gamma
emitters, cobalt-60 or cesium-137.
 these radiation sources emit gamma rays that are directed as a
fine beam to the cancer site, destroying the cancer cells.
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Uses of Radiation in Medicine
• Systemic radiation therapy/ Chemotherapy
 In systemic radiation therapy, a patient swallows or receives an
injection of a radioactive substance, such as radioactive iodine or a
radioactive substance bound to a monoclonal antibody.
 Chemotherapy is the use of medication (chemicals) to treat disease.
 Chemotherapy (chemo) drugs interfere with a cancer cell's ability to
divide and reproduce.
 Chemo drugs may be applied into the bloodstream to attack cancer
cells throughout the body, or they can be delivered directly to
specific cancer sites.
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Radiation detection
 Radiation-detecting instruments used to determine presence, type,
intensity and energy of radiations emitted by radionuclides.
 commonly used devices: gas-filled detectors, scintillation detectors &
semiconductor detectors
• Gas-filled detectors consist of a volume of gas between two
electrodes
 operation of gas-filled detector based on ionization of gas molecules
by radiations  collection of ion pairs as current with application of
Voltage between 2 electrodes
 measured current proportional to applied voltage and amount of
radiations
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Not for exam
purposes.
IONIZATION CHAMBER:
 detects ionizing radiation by measuring electric current generated
when radiation ionizes the gas in the chamber  electrically
conductive
 max. efficiency of operation, V betw. electrodes sufficient to
ensure complete collection of ions & electrons.
 V too low  ions and electrons recombine w/o contributing to
electrical current flow
E.g. ionization chambers, proportional counters & Geiger-Müller
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gas-filled detectors i.e. ionization chamber
Not for exam
purposes.
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 in scintillation detectors, the interaction of ionizing
radiation produces UV and/or visible light. E.g.
Photomultiplier tube (PMT).
 PMTs perform 2 functions:
Not for exam
purposes.
 conversion of ultraviolet and visible light photons into an electrical signal
 signal amplification, on the order of millions to billions
 consists of an evacuated glass tube containing a
photocathode, typically 10 to 12 electrodes called dynodes,
and an anode
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150
Not for exam
purposes.
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151
Not for exam
purposes.
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Radiation detection
Not for exam
purposes.
 semiconductor detectors: pure crystals of Si, Ge or other materials to
which trace amounts of impurity atoms have been added so that they act as
diodes.
 E.g. Germanium detectors are mostly used for spectroscopy (analysis of
white light by dispersing it with a prism) in nuclear physics.
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Radiation detection
• Detectors may also be classified by the type of information
produced:
 detectors, such as Geiger-Mueller (GM) detectors, that
indicate the number of interactions occurring in the detector
are commonly called counters.
 detectors that yield information about the energy distribution
of the incident radiation, such as NaI scintillation detectors,
are called spectrometers.
 detectors that indicate the net amount of energy deposited in
the detector by multiple interactions are called dosimeters.
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Radiation detection
survey meters: battery operated & portable radiation
detection and measurement instr.
 to check personnel, equipment and facilities for radiation
contamination.
 GEIGER-MULLER COUNTERS: used to detect ionizing
radiation (usually β and γ particles).
• consists of a glass tube with a wire electrode (cathode)
surrounded by a cage-like anode.
• an inert gas-filled tube (helium, neon or argon) conducts
electricity when β rays or electrons ionize the gas causing a
current to flow between the electrodes
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Geiger-Mueller (GM) detectors
Not for exam
purposes.
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Geiger-Mueller (GM) detectors
Not for exam
purposes.
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157
Geiger-Mueller (GM) detectors
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Not for exam
purposes.
158
Radiation detection
Not for exam
purposes.
• The current in all these survey meters is amplified and fed
into a loudspeaker and a rapid series of clicks can be heard
and the rate of clicking per minute counted.
• as the radiation increases, the signal becomes a rapid
crackle, becoming a loud buzz and finally, a high pitched
whistle at high radiation concentrations.
• meter dial records the radiation in rads.
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 dose calibrators: assay activity levels in syringes, vials
containing materials to be administered to patients.
cyl. shaped, sealed chamber with a central well & filled with
argon and traces of halogen @ high Pressure.
calibration factor set for radionuclide  syringe/vial placed
inside chamber & reading displayed on the digital meter.
 dose calibrators consist of ion chambers and electrometers
 small amounts of electrical currents measured using sensitive
devices (electrometers)
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Dose calibrator
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161
Radiation detection
 X-ray and radiotherapy workers wear a small photographic
film badge (dosimeter used for monitoring cumulative
exposure to ionizing radiation).
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Radiation detection
• worn on lapels or breast pockets or around waist  film
sensitive to electrons or beta and X-rays.
