Higher Unit 1

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Higher Unit 1

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Title: Higher Unit 1

1
In unit 4 we will learn about energy from the
nucleus and its applications.

2
What do you know? How do we get energy from the
nucleus? What do we mean by energy? What do we
mean by nucleus? What do we use it for? What do
we know?

3
Ionising Radiations are used in many medical
applications including X-rays and sterilising
hospital equipment. They are also used in many
non medical applications and it is important in
many fields of work to understand radiation dose
and safety. Nuclear reactors are used in the
production of around 11 of the worlds energy
production, and to power some military ships and
submarines.
4
(No Transcript)
5

Key words atom, protons, neutrons, electrons,
radiation
energy, absorption, alpha, beta, gamma,
ionisation
By the end of this lesson you will be able to
Describe a simple model of the atom which
includes protons,
neutrons and electrons.
State that radiation energy may be absorbed in
the medium
through which is passes.
State the range through air and absorption of
alpha,
beta and gamma radiation.
Explain what is meant by an alpha particle, beta
particle and gamma
radiation.
Explain the term ionisation.
State that alpha particles produce much greater
ionisation
density than beta particles or gamma rays.

6
Useful Radiation

Radiation has many uses in medical Physics
different types of radiation are used
for different things.

7
Baggage scanning
8
Smoke Detectors
Smoke alarms contain a weak source made of Americium-241. Alpha particles are emitted from here, which ionise the air, so that the air conducts electricity and a small current flows. If smoke enters the alarm, this absorbs the alpha particles, the current reduces, and the alarm sounds. Am-241 has a half-life of 460 years.
9
Radioactive Dating
Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues. When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate (Carbon-14 has a half-life of 5700 years). The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left.
10
Leaking Pipes
Radioactivity is used in industry to detect leaks in pipes.
11
To have a good understanding of radioactivity we
need to know a bit about the structure of the atom
12
What do atoms look like?
They are very small!
Atoms are the smallest possible particles of the
elements which make up everything around us
13
Structure of the atom
nucleus
proton
neutron
electrons
14
Structure of the atom
nucleus
proton
neutron
electrons
15
The relative masses and charges of these
particles are given below
PARTICLE CHARGE MASS
Proton 1 1
Neutron 0 1
Electron -1 1/ 2000
16
Relative size of the atom and the nucleus.

The ratio of the diameters is
10 000 1 !
If the diameter of a particular atom was 10
metres, its nucleus would be 1 millimetre across!!

17
The atoms of a particular element are identical
All carbon atoms have 6 protons in the nucleus
and 6 orbiting electrons.

18

Atoms usually have the same number of
protons and electrons so an atom has no
overall charge.

Six protons charge?
6
Six electrons charge?
-6
0
Overall charge?
19
Ionisation

We will learn
about types
of radiation
which cause
ionisation.

20
Ionisation

Ionisation means adding or
removing an electron from an
atom to produce a charged
particle.
What happens to the charge
on an atom when an electron is
added or removed?

21
Atoms contain protons, which are positive as well
as electrons, which are negative
22
Normally atoms have equal numbers of protons and
electrons and are therefore neutral
23

Atoms usually have the same number
of protons and electrons so an atom
has no overall charge.

Six protons charge?
6
Six electrons charge?
-6
0
Overall charge?
24

If you add an electron

Six protons charge?
6
Six electrons charge?
-6
-1
Add one more electron charge?
-1
Overall charge?
25

If you remove an electron

Six protons charge?
6
Six electrons charge?
-6
-5
Take away an electron charge?
1
Overall charge?
26

Ionisation means the addition
or removal of an electron from
a neutral atom to produce a
charged particle.

Virtual Int 2 Physics -gt Radioactivity -gt
Ionising Radiations -gt Model of the Atom

27
The picture below shows an ALPHA PARTICLE,
consisting of 2 protons and 2 neutrons
28
Imagine that an ALPHA PARTICLE passes through a
neutral atom this will be shown in slow motion!
29
An electron has been knocked out of the atom.
This atom is now positively charged it is a
POSITIVE ION.
30
There are three types of ionising radiation.

Alpha radiation (a)
Beta radiation (ß)
Gamma radiation (?)
Virtual Physics Int 2 Radioactivity -gt Ionising
Radiations -gt Alpha, Beta, Gamma

31
Alpha radiation (a)

An alpha particle is made up of two
protons and two neutrons. It is the same
as a helium nucleus.
It is positively charged.
It is largest of all the three types of
radiation.

32
A big atom releases an alpha particle to make
itself more stable.
The alpha particle that is emitted has a lot of
energy and can damage human cells.

33
Alpha radiation (a)

An alpha particle is given the symbol

34
Summary
What are alpha particles? An alpha particle is made up of two protons and two neutrons. It is the largest of the three ionising radiations. It has a lot of energy.

35
Beta radiation (ß)

a fast moving high
energy electron
released from the
nucleus
it is very very small
Virtual Physics Int 2 Radioactivity -gt Ionising
Radiations -gt Alpha, Beta, Gamma

36
Beta radiation (ß)

A beta particle is given the symbol

37
This is what happens inside the nucleus.

38
Summary
What are beta particles? A beta particle is a fast moving, high energy electron. The electron is released from the nucleus when a neutron changes into a proton plus electron. It is very very small.

39
Gamma Radiation (?)

A wave of energy.
High frequency electromagnetic wave (so
travels at the speed of light)
No significant mass.
No charge.
Has the greatest amount of kinetic energy.

40
Gamma ray
It is the most energetic of all three radiations.
It is therefore the most penetrating the most
difficult to stop.

41
What are gamma rays? Gamma rays are high energy electromagnetic waves. They travel at the speed of light.

42
Radiation Ionisation

These three radiations (a, ß, ?) are called
ionising radiations because they
cause ionisation of living cells.
Radiations can kill or change living cells.
This is what makes them dangerous.

43
Ionisation Density

We can think about how much damage
a type of radiation will cause in terms
of ionisation density.

Alpha particles are heavy and slow moving.
They cause a lot of ionisation.
Beta particles are light and cause less
ionisation.
Gamma rays have no mass. They cause
little ionisation.

45
ALPHA PARTICLES are relatively large and cause a
lot of ionisation

-
-
-
-

-
-

-
-
-
-

-

-
46
BETA PARTICLES are smaller, so they cause less
ionisation

-

-
-

-
47
GAMMA RAYS cause least ionisation of all
-

48
Ionisation Density Range of Particles

Each time a particle causes ionisation it
loses energy. The energy is absorbed by
the medium through which it passes.
Alpha particles cause a lot of ionisation,
therefore lose a lot of energy. This means
they have a short range in air.

49
Ionisation Density Range of Particles

Beta particles cause less ionisation,
therefore lose less energy. This means
they have a longer range in air than alpha
particles.
Gamma particles have the lowest
ionisation density. This means they have
the longest range in air.

50
Identifying Radiations

We can tell which radiation is
which by testing to see what
happens when they reach
different materials.
Virtual Int 2 Radioactivity -gt Ionising
Radiations -gt Absorption of Ionising Radiations

51
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52
What material is sufficient to absorb alpha particles? Paper
What material is sufficient to absorb beta particles? A few millimetres of aluminium
What material is sufficient to absorb gamma rays? Several cm of lead
53
How much ionisation do alpha particles cause? The greatest amount. Alpha particles are most dangerous when inside the body (but least dangerous outