Radioactivity lectures level 200 radiology st louis unihebs MDIRT Nchanji Nkeh Keneth

RADIOACTIVITY
lecture 1
THE ATOMIC THEORY AND NUCLEAR REACTIONS
Second Semester 2015/2016 Academic Year
Level 200 Radiology
NCHANJI NKEH KENETH
kennchanji@yahoo.com
Radiology Dept.
ST LOUIS UNIHEBS, MILE 3 NKWEN BAMENDA
TH
RADIOACTIVITY. LECTURE NOTES .ST LOUIS UHIHEBS,2015/2016 ACADEMIC YEAR,
LEVEL 200 RADIOLOGY.COMPILED BY Nchanji Nkeh Keneth 1

Life and atoms
Every time you breathe you are
taking in atoms. Oxygen
atoms to be exact. These
atoms react with the blood
and are carried to every cell
in your body for various
reactions you need to
survive. Likewise, every time
you breathe out carbon
dioxide atoms are released.
The cycle here is interesting.
TAKING SOMETHING IN.
ALLOWING SOMETHING
OUT!
RADIOACTIVITY. LECTURE NOTES .ST LOUIS
UHIHEBS,2015/2016 ACADEMIC YEAR,
LEVEL 200 RADIOLOGY.COMPILED BY
2

The atom
As you probably already
know an atom is the
building block of all
matter. It has a nucleus
with protons and
neutrons and an electron
cloud outside of the
nucleus where electrons
are orbiting and MOVING.
Depending on the ELEMENT,
the amount of electrons
differs as well as the
amounts of orbits
surrounding the atom.
3

RELATIONSHIP BETWEEN Z AND A(the below
formula is used to approx the stability of nuclei.
Z=
A
1.98+0.0155A
2/3
4

To help visualize the atom think of it like a ladder. The bottom of the
ladder is called GROUND STATE where all electrons would like to
exist. If energy is ABSORBED it moves to a new rung on the ladder
or ENERGY LEVEL called an EXCITED STATE. This state is AWAY
from the nucleus.
As energy is RELEASED the electron can relax by moving to a new
energy level or rung down the ladder.
5

ENERGY LEVELS
Yet something interesting happens as
the electron travels from energy
level to energy level.
If an electron is EXCITED, that means
energy is ABSORBED and
therefore a PHOTON is absorbed.
If an electron is DE-EXCITED, that
means energy is RELEASED and
therefore a photon is released.
We call these leaps from energy level
to energy level QUANTUM LEAPS.
Since a PHOTON is emitted that
means that it MUST have a
certain wavelength.
6

What energy does the emitted photon
has?
We can calculate the ENERGY of the released or
absorbed photon provided we know the initial
and final state of the electron that jumps
energy levels.
7

Energy level diagrams
• Note: It is very important
to understanding that
these transitions DO NOT
have to occur as a single
jump! It might make TWO
JUMPS to get back to
ground state. If that is the
case, TWO photons will
be emitted, each with a
different wavelength and
energy.
8

RADIOACTIVITY
9

discussion
10

What can you say?
11

Any difference?
12

13

14

RADIOACTIVITY CON’T
• Radioactivity - a natural and spontaneous process by
which the
• unstable atoms of an element emit or radiate excess
energy in the form of particles or waves.
• After emission the remaining daughter atom can either be
a lower energy form of the same element or a completely
different element.
• The emitted particles or waves are called ionising radiation
because they have the ability to remove electrons from the
atoms of any matter they interact with.
• NB) RADIOACTIVITY IS A STATISTICAL PROCESS
15

Properties of radioactive decay
• Statistical process
• Spontaneous emission of particle or
electromagnetic radiation from the atom
• Unaffected by temperature, pressure, physical
state, etc
• Exoergic process
• Conserves total energy, linear and angular
momentum, charge, mass number, lepton
number, etc.
16

Some basic decay modes:
• Alpha decay
• Beta decay
• Gamma decay
• Spontaneous fission
• Delayed neutron and proton emission
• Two-proton decay
• Composite particle emission
• Double beta decay
• Prompt proton decay (new)
to be seen ahead in the course of the lectures
Nchanji Nkeh Keneth
17

Review of atomic theory
• The Bohr Model (1913) – negatively charged
electrons orbiting a positively charged
nucleus. Electrons only in ‘allowable’ orbits.
18
•Only works for hydrogen atom
• electrons are not ‘point like’
particles
• electrons do not ‘orbit’ the
nucleus in a traditional sense
• electrons carry one unit of (-
ve) electrical charge

The Nucleus:
Two particles: protons & neutrons (hadrons)
Proton mass = 1.673 x10-27 kg = 1.00728 amu
Neutron mass = 1.675 x10-27 kg = 1.00866 amu
amu = atomic mass unit, defined relative to carbon 12
19
Charge: protons carry one (+ ve) unit of electrical charge
neutrons are chargeless
Forces: electrical – protons repel each other – infinite range
strong nuclear – short range (~10-15m) attractive force
between quarks – is 137x stronger than electrical forces
the nucleus is held together by a balance of these forces
when the nucleus is in balance it is called stable
the key to the balance is the neutron:proton ratio

• Summary:
• Size of atom 10-10m, size of nucleus 10-15m
• Made up from 3 particles – proton, neutron, electron
• Electrons exist outside of nucleus in discrete allowable orbits
• Electrons can move between orbits by absorbing/emitting energy
• Electrons carry one unit of electrical charge (-ve)
• Protons and neutrons exist within the nucleus
• They have roughly the same mass
• Protons carry one unit of electrical charge (+), neutron has no
charge
• Stable nucleus there is a balance between SNF and electrical force
• When the balance is upset the nucleus is unstable
20

definitions
• Atoms with the same number of protons/electrons have
the same physical and chemical properties, these are
called elements e.g. all oxygen atoms have 8 protons.
• Elements are arranged in order of increasing proton
number and are characterised with the symbol
- Periodic Table
• Elements can have different numbers of neutrons and
these are called isotopes
• Isotopes can be stable or unstable
21
XA
Z

• Isotones: these are atoms that have the same
number of neutrons such as (V 51,23; Cr 52,
24)
• Isobars: these are atoms of different elements
with the same mass number. Ex A40,18; K
40,19, etc
• for all natural nuclides, A ranges from 1 to
238.
• A group of atoms having the same
number of protons/ electrons and
same chemical and physical properties
is called an element
22

Isotope - atoms of the same element with different numbers of
neutrons.
Isotopes of Hydrogen
Hydrogen - 1 proton + 1 electron - stable
Deuterium - 1 proton + 1 neutron + 1 electron - stable
Tritium - 1 proton + 2 neutrons + 1 electron - unstable
Stability - related to n:p ratio
low atomic number - n:p ~ 1:1
high atomic number - n:p rises to ~ 1.6:1
Stability regained by radioactive decay processes
23

Th The concept of isotopy
An isotope is when you have
the SAME ELEMENT, yet
it has a different MASS.
This is a result of have
extra neutrons. Since
Carbon is always going to
be element #6, we can
write Carbon in terms of
its mass instead.
Carbon - 12
Carbon - 14
24

Mass energy relationship
• In 1905, Albert Einstein published a 2nd major
theory called the Energy-Mass Equivalence in
a paper called, “Does the inertia of a body
depend on its energy content?”
25

• Looking closely at Einstein’s equation we see
that he postulated that mass held an
enormous amount of energy within itself. We
call this energy BINDING ENERGY or Rest mass
energy as it is the energy that holds the atom
together when it is at rest. The large amount
of energy comes from the fact that the speed
of light is squared.
26

Radioactive decay
When an unstable nucleus releases energy
and/or particles.
27

Basic types of decay processes
There are 4 basic types of
radioactive decay
• Alpha – Ejected Helium
• Beta – Ejected Electron
• Positron – Ejected Anti-
Beta particle
• Gamma – Ejected Energy
You may encounter protons
and neutrons being
emitted as well
28
n
p
e
e
He
1
0
1
1
0
0
0
1
0
1
4
2



1) BETA( β- DECAY)
• EXAMPLE:
29
max32.140
20
40
19

 MeVCaK
  
 YX A
Z
A
Z 1

Beta minus decay continued
• A beta particle is a fast moving electron which
is emitted from the nucleus of an atom
undergoing radioactive decay.
30
Beta decay occurs when a neutron
changes into a proton and an electron.

As a result of beta decay, the nucleus has one less neutron, but
one extra proton.
• The atomic number, Z, increases by 1 and the
mass number, A, stays the same.
31
Po
218
84

0
-1
At
218
85

32

33

• What of molybdenum 99
decaying to yield technetium
99 metastable
34

2) BETA (PLUS) DECAY
35
  
MeVFNe 22.219
9
19
10
+ annihilation radiation
  
 YX A
Z
A
Z 1
annihilation radiation = mec2 = 0.511 MeV (x2)
Daughter
Nucleus
Fluorine - 19

36

Other examples of beta decay
37
Other examples of beta decay
14C6  14N7 + – + n (t½, 5720 y)
40K19  40Ca20 + – + n (1.27e9 y)
50V23  50Cr24 + – + n (6e15 y)
87Rb37  87Sr38 + – + n (5.7e10 y)
115In49  115Sn50 + – + n (5e14 y)

38

3) ELECTRON CAPTURE
• Electron capture:
• Excess of protons, stability reached by different process than +
• Orbital electron is captured by the nucleus, neutrino emitted.
• Commonly nucleus is left in an ‘excited’ state and returns to its
• ground state by emitting a gamma-ray photon from the nucleus
• In all cases a characteristic X-ray photon is emitted by the
atom.
39
)(1 possiblyraysraysXYeX A
Z
A
Z   
raysMeVMevraysXTelluriumTeeI  035.0)027.0(125
52
125
53

4) GAMMA DECAY
40
60Co27
60Ni28 + 0.318 MeV - +
1.17 MeV  + 1.33 MeV 
Nucleons have quantised energy levels - emitted -ray photons
from a particular nucleus have a unique -ray spectrum.
-ray spectrum can be used to identify unknown isotopes and
calibrate instruments.

41

5) ALPHA DECAY
• An alpha particle is a helium nucleus.
• It has a mass of 4 and a charge of +2.
• It is very heavy and moves slowly causing very
thick and dense ionisation tracks in the
medium it traverses.
• Due to its large mass, alpha particles unlike
beta particles do not travel long distances.
They can easily be stopped by few millimeters
of air or tissue paper
42

ALPHA DECAY cont
• They are deflected in both magnetic and
electric fields to a lesser extent compared to
the beta particles due to their heavy charge.
• Alpha particles are very effective for the
superficial treatment of tumors but can be
very dangerous if the source emitting these
particles is ingested
• This is due to the high LET of these particles
43

What happens to the parent atom
during alpha decay?
• The mass number reduces by 4 while the
proton( atomic number reduces by 2)
• The general decay equation is as given below:
44

45

46
• Decay chain:
• Generally, unstable heavy elements require a series of alpha and
• beta decays until a lighter more stable element is reached

47
HeUPu 4
2
236
92
240
94 

Alpha decay applications
Americium-241, an
alpha-emitter, is used
in smoke detectors.
The alpha particles
ionize air between a
small gap. A small
current is passed
through that ionized
air. Smoke particles
from fire that enter
the air gap reduce the
current flow, sounding
the alarm.
48
?4
2
241
95
A
ZHeAm 

• The daughter nuclide formed is Neptunium,
with symbol as Np. It has a Z of 93 and A of
237.
• Am stands for Americium. These are all
transition metals
49

Comparison of the penetrability of
matter by the various particles and
photons emitted during nuclear
decays
t
50

Any difference?
51

Influence of E and M fields on alpha,
beta particles and y rays
52

• Radiation passing through a magnetic field
shows that massive, positively charged alpha
particles are deflected one way, and less
massive beta particles with their negative
charge are greatly deflected in the opposite
53

The radioactive decay laws and half
life estimation.
• Quantifying the number of atoms remaining after
decay can be pretty challenging; it is rather easier to
measure the effect of the nuclear disintegration such
as counting the number of gamma rays emitted.
• The decay laws are those which govern radioactive
decay and they are all exponential.
• They can be used to calculate the number of
radioactive atoms present after a time interval reason
why radioactivity is a statistical process
• They also enable the calculation of half life of the
radionuclide
54

Exponential laws
• Exponential laws enable us to determine the
rate of change of a quantity with another.
For example, a quantity A varies exponentially
with B if any change in quantity B produces the
same fractional or % changes in A.
i.e, A’=kA….
Exponential laws have wide applications in
medicine, banking and finance , etc
55

Radioactive decay and the ISL
• When radionuclides change from one form to
another, they are said to decay.
• Such decay processes lead to the emission of
particles such as alpha from the nucleus and
also energetic photons such as gamma rays.
• These and other decay modes are valid in
some cases with the exponential law
56

Definition of the Exponential law
• It states that the rate of decay of a particular
nuclide is directly proportional to the number
of such nuclides left in the sample.
That is, if there are N atoms in a sample, the
rate of decay of these atoms (dN/dt) is
proportional to N.
57

Mathematical illustration
-dN/dt = λN0
58
dN= -N dt .
-λt
0N=-N e .Gotten after integration
Where -dN stand for
the number of nuclides
present after time t.
λ stands for the decay
constant
No, stands for the
initial number of atoms

The above decay process can be
graphically represented as:
59

60
N
t

Measurement of radioactivity
• This is done by calculating the activity of the
radionuclide.
• ACTIVITY refers to the number of
disintegrations per second. Measured in
Becquerels where in
• 1Bq=1nuclear transformation/1 second.
• It means that 1Bq=s-1
61

• 1 Bq = 1 disintegration per second
• this is a small unit, activity more usually
easured in:
• kilobecquerel (kBq) = 103 Bq
• Megabecquerel (MBq) = 106 Bq
• Gigabecquerel (GBq) = 109 Bq
• Terabecquerel (TBq) = 1012Bq
62

Units of Activity continued
• Units:
• Old units still in use:
• Curie (Ci) = 3.7 x 1010 disintegration per second
• therefore:
• 1 Ci = 3.7 x 1010 Bq = 37 GBq
• 1 mCi = 3.7 x 107 Bq = 37 MBq
• 1 Ci = 3.7 x 104 Bq = 37 kBq
• 1 MBq ~ 27 Ci
63

When the activity is plotted against
time, similar graphs as that of the
decay atoms above is obtained
Such plots can demonstrate slow and
fast activities
64

Mathematical illustration of Activity
• If N(t) is the number of atoms present at
a time t, then the activity R is
65
dN
R = - .
dt
dN/dt is negative, so the activity is a positive
quantity.
NB: A=A0e also denotes activity.
-λt

Half life of a radionuclide.
• Several types of half lives exist and depends
on the context.
• There are basically three types of half life
which are:
1. Physical half life
2. Biological half life
3. Effective half life
66

Definition
• HALF LIFE of a radioactive element refers to
the time taken for its activity to reduce to
half(1/2) its initial value.
• In this case, the half life obtained is that of the
radionuclide measured in the lab; otherwise
known as the physical half life.
67

Biological half life
• This refers to the time taken for the
concentration of the radioactive element to
reduce to half its initial concentration in the
organ of a living organism.
• This type of half life is affected by excretory
processes of the body such as respiration,
perspiration, urination etc.
68

Effective half life
• This refers to the time taken for the activity of
a radionuclide in an organ to reduce to half its
initial value.
NB: it should be noted that the same radioactive
atom can have three different half lives.
A plot of the graph of the activity or the number
of atoms of the radionuclide with time can be
used to determine the half life.
69

70
original activity
1/2-λΤ0
0
R
= -R e
2
1/2-λΤ1
= e
2
1/2+λΤ
e =2
 1/2Τ =ln 2
 
1/2 1/2
ln 2 0.693
= =
Τ Τ

activity after
T½

71

72

Relationship between physical,
effective and biological half lives
1/Te = 1/Tp + 1/Tb
73

Examples 1
•
74

Example 2
75

Some common laboratory isotopes
• 3H: 1/2 = 12.3 yrs, - emitter (19 keV, ‘soft’)
• Cannot be detected using Geiger counter
– Bremsstrahlung radiation may be significant
– Shielding < 0.1 mm plastic
76
14C: 1/2 = 5730 yrs, - emitter (157 keV, ‘soft’)
Can be detected using Geiger counter
Bremsstrahlung radiation may be significant
Shielding ~ 3 mm plastic

Lab isotopes continued
• 32P: 1/2 = 14.3 days, - emitter (1.71 MeV,
‘hard’)
• Can be detected using Geiger counter
• Shielding ~ 6.3 mm plastic
77
125I: 1/2 = 60 days, X-ray emitter
Can be detected using a portable
scintillation counter
Shielding ~ 1 mm lead

• Summary:
• Unstable atoms (excess p or n) can regain stability by emitting
• radiation
• Two types – particle and electromagnetic
• Particle: β minus – electrons (-1 charge)
• β plus – positrons (+1 charge)
• α – helium nuclei (+2 charge)
• neutrons (chargeless)
• EM: γ – ray – originate from inside nucleus
• X – ray – originate outside nucleus or man made
• Shielding: charged particles – low density materials
• γ/X rays – high density materials
• Units Becquerel (Bq) old unit Curie (Ci)
• Excellent physics website:
• http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htm
78

• Common laboratory isotopes
• 32P – pure beta (minus)
• 33P - pure beta (minus)
• 14C - pure beta (minus)
• 3H - pure beta (minus)
• 35S - pure beta (minus)
• 125I – electron capture – gamma and X-rays
• 131I – beta (minus) + gamma
79

80
no Name/ symbol Half life Energy in MeV/KeV comments
1 HYDROGEN-3 12.3YRS 19kEv BETA MINUS SHIELDING LESS THAN
0.1MM, CANNOT BE
DETECTED BY GM TUBES,
BREAKING RAD IS SIGNIF
2 CARBON -14 5730YRS 157KEV, B MINUS DETECTED USING GM
TUBES,SHIELDING =3MM
PLASTIC
3 PHOSPHORUS-
32
14.3DAYS 1.71 MeV B MINUS SHIELDING IS 6.3MM
PLASTIC. DETECTED USING
GM COUNTERS
4 IODINE-125 60DAYS X RAY EMITTER DETECTED USING
PORTABLE SCIN COUNTERS,
SHIELDING IS 1MM LEAD

Home work
• study carbon dating
• Read about medical applications of
radioactivity.
• Methods of shielding in Brachy therapy from
implanted radioactive sources.
• Milking of Technetium 99m
• Hardware of the Co-60 teletherapy machine
and how it prevents exposure to the
radioactive source
81

References
1. Atomic Physics by IAEA
2. Principles of Physics
3. Radiological Physics by Donald T.
Graham, Paul Cloke and Martin Vosper
(sixth Edition)
RADIOACTIVITY. LECTURE NOTES .ST LOUIS UHIHEBS,2015/2016 ACADEMIC YEAR, LEVEL 200
RADIOLOGY.COMPILED BY Nchanji Nkeh Keneth
82

Radioactivity lectures level 200 radiology st louis unihebs MDIRT Nchanji Nkeh Keneth

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