91172 – AS 2.5
Demonstrate an understanding of
atomic and nuclear physics
3 credits - Internal
Topics to cover
1. Structure of the Atom
2. Atomic number and Mass number
3. Isotopes
4. Nuclear reactions α,β, and γ, equations, rate
5. Reaction types
6. Applications/ Nuclear energy
Historical Discoveries
From Ancient Greeks to nowadays
Ancient Greeks – Aristotle
around 500BC
The smallest particle of an
element that still retains the
identity of that element is the
atom.
Atom comes from the Greek
ατομος = ‘indivisible’
“The Dark Age”
Making gold out of lead
4 basic ‘elements’
for Ancient Greeks
John Dalton (1766-1844)
Around 1800, Dalton, an English chemist,
revived the proposal that all matter
was made up of atoms.
He also suggested that all atoms of a
given element are identical to one another,
but differ from the atoms of other
elements.
Antoine Henri Becquerel
(1852-1908)
French
physicist
In march 1896,
he put his wrapped photographic
plates in a dark drawer along
with crystals containing uranium.
The plates were exposed by invisible
emanations from the uranium.
Discovery of radioactivity
Joseph Thomson (1856-1940)
In 1897, discovered the electron,
negative charge carrier.
First model of atom:
the “plum-pudding” model.
British
physicist
The “plum-pudding” model from Thomson
Sphere of positive charge
Electrons dotted evenly through it like currants in a
plum pudding. Overall, the atom is electrically neutral.
Ernest Rutherford (1871-1937)
Attempted to prove
Thomson’s model of the atom
was correct
with an experiment:
the gold foil experiment.
New Zealand
physicist
Rutherford’s gold foil experiment
Very thin gold foil
ToProviding
prevent the
a α
particles
from
thin
Preventing
α
Zinc
sulfide
being
stopped
stream
of
particles
screen
that
by gas
particles
from
radiating
would
flash in
molecules
other hit
directions
when
by an
(shield)
alpha
particle
Lead casing
with narrow
slit
α
Vacuum
Scintillation screen α particle source
Rutherford’s gold foil experiment
Gold atom
Aim
-
-
-
-
-
-
-
Alpha particle
path
-
-
Prove that Thomson’s model of
the atom is correct.
Rutherford’s gold foil experiment
Observations
Gold atom
Alpha particle
path
Alpha particle
Alpha particle
path
path
Conclusions
3. A few α particles rebounded
1. Significantly,
2.
Most
of the α back)
about
particles
1 inwent
every
8000
(bounced
from
the straight
gold
through
were
deflected
the gold
at atoms
a largeundeflected.
angle.
atoms.
The nucleus
nucleus must
must be
be tiny
The
positively
charged
to
Most
of the
atom
is α
and
dense
togold
cause
a few
cause
α
emptythe
space.
particles
topositive
rebound.
particles to repel.
Rutherford’s gold foil experiment
Rutherford’s model of the atom
Mainly empty space
Tiny, dense, positive nucleus
Small negatively charged
electrons orbiting the
nucleus
Overall neutral
Niels Bohr (1885-1962)
Permitted orbits
In 1919, proposed that electrons
revolve around an atom’s nucleus
in particular orbits and have
definite energies.
Danish
physicist
James Chadwick (1891-1974)
In 1932, discovered the neutron.
neutrons
protons
Electrons in
defined orbits
British
physicist
The atom today
Quantum theory and discoveries of new particles.
neutrons
Positively
charged nucleus
neutrons + protons = nucleons
protons
electrons in
shells around
the nucleus
The nucleus
2x10-14m
2x10-10m
The whole atom, including electrons
So, what is radioactivity?
No more Americium.