• daily chemical development of the film and subsequent
examination will give an indication of whether the person
has been exposed to excessive radiation.
• person injected or swallowed radioactive material will emit
beta rays that will in turn affect a photographic film badge
placed near the body.
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Safety measures against radiation
• Penetrating Properties of
Radiation
 Radiations from
radioactive materials can
be dangerous and pose
health hazards.
 By knowing the ability of
the different types of
radiation to penetrate
matter allows us to gain
an understanding on how
best to protect ourselves.
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164
Safety measures against radiation
• Penetration of Alpha Particles
• Alpha particles can be absorbed by a thin sheet of paper or by a few
centimetres of air. As alpha particles travel through air they collide with
nitrogen and oxygen molecules. With each collision they lose some of their
energy in ionizing the air molecule until eventually they give up all of their
energy and are absorbed. In a sheet of paper the molecules are much close
together so the penetration of alpha particles is much less than in air.
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Safety measures against radiation
• Penetration of Beta Particles
• Beta particles travel faster than alpha particles and carry less charge
(one electron compared to the 2 protons of an alpha particle) and so
interact less readily with the atoms and molecules of the material
through which they pass. Beta particles can be stopped by a few
millimeters of aluminium.
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Safety measures against radiation
• Penetration of Gamma rays
• Gamma rays are the most penetrating of the radiations. Gamma rays
are highly energetic waves and are poor at ionizing other atoms or
molecules. It cannot be said that a particular thickness of a material
can absorb all gamma radiation. Many centimetres of lead or many
meters of concrete are required to absorb high levels of gamma rays.
Modern Physics By RW Letsoalo 2015
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Safety measures against radiation
•
•
•
•
•
•
•
increase distance from the radiation source as much as
possible;
decrease time spent near the source to a minimum;
place shielding between you and the source;
wear protective clothing when working with radiation; e.g.
lead apron, lead gloves, lead goggles, thyroid collar etc.
special lead gloves must be used when handling radioactive
material to prevent contamination
use a film badge or other monitoring device to record the
radiation dose;
radioactive material must be collected in special lead
containers for disposal;
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• Test (17 September @ 16:00)
• Venue: Sports Complex
1. Equations and constants will be provided.
2. Remember to write the SI units.
3. Treat all the tutorials as well as the theory from
the lecture notes provided as well as the ones from
the prescribed text book..
• Thank you for your co-operation and Good – Luck in
the coming examinations.
Modern Physics By RW Letsoalo 2015
170
REFERENCES:
Introduction to physics. 8th Ed. Cutnell JD and Johnson KW
Physics. 3rd Ed. Kane JW and Sternheim MM.
http://www.eskom.co.za/nuclear_energy/fuel/fuel.html
http://regentsprep.org/Regents/physics/phys05/catomodel/cloud.htm
http://education.jlab.org/glossary/isotope.html
http://www.bmb.psu.edu/courses/bisci004a/chem/atoms.jpg
http://www.bmb.psu.edu/courses/bisci004a/chem/basechem.htm
http://ocw.mit.edu/courses/nuclear-engineering/22-01-introductionto-ionizing-radiation-fall-2006/lecture-notes/lecture2the_nucl.pdf
9. http://www.anl.gov/images/ARRA_AGHCF-200.JPG
10."The Technetium-99m Generator". Bnl.gov.
11.The use of technetium 99m as a clinical tracer element. Herbert, R.;
Kulke, W; Shepherd, R.T. (1965). Postgraduate Medical Journal 41
(481): 656–62. PMC 2483197. PMID 5840856.
1.
2.
3.
4.
5.
6.
7.
8.
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12.Technetium-99 in generator systems. Moore, P.W. (April 1984). Journal of
nuclear medicine : official publication, Society of Nuclear Medicine 25 (4):
499–502. PMID 6100549. Retrieved 27 August 2013.
13.A review of 99mTc generator technology. Molinski, Victor J. (1 October
1982). The International Journal of Applied Radiation and Isotopes 33
(10): 811–819. doi:10.1016/0020-708X(82)90122-3.
14.The Encyclopedia of the Chemical Elements. Rimshaw, S. J. (1968).
Hampel, Cifford A., ed. New York: Reinhold Book Corporation.
15.http://www.passmyexams.co.uk/GCSE/physics/penetrating-properties-ofradiation.html#2
16.Quantities and units in clinical chemistry: Nebulizer and flame properties
in flame emission and absorption spectrometry. (Recommendations 1986)".
Herrmann, R.; C. Onkelinx (1986). Pure and Applied Chemistry 58 (12):
1737–1742. doi:10.1351/pac198658121737
17. The physics of Radiation Therapy. Khan FM. (2010). 4th ed.
18.Radioactive and human health Handouts by Mandiwana N (2013)
19.http://www.who.int/ionizing_radiation/about/what_is_ir/en/
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