Radioactivity

Radioactivity
Peter Huruma Mammba
Department of Physical Science
DODOMA POLYTECHNIC OF ENERGY AND EARTH RESOURCES MANAGEMENT (MADINI INSTITUTE) –D
peter.huruma2011@gmail.com

Radioactivity
• The nuclei of naturally occurring heavy
elements like U, Th, Ra and Po are
unstable and keep on emitting
spontaneously invisible rays or radiations
(α, β, γ -rays) and give more stable
elements.

Radioactivity
• These heavy elements are called
radioactive elements.
• The property of emitting these rays is
called radioactivity of the elements.

Radioactivity cont….
• It is the nucleus of an atom of an element
which spontaneously disintegrates to emit α,
β or γ-rays.
• The rays emitted by radioactive element are
called radioactive rays

Thus radioactivity can be
defined as:-
The phenomenon in which the nucleus of the
atom of an element undergoes spontaneous
and uncontrollable disintegration (or decay)
and emit α, β or γ-rays.

Also, radioactivity can be
defined as:-
• Is the Process of spontaneous
disintegration of the nuclei of
heavy elements with the emission
of certain types of radiations.

The emitted α, β or γ-rays from unstable nuclei are
collectively called ionizing radiations.
Depending on how the nucleus loses this excess
energy either a lower energy atom of the same
form will result, or a completely different
nucleus and atom can be formed.

Ionization
is the addition or removal of an electron to create
an ion.
Ionizing radiation
is any type of particle (α, β) or electromagnetic
wave (γ) that carries enough energy to ionize or
remove electrons from an atom.
.

• These radiations are of such high energy
that when they interact with materials, they
can remove electrons from the atoms in
the material. This effect is the reason why
ionizing radiation is hazardous to health

• Radioactivity is of the following two types
which are:
a) Natural radioactivity
b)Artificial Radioactivity

Natural Radioactivity
is the process of spontaneous (i.e. without
external means, by it self) disintegration of the
nuclei of heavy elements with the emission of
radiation.
-these are unstable nuclei found in nature.

Natural radioactivity cont…..
• All heavy elements above Z=82 show the
phenomenon of radioactivity. the emission
of radiation changes the structure of the
nucleus and transforms the atom into a
lighter atom.

Natural radioactivity cont…..
• The heavy element are unstable
therefore they disintegrate to acquire a
more stable state.

continue
• Since radioactivity is practically unaffected by
temperature, pressure and other conditions, we
conclude that it is a nuclear property. Therefore
α,β-particles and γ-rays are emitted from the
nucleus.
• It may be noted that electrons revolving around
the nucleus are not responsible for radioactivity.

Artificial Radioactivity
• Is the process in which a stable (non-
radioactive) nucleus is changed into an
unstable (radioactive) nucleus by
bombarding it with appropriate atomic
projectiles like α, neutron, proton.

Example of Artificial
radioactivity

The differences between natural
and artificial radioactivity
Natural radioactivity Artificial radioactivity
Is spontaneous, since in natural
radioactivity, the nuclei of heavy
atom disintegrate on their own
accord, forming slightly lighter and
more stable nuclei and emitting
α,β,ᵞ radiations.
Is not spontaneous, since in it the
nuclei of the atoms have to be
bombarded by fast moving particles
like α, neutrons, protons, deuterons.
Is uncontrolled and hence it can not be
slowed down or accelerated by any
means.
Can be controlled by controlling the
speed of the bombarding particles used
for bringing about the artificial
radioactivity
Is usually shown by heavy elements. Can be induced even in light element.

Equivalent dose
Is a dose quantity representing the stochastic health
effects of low levels of ionizing radiation on the human
body.
It is derived from the physical quantity absorbed dose,
but also takes into account the biological effectiveness of
the radiation, which is dependent on the radiation type
and energy.
The SI unit of measure is the Sievert (Sv).

Other Common Radiation Units –
SI
1. Gray (Gy)
• To measure absorbed dose. (the amount of energy
actually absorbed in some material) and is used for
any type of radiation and any material (does not describe
the biological effects of the different radiations)
• Gy = J / kg (one joule of energy deposited in one
kg of a material)

2. Roentgen (R)
-Is used to measure exposure but only to
describe for gamma and X-rays, and only in
air.
• R = depositing in dry air enough energy
to cause 2.58 𝑥10−4
coulombs per kg

3. Rem
(Roentgen Equivalent Man)
- to derive equivalent dose related the
absorbed dose in human tissue to the
effective biological damage of the radiation.

4. Sievert (Sv)
• To derive equivalent dose (the absorbed
dose in human tissue to the effective
biological damage of the radiation).
• Sv = Gy x Q (Q = quality factor
unique to the type of incident radiation)

5. Becquerel (Bq)
- to measure a radioactivity (the quantity of a
radioactive material that have 1
transformations /1s)
• Bq = one transformation per second, there
are 3.7 x 1010 Bq in one curie.

Detection and Measurement of
Radioactivity
• The radioactivity of the radioactive
substance is detected and measured by
instruments like:-
Geiger-Muller (G-M) counter
Wilson Cloud Chamber.
Scintillation Counters.
Dosimeter.

Types of Radioactive Rays
• There are three types of radioactive rays
which are:-
– Alpha (α)
– Beta (β)
– Gamma (ᵞ) rays

Alpha ( 4
2He)
• An alpha particle is a helium
nucleus whose mass number is 4
and nuclear charge (Atomic
number) is +2.

Alpha- Particle Decay
• For proton- rich heavy nuclei, a
possible mode of decay to a more
stable is by alpha particle emission.

Alpha decay
• Has largest ionizing power
Ability to ionize molecules & atoms due to
largeness of -particle
• has lowest penetrating power
Ability to penetrate matter
• Skin, even air, protect against -particle radiation

X
A
Z
Y
A - 4
Z - 2
+ He
4
2
Alpha Decay
unstable atom
more stable atom
alpha particle

Alpha Decay
Ra
226
88
Rn
222
86
He
4
2

X
A
Z
Y
A - 4
Z - 2
+ He
4
2
Ra
226
88
Rn
222
86
+ He
4
2
Alpha Decay

Rn
222
86
He
4
2
+Po
218
84
He
4
2
Rn
222
86
+Y
A
Z
He
4
2
Alpha Decay

He
4
2
U
234
92
+Th
230
90
He
4
2
X
A
Z
+Th
230
90
He
4
2
Alpha Decay

Th
230
90
+Y
A
Z
He
4
2
Alpha Decay
He
4
2
+Ra
226
88
He
4
2
Th
230
90

X
A
Z
+Pb
214
82
He
4
2
Alpha Decay
He
4
2
+Pb
214
82
He
4
2
Po
218
84

Beta Decay
A beta particle (Denoted by 𝛽) is a fast moving electron
which is emitted from the nucleus of an atom
undergoing radioactive decay.
Beta decay occurs when a neutron changes into a
proton and an electron.

Beta Decay
• Many neutron-rich radioactive nuclides
decay by changing a neutron in the parent
nucleus into a proton and emitting an
energetic electron.

Beta Decay
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.

Beta Decay
• Many different names are applied to this
decay process:
• Electron decay, beta minus decay, negatron
decay, negative electron decay, negative
beta decay or simply Beta Decay

Beta Decay
Po
218
84
b
0
-1
At
218
85

X
A
Z
Y
A
Z + 1
+ b
0
-1
Beta Decay
Po
218
84
Rn
218
85
+ b
0
-1

Th
234
90
Y
A
Z
+ b
0
-1
Beta Decay
Th
234
90
Pa
234
91
+ b
0
-1

X
A
Z
Pb
210
82
+ b
0
-1
Beta Decay
Tl
210
81
Pb
210
82
+ b
0
-1

Bi
210
83
Y
A
Z
+ b
0
-1
Beta Decay
Bi
210
83
Po
210
84
+ b
0
-1

X
A
Z
Bi
214
83
+ b
0
-1
Beta Decay
Pb
214
82
Bi
214
83
+ b
0
-1

Comparison between a β-
particle and electron
a) Both the particles are negatively charged
which is equal to -1.

Comparison ….
b) When an atom lose an electron, a cation of
the same element is formed. On the other
hand, when the nucleus lose a β –particle a
new neutral element is obtained.

Gamma Decay
Gamma rays are not charged particles like  and b
particles.
Gamma rays are electromagnetic radiation with high
frequency.
When atoms decay by emitting  or b particles to form a
new atom, the nuclei of the new atom formed may still
have too much energy to be completely stable.
This excess energy is emitted as gamma rays (gamma ray
photons have energies of ~ 1 x 10-12 J).

Absorption of 𝜸 rays
• Many nuclides emit 𝛾 rays of more than one
wavelength. If 𝛾-rays of a 𝜆 are selected,
their absorption is an exponential faction of
absorber thickness, i.e.
𝐼 = 𝐼 𝑜 𝑒−𝜇𝑑

Absorption…
𝐼 = 𝐼 𝑜 𝑒−𝜇𝑑
I = the intensity transmitted by a thickness by a
thickness d of absorber.
𝐼0 = the intensity of the 𝛾 − rays incident on the
absorber
𝜇 = the linear absorption coefficient (or
attenuation coefficient) of the absorber (Unit
= 𝑚−1
)

• Also hold for x- rays
𝛾 −rays passing through an absorber

Absorption…
Note.
(i) The absorption of 𝛾-rays increases with the
atomic number of the material of the absorber.

Note.
(ii) The exponential nature of 𝛾- ray absorption
arises because, in most cases, a 𝛾- ray quantum
loses all its energy in a single event, and therefore
the fractional intensity of the beam falls by a fixed
amount each time it traverses any given small
thickness of absorber.

Photon Emission
Difference
Between
X-Rays and
Gamma Rays
63

X rays
• X Rays are electromagnetic waves /
photons emitted not from the nucleus, but
normally emitted by energy changes in
electrons. These energy changes are either in
electron orbital shells that surround an atom
or in the process of slowing down such as in
an X-ray machine.

Properties of α, β and ᵞ rays

Positron Decay
• Nuclei that have too many protons for stability
often decay by changing a proton into a
neutron.
• In this decay mechanism an anti-electron or
positron 𝛽+
or 1
0
𝑒, and a neutrino 𝜐 are
emitted.

Positron Decay
• The 𝛽+
decay reaction is written as
𝑍
𝐴
𝑃 → 𝑧 −1
𝐴
𝐷 + +1
𝑜
𝑒 + 𝜐
The positron has the same physical
properties as an electron, except that it has
one unit of positive charge.

Neutron Decay
• A few neutron-rich nuclides decay by emitting a
neutron producing a different isotope of the
same parent element.
• Generally, the daughter nucleus is left in an
excited state which subsequently emits gamma
photons as it returns to its ground state.

Neutron Decay
• This decay reaction is
𝑍
𝐴
𝑃 → 𝑧
𝐴−1
𝑃∗
+ 0
1
𝑛
An example of such neutron decay reaction is;-
54
138
𝑋𝑒 → 54
137
𝑋𝑒∗
+ n

Proton Decay
• A few proton-rich radionuclides decay by
emission of a proton.
• In such decays, the daughter atom has an
extra electron (i.e., it is a singly charged
negative ion)

Proton Decay
• This extra electron is subsequently ejected
from the atom’s electron cloud to the
surroundings and the daughter returns to an
electrically neutral atom.

Proton Decay
• The proton decay reaction is thus;-
𝑍
𝐴
𝑃 → 𝑧−1
𝐴−1
𝐷∗ −
+ 1
1
𝑝
• In this reaction P and D refer to atoms of the
parent and daughter.

Electron Capture
• In the quantum mechanical model of an
atom, the orbital electrons have a finite (but
small) probability of spending some time
inside the nucleus, the innermost K-shell
electrons having the greatest probability.

Electron Capture
• It is possible for an orbital electron, while
inside the nucleus, to be captured by a
proton, which is thus transformed into a
neutron.

Electron Capture
• Conceptually, we can visualize this
transformation of the proton as
𝑝 + −1
0
𝑒 → 𝑛 + 𝜐,
Where the neutrino is again needed to
conserve the energy and momentum.

Electron Capture
• The general electron capture (EC) decay
reaction is written as
𝑍
𝐴
𝑃 → 𝑍−1
𝐴
𝐷∗
+ 𝑣
Where the daughter is generally left in an
excited nuclear state with energy E above a
ground level.

Electron Capture
• The following nuclear reactions are electron
capture reactions.
26
55
𝐹𝑒29 + −1
0
𝑒 → 25
55
𝑀𝑛30 + 𝑣
56
131
𝐵𝑎75 + −1
0
𝑒 → 55
131
𝑀𝑛76 + 𝑣

Some important particles contin...

Example 1
• 90
234
𝑇ℎ disintegrates give 82
206
𝑃𝑏 as
the final product. How many alpha
and beta particles are emitted
during this process?

Solution
• If the number of ∝, 𝛽 particles which are emitted
from 90
234
𝑇ℎ is x and y respectively, then the
formation of 82
206
𝑃𝑏 can be represented as;
90
234
𝑇ℎ
−𝑥2
4
𝐻𝑒
→
.
90−2𝑥
234−4𝑥
𝐴
−𝑦−1
0
𝑒
→
.
90−2𝑥+𝑦
234−4𝑥
𝑃𝑏 = 82
206
𝑃𝑏

Solution
234 - 4x = 206 or x = 7 and 90 - 2x + y = 82
90 - 14 + y = 82
y = 6
Thus 7𝛼 and 6𝛽 particles are emitted

Exercise 1
• In the following natural radioactive
series where only the first and last
elements are given, calculate the
number of ∝, 𝛽- particles emitted in
each case.

Exercise 1
(a) 92
238
𝑈 → 82
206
𝑃𝑏
(b) 90
232
𝑇ℎ → 82
208
𝑃𝑏
(c) 92
235
𝑈 → 82
207
𝑃𝑏

Exercise 2
• The Uranium atom 92
238
𝑈 emits an 𝛼 -
particle to become thorium, which then
emits a 𝛽- particle to become protactinium.
What are the atomic and mass numbers of
protactinium?

Exercise 3
• Radon has an atomic number of 86 and a mass
number of 220. it emits an 𝛼 − particle to
become Thorium A (polonium), which emits
another 𝛼 − particle to become Thorium B
(radioactive lead). Thorium B then emits a 𝛽 −
particle to become Thorium C (bismuth). What is
(a) the atomic number and (b) the mass number
of Thorium C?

Exercise 4
• Radioactive Uranium 92
238
𝑈 emits an 𝛼 −
particle to become Thorium. Thorium emits
a 𝛽 − particle to become Protactinium
which then emits another 𝛽 − particle.
What is the atomic number, mass number
and name of the final atom produced?

Exercise 5
• In each of the nuclear reactions listed below, what
is the atomic number, mass number and name of
the particle produced?
• Boron 5
10
𝐵 bombarded with a neutron gives
Lithium 3
7
𝐿𝑖 + particle.
• Aluminum 13
27
𝐴𝑙 bombarded with an 𝛼 − particle
gives Silicon 14
30
𝑆𝑖 + particle.
• Sodium 11
23
𝑁𝑎 bombarded with an 𝛼 − particle
gives Aluminum 13
26
𝐴𝑙 + particle.

Disintegration constant
or
Decay Constant (K)

Decay Constant (K)
• Suppose a radioactive element A (i.e. at t = 0
be 𝑁0) disintegrates into another substance B.
• Now as the time passes, the element A
disintegrates and hence the amount of A goes
on decreasing while that of B goes on
increasing.

Decay Constant (K)
• Suppose that after t time, the amount of A left
undisintegrated is N.
• ( 𝑁0 - N ) is the amount of A that gets
disintegrated into B after time t.

Decay Constant (K)
• Now if a small amount, 𝑑𝑁 of A gets
disintegrated into B in a small time 𝑑𝑡, then the
rate of a disintegration (i.e. rate of decrease) of
A into B is equal to − 𝑑𝑁
𝑑𝑡 which is
proportional to the amount of A left
undisintegrated (N).

Decay Constant (K)
− 𝑑𝑁
𝑑𝑡 ∝ N or − 𝑑𝑁
𝑑𝑡 = KN
Where
K = is amount of proportionality which is
called disintegration or decay constant
-
𝑑𝑁
𝑁
= 𝐾. 𝑑𝑡 ………….. (i)

Decay Constant (K)
• Decay Constant (K)
Can be defined as the fraction of the total amount
of the radioactive substance 𝑑𝑁
𝑁 which
disintegrates in unit time.
K is expressed in 𝑡𝑖𝑚𝑒−1
units i.e. in 𝑠−1
, 𝑚𝑖𝑛−1
,
ℎ𝑟𝑠−1
, 𝑑𝑎𝑦𝑠−1
, 𝑦𝑟𝑠−1

Decay Constant (K)
• Integrating equation (i) over limit 𝑁0 and N (for
the left hand side) and 0 and t (for the right hand
side), we get;
𝑁0
𝑁 𝑑𝑁
𝑁
= - 0
𝑡
𝐾𝑑𝑡
𝐼𝑛
𝑁
𝑁0
= - Kt …………………… (ii)

Decay Constant (K)
𝑁
𝑁0
= 𝑒−𝐾𝑡
𝑵 = 𝑵 𝟎 𝒆−𝑲𝒕

Radioactive Decays 99
Variation of N as a function of time t
N
No
t
N = No e - t
Also A = Ao e - t
Radioactive Decay Kinetics - plot
Number of
radioactive nuclei
decrease
exponentially with
time as indicated by
the graph here.
As a result, the
radioactivity vary in
the same manner.
Note  N = A
 No = Ao

Decay Constant and Half-life
Variation of N as a function of time t
N
No
t
N = No e - t
Also A = Ao e - t
Be able to apply
these equations!
N = No e– t
A = Ao e – t
ln N = ln No –  t
ln A = ln Ao –  t
Determine half life,
t½
Ln(N or A)
t
ln N1 – ln N2
 = –––––––––––
t1 – t2
t½ *  = ln 2

Half-Life
• The half-life of a radioactive nuclide
– Is the time taken for half the nuclei present to
disintegrate.
If the half-life is represented by 𝑻 𝟏
𝟐
, then when t = 𝑻 𝟏
𝟐
,
𝑁 = 𝑁 𝑜
2, and therefore by equation 𝑁 = 𝑁0 𝑒−𝐾𝑡
𝑁0
2= 𝑁0 𝑒
−𝐾𝑇1
2
∴ 𝑇1
2
= 0.693
𝐾

Half-life
• Most radioactive
materials decay in a
series of reactions.
• Radon gas comes from
the decay of uranium in
the soil.
• Uranium (U-238)
decays to radon-222
(Ra-222).

The Decay Path of 4n + 2 or 238
U Family 238
U234
U
234
Pa
234
Th230
Th
226
Ra
222
Rn
218
At
218
Po214
Po
214
Bi
214
Pb
210
Po
210
Bi
206
Pb 210
Pb
206
Tl 210
Tl
206
Hg
Minor route
Major route
 decay
b decay
Radioactivity - 238U radioactive decay series

Decay Constant for some Elements

ACTIVITY
• Activity is the rate of disintegration in a
radioactive substance.
Activity of a substance, A = -
𝑑𝑁
𝑑𝑡
The minus sign shows that the activity decreases
with the passage of time.

ACTIVITY
• According to decay law, the rate of disintegration
is directly proportion to the number of atoms
present. i.e.
−
𝑑𝑁
𝑑𝑡
∝ 𝑁
−
𝑑𝑁
𝑑𝑡
= 𝐾𝑁
∴ 𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦, 𝐴 = 𝐾𝑁

Examples…
4. A sample of radioactive material contains 1018
atoms. The half-life of the material is 2000 days.
Calculate;-
(i) The fraction remaining after 5000 days
(ii) The activity of the sample after 5000 days.

Exercise 6
• Half-file of .
210
𝑃 is 140 days. Calculate
the number of days after which 1
4g of
.
210
𝑃 will be left undisintegrated from
1g of the isotope.

Exercise 6
The mass number of radium is 226. It is
observed that 3.67 𝑥 1010
∝ − particles
are emitted per second from 1g of
radium. Calculate the half-life of
radium.

Exercise 7
The count rate meter is used to measure the activity
of a given amount of a radioactive element. At one
instant, the meter shows 475 count/ minutes.
Exactly 5 minutes later, it shows 270
counts/minutes. Find
(i) The decay constant (K)
(ii) Mean life
(iii)The half life of the element.

Exercise 8
• The half-life of radium is 1500 years. After
how many years will 1 g of pure radium (i)
reduce to 1 centigram (ii) lose 1 mg?

Exercise 9
• A sample of radioactive material has an
activity of 9 𝑥 1012
𝐵𝑞. The material has
a half-life of 80 s. How long will it take
for the activity to fall to 2 𝑥 1012
𝐵𝑞?

Exercise 10
• Biologically useful technetium nuclei (with
atomic weight 99) have a half life of six hours. A
solution containing 10−12
g of this is injected
into the bladder of a patient.
• Find its activity in the beginning and after one
hour.

Exercise 11
• An isotope (36
87
𝐾𝑟) has a half-life of 78
minutes. Calculate the Activity of 10
𝜇𝑔 of 36
87
𝐾𝑟. The (Avogadro's constant,
𝑁𝐴 = 6.0 𝑥 1023
𝑚𝑜𝑙−1
)

Exercise 12
• Carbon-14, 6
14
𝐶, is a radioactive isotope of
carbon that has a half-life of 5730 years. If
you start with a sample of 1000 carbon-14
nuclei, how many will still be around in
22920 years?

Exercise 13
• The half-life of the radioactive nucleus
88
226
𝑅𝑎 is 1.6 𝑥 103
𝑦𝑒𝑎𝑟𝑠. If a sample
contains 3 𝑥 1016
such nuclei, determine
the activity of this time.

Exercise 14
• A radioactive sample contains 3.50 𝜇𝑔 of
pure 6
11
𝐶, which has a half-life of 20.4 min.
(a) Determine the number of nuclei present
initially.

Exercise 15
• A sample of the Isotope 53
131
𝐼, which has a
half-life of 8.04 days, has a measured activity
of 5 𝑚𝐶𝑖 at the time of shipment. Upon
receipt in a medical laboratory, the activity is
measured to be 4.2 𝑚𝐶𝑖. How much time has
elapsed between the two measurements?

Exercise 16
• A piece of charcoal of mass 25 g is found in
some ruins of a ancient city. The sample
shows a 6
14
𝐶 activity of 250 decays/min.
How long has the tree that this charcoal
came from been deed?

Exercise 17
• Why are heavy nuclei unstable?

Exercise 18
• If a nucleus has a half-life of 1 year, does
this mean that it will be completely decayed
after 2 years? Explain

Exercise 19
• What do you understand by mass defect of
an atom.
• Distinguish between binding energy of an
atom and binding energy of the nucleus.
Give the expressions for binding energy in
MeV and binding energy in Joules (J)

Exercise 20
• What is the difference between a neutrino
and photon?

Exercise 21
• Explain why many heavy nuclei undergo
alpha decay but not spontaneously emit
neutrons or protons.
• Pick any beta decay process and show that
the neutrino must have zero charge.

Isotopes
Isotope are the atoms of the same elements which have the same
atomic number (Z) or the same number of protons (p) but different
mass number s(A). For example
(A = 2,Z =1), (A = 1, Z= 1), these
are isotopes of hydrogen.
Isotopes of chlorine
35Cl 37Cl
17 17
chlorine - 35 chlorine - 37 129

Isotopes
+
+ +
+
+
+
Nucleus
Electrons
Nucleus
Neutron
Proton
Carbon-12
Neutrons 6
Protons 6
Electrons 6
Nucleus
Electrons
Carbon-14
Neutrons 8
Protons 6
Electrons 6
+
+
+
+
+
+
Nucleus
Neutron
Proton

3 p+
3 n0
2e– 1e– 3 p+
4 n0
2e– 1e–
6Li 7Li
+ +
+Nucleus
Electrons
Nucleus
Neutron
Proton
Lithium-6
Neutrons 3
Protons 3
Electrons 3
Nucleus
Electrons
Nucleus
Neutron
Proton
Lithium-7
Neutrons 4
Protons 3
Electrons 3
+ +
+

Mass Spectrophotometer
electron
beam
magnetic field
gas
stream
of ions of
different
masses
lightest
ions
heaviest
ions
Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138

.
• mass spectrometry is used to experimentally determine isotopic masses
and abundances
• interpreting mass spectra
• average atomic weights
- computed from isotopic masses and abundances
- significant figures of tabulated atomic weights gives some idea
of natural variation in isotopic abundances
Weighing atoms
gas sample
enters here
filament current
ionizes the gas
ions accelerate
towards charged
slit
magnetic field
deflects lightest ions
most
ions separated by mass
expose film
The first mass spectrograph was
built in 1919 by F. W. Aston, who
received the 1922 Nobel Prize for
this accomplishment