Americium
Neptunium
Strong nuclear force
about 1000 times
stronger than
electromagnetic
force
Radioactive material
No radioactive matter found
Except:
Long time to decay
Created continuously
Uranium-238
92 protons
4.5
billion
years
Atomic notation
Nucleon number
or
Mass number
(represents the
number of nucleons
in the nucleus)
neutrons + protons = nucleons
A
Z
Charge number
or
Atomic number
(represents the number of
protons in the nucleus)
X
Symbol of the
element
Example Iron-56
56 nucleons
56
26 protons
26
Fe
It has 56 nucleons, 26 of which are protons.
This means that there are 56 – 26 = 30 neutrons.
There will also be 26 electrons in the shells (atom
electrically neutral).
Isotopes
Atoms that contain the same number of protons
but different number of neutrons are known as
isotopes.
Different numbers
Same number
A
Z
X
Same element
Examples
12
6
C
Carbon-12
6 protons and 6 neutrons in the
nucleus.
Most common form of carbon.
14
6
C
Carbon-14
6 protons and 8 neutrons in the
nucleus.
This is a ‘radioactive’ isotope of carbon.
Examples
1
1
Hydrogen
H
1 proton and 0 neutron in the
nucleus.
2
1
Deuterium
H
1 proton and 1 neutron in the
nucleus.
3
n
np
p
1
n
p
Tritium
H
1 proton and 2 neutrons in the
nucleus.
Nuclear reactions
Nuclei may break down or react to form new, more stable nuclei.
3 forms of nuclear activity: • nuclear decay
• nuclear fusion
• nuclear fission
Fe
Products are more stable
Energy is released.
56
Nuclear decay
Early discoveries
Spontaneous decay of unstable substances:
radium, uranium, carbon-14
Emission of:
• alpha particles, α
• beta particles, β
α
• gamma rays, γ
β
γ
Detection with Geiger-Müller
counter, records the rate of
nuclear activity in becquerel (Bq).
Alpha particles, α
Neutron
Proton
Nucleus
4
2
He
Helium nuclei
np
pn
Positively charged
Travels at speed of around 0.1c (3x107m.s-1)
Strong ioniser
Can travel a few centimetres in air
Easily stopped by a sheet of paper
Alpha particle
Nucleus with two fewer
protons and two fewer
neutrons
4
2
α
Alpha particle
Ionisation
electrons
The atom loses an
electron, it becomes a
positive ion.
Atom from a
gas
Beta particles, β
Nucleus
Fast-moving electron
emitted from a nucleus
Negatively charged
Travels at speed of around 0.9c
Can travel up to half a metre in air
Can penetrate paper
Stopped by a sheet of aluminium a
few millimetres thick
Beta particle
0
Nucleus with one fewer
neutron and one more proton
-1
β
Gamma rays, γ
Very high frequency
High energy
Electromagnetic radiation (light)
Carries no charge
Travels at c (3x108m.s-1) in a vacuum
Stopped by several centimetres of lead
γ
Sorting Radiation
• Identify each type of radiation
Sorting by Absorbtion
Paper
1mm Lead
Comparison
Alpha particles
Beta particles
Gamma rays
Nature
Helium nuclei
Electron
Electromagnetic
radiation
Charge
+2
-1
0
Relative mass
4
Relative speed
0.1c
Effect of
Deflected towards
electromagnetic
negative
field
Low
Penetrating
power
Ionising
effect
High
1/2000
0
0.9c
c
Deflected towards
positive
Unaffected
Moderate
High
Moderate
Low
+++++++++++
β
α
-----------------
γ
Equation of nuclear reactions
In nuclear reaction:
• Mass number (nucleon number) conserved.
• Atomic number (charge number) conserved.
Examples
230
90
226
Th
Ra
88
Represents the
nuclei, not the
whole atom
14
6
C
Denotes the
direction in
which reaction
proceeds
4
+
2
14
7
A: 230
Z: 90
α
N
60
+
Ni*
28
226 + 4
88 + 2
0
β
-1
60
Ni
28
+
γ
Number of
undecayed nuclei
remaining
N0
Half-life
The time taken for half the original nuclei to have decayed.
Rate of decay is expressed using the half-life
N0
Isotope
Half-life
Carbon-14
5730 years
Cobalt-60
5 years
Iodine-131
8 days
Carbon-11
21 minutes
2
N0
4
N0
8
t1/2
2t1/2
3t1/2
Time
Half-life
Start
N
After 1 half-life
N/2
After 2 half-lives
N/4
After 3 half-lives
N/8
Activity
Activity
(Bq)
A lump of radioactive matter gives the following activity
against time:
200
1 Bq = 1 count per second
The activity of the sample after 3 weeks is:
100
3 weeks = 21 days
50
33
25
8
16
21
24
Time
(days)
Sample Question.
• A radioactive isotope has a half life of 3 years.
• A 5 g sample of the isotope produces 30
decays per sec.
• What will the decay rate of a 1 g sample be in
9 years time?
Geiger counter
α
β
Ionisation of the
gas molecules
Tube containing gas
molecules
Electrical current
detected
Nuclear Fusion
Nuclear fusion is the joining together of smaller isotopes to
become larger, more stable nuclei.
Example:
2
1
H
3
+
1
4
H
Fe
He
2
1
+
0
n
+
Energy
Amount of energy released very large considering masses involved
Reaction typical of fusion reactions that occur on the
surface of the sun and in a hydrogen bomb.
Products of the reaction more stable than the reactants
A controlled fusion reaction as not yet been possible.
One of the biggest problem is required temperature: 5 x 109 K
In Theory, very efficient source of energy with cheap and abundant
reactants. Deuterium and Tritium can be extracted from sea water.
Nuclear Fission
Nuclear fission is the breaking down of large (parent) nuclei
into small (daughter) nuclei.
The nuclear reactions that can be controlled and are used for
energy production are fission reactions.
238
92
U
1
+
n
0
Fe 144
92
1
n
Kr
3
Ba
+
+
0
56
36
56
+ Energy
Nuclear Fission – Chain Reaction
Energy
released
Chain Reaction
Controlled breakdown of uranium
absorbed
absorbed
Energy
released
Comparison
Fusion
Fission
Energy released
Energy released
More stable products
More stable products
(but still radioactive)
Larger products
Cannot be controlled
Smaller products
Can be controlled
In our lives
Applications of Nuclear Physics
The heat from the sun!
Positives and Negatives
Natural radioactivity
Minerals
Food
Cosmic rays
Radon
Risk because
principal source
of radioactivity
exposure for
human beings.
Causes lung
cancers.
Prevent risks with
good ventilation
and good isolation
from soil.
Artificial radioactivity
Carbon-14 dating
Medical examination – Scintigraphy
Tracers
Radiotherapy
Nuclear activity and industry
Nuclear weapons
Electricity
production from
nuclear sources
Radiation
and You
eg from sun or
collapsing
stars
87% natural!
External/Internal
̴ 1:3
Measured in Sv (sievert)
Dosage guidelines
1 mSv general public
50 mSv radiation worker
eg from radon gas
produced by
radioactive rocks
eg from potassium,
lead or polonium
Effects of Radiation
Home and Away Activity
Radioactive
decay
eg 238Uranium
Nuclear Fission
eg 235Uranium
typically unequal daughter nuclei, around 130-140 and 90-100
another example
Energy from fission
Energy released by typical fission reaction
nucleus
2.6 x 10-11 J per
1kg uranium-235 contains 2.6 x 1024 nuclei
Therefore energy per kg
2.6 x 1024 x 2.6 x 10-11 = 6.8 x 1013 J
equivalent to 3 million kg coal
Control
rods
Shielding
Containment building
Coolant eg
pressurised water
Thick concrete
walls
Steel reactor
vessel
Fuel
rods
Moderator
to turbines
Issues of Nuclear Reactors
Waste disposal
Accidents
Classification of waste
Low
Intermediate:
High
metal reactor fuel cladding, as well as contaminated
materials from reactor decommissioning. It may be
solidified in concrete or bitumen for disposal.
Forsmark, Sweden
A Boiling Water Reactor