Natural uranium, atomic weight = 238.029 g/mol
Density is 19 g/cm3. Melting point 1000oC.
Two main isotopes:
U
238
92
U
235
92
99.3%
0.7%
Because isotopes are chemically identical
(same electronic structure), they cannot be
separated by chemistry.
So Physics separates them by diffusion or
centrifuge
Separation of Isotopes
(238 amu) x (0.993) + (235 amu) x (0.007)
236.334 amu + 1.645 amu
237.979 amu
U
238
92

Characteristics of Isotopes
i. Since the atomic number of the
isotopes are the same, they contain the
same number of protons in the nucleus
and the same number of electrons
revolving around the nucleus

ii. Since the mass number of isotopes are
different, the sum of protons and
neutrons in the nucleus is also different.

iii. Since isotopes of a given element have different
number of neutrons, they show different physical
properties (e.g.. 𝜌, MP, BP etc..).
• Since isotopes have the same atomic number,
they have the same electronic configurations and
hence have the same chemical properties.

iv. Isotopes have different radioactive
properties, since the composition of their
nuclei is different.

v. All the isotopes are placed in the
same group of the periodic table, since
they have the same atomic number.

Types of isotopes
Radioactive (unstable) as well as non-radioactive
(stable) elements give two types of isotopes as
follows:-
I. Radioactive or Unstable isotopes
II. Non-radioactive or stable isotopes

Radioactive or Unstable isotopes

Non-radioactive or Stable isotopes

Production of an isotopes by the
emission of one α and two β-particles

Uses of Radioactive Isotopes
In Medical Field
(i) In order to find out if blood is circulating to a
wound or not, a radiotracers is introduced in to the
body and after suitable time, some quantity of
blood is taken from the wound and its radioactivity
is measured by means of a Geiger-Muller Counter.

(i)
• The Geiger-Muller Counter detect the exact
position where the blood clot in that human
body.

In Medical Field conti…
• Diagnosis of diseases.
Isotopes with a short half-life give off lots of
energy (γ-rays) in a short time has been used to
detect the exact position of the tumour in the
human body.
Therefore, Isotopes are useful in medical imaging.

Single-photon emission computed
tomography (PET)

Image produced by PET scanner
during diagnosis

Image produced by PET scanner
during diagnosis
PECT can be used for nuclear
cardiology
SPECT modality can be used
to assess cerebral perfusion.

• The isotopes with high energy γ rays has
been used for the treatment of cancer.
The treatment of diseases by the use of
radioactive isotopes is called
Radiotherapy

The use of linear Accelerator for the
treatment of different kinds of Cancer

Treatment of Cervical Cancer using
High Dose Rate Intracavitary
Brachytherapy (HDR-ICBT)

Treatment of Prostate Cancer
using High Dose Rare Interstitial
Brachytherapy

Sterilization and irradiation
• Syringes, dressings, surgical gloves and instruments, and
heart valves can be sterilized after packaging by using
radiation.
• Radiation sterilization can be used where more
traditional methods, such as heat treatment, cannot be
used, such as in the sterilization of powders and
ointments and in biological preparations like tissue
grafts.

Medical Sterilization Machines

In Agriculture
• In agriculture, radioactive materials are used to
improve food crops, preserve food, and control
insect pests.
• They are also used to measure soil moisture
content, erosion rates, salinity, and the efficiency
of fertilizer uptake in the soil.

Application of radiation during
protection of Agricultural materials

Environment
• Radioactive materials are used as tracers to measure
environmental processes, including the monitoring of silt, water
and pollutants.
• They are used to measure and map effluent and pollution
discharges from factories and sewerage plants, and the
movement of sand around harbours, rivers and bays.
Radioactive materials used for such purposes have short half-
lives and decay to background levels within days.

In Industry
• Radioactive materials are used in industrial
radiography, civil engineering, materials
analysis, measuring devices, process control in
factories, oil and mineral exploration, and
checking oil and gas pipelines for leaks and
weaknesses.

In industry…
• Examples on the uses of industrial
measuring devices which containing
radioactive materials are:-
(i) They are used for testing the moisture
content of soils during road construction.

In industry….
(ii) The are used to measure the thickness of
paper and plastics during manufacturing.
(iii) To checking the height of fluid when filling
bottles in factories.
NB: Radioactive materials are even used in
devices designed to detect explosives.

In Our Homes
One of the most common uses of radioactive materials in
the home is in smoke detectors. Most of these life-saving
devices contain tiny amounts of radioactive material
which make the detectors sensitive to smoke.
The radiation dose to the occupants of the house is very
much less than that from background radiation.

Other Applications of radioactive
isotopes
• Many satellites use radioactive decay from
isotopes with long half-lives for power
because energy can be produced for a long
time without refueling.
• The isotope carbon-14 is used by
archeologists to determine age.
• Radioactive isotopes are used to detect the
leakage or crack in the underground oil
pipes, gas pipes and water

Exercise 22
• Explain in detail how you can determine the
age of a sample using the technique of carbon
dating.
• Why is carbon dating unable to provide
accurate estimates of very old material?

Nuclear stability
• It has been observed that some isotopes are
stable while others are unstable. Stable
nuclides are not undergo spontaneous
disintegration. This is due to the stability of
their nuclides of an atom called nuclear
stability.

Factors Affecting Nuclear
stability
i. Even and Odd number of protons and
neutrons.
ii. Neutron-to-proton ratio.
iii. Packing fraction.
iv. Binding energy.
v. Magic numbers.

Exercise 23
Naturally occurring carbon consists of three isotopes,
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of these carbon atoms.
12C 13C 14C
6 6 6
#P _______ _______ _______
#N _______ _______ _______
#E _______ _______ _______
LecturePLUS Timberlake 168

Nuclear reactions
• These are the reactions in which one
element is converted into the other either by
emitting α or β particles (spontaneous
disintegration) or by bombarding it with
suitable bombarding particle(artificial
transmutation of element).

Differences between chemical
reaction and Nuclear reaction

Nuclear Fission
• Is a nuclear reaction in which a heavy
nucleus, when bombarded with slow
moving neutrons, split into two nuclei of
near equal mass with the release of
anomalous amount of energy.

Example of nuclear fission
A
B
C
D
Where
A = Thermal neutron
B = Fission
C = Fission product
D = Huge amount of energy

Types of fission reaction
• We have two types of nuclear fission
reaction. These are :-
a. Uncontrolled or explosive fission reaction
b. Controlled or artificial fission reaction

Uncontrolled or explosive fission
reaction

Nuclear Reactor or Atomic Reactor
• Is a kind of furnace in which controlled
fission of a radioactive material like U-235
takes place and a manageable amount of
nuclear energy (atomic energy) is produced
at a steady slow rate.

Uses of a Nuclear Reactor
(i) Radioactive isotopes of various elements are
produced
(ii) Heat energy produced in the fussion of u-235
nucleus by slow neutrons taking place in the
atomic reactor has been used to generate
electricity

Uses of a Nuclear Reactor
iii. It produces fissionable material like
plutonium which is used in atomic bomb.
iv. It is also used for the production of fast
neutrons that are needed for nuclear
bombardment

The Uses of Nuclear reactor for the
production of electricity

Nuclear Fusion
Also called Atomic Fusion
• Is the nuclear reaction in which lighter
nuclei combine together to form a single
heavy and more stable nucleus and large
amount of energy is released.

Example of nuclear fusion
reaction
A B C D E
A= Deuterium
B= Tritium
C = α- Particle
D= Neutron
E = Anomalous amount of energy

Our sun undergos nuclear fusion

Differences between Nuclear
fusion and Nuclear fission

Exercise 24
• Distinguish between nuclear fission and
nuclear fusion. Give some equations to
support your answer.

The after-effects of atomic bomb

1. Can a single nucleus emit 𝛼-particle, 𝛽- particle
or 𝛾-ray together? (1 Mark)
2. When does 𝛼-decay occur? (1 Mark)
3. The half-life of radium is 1600 Years. After how
much time 1
20 𝑡ℎ part of radium will remain
undisintegrated in the sample. (3 Marks)

Radioactivity

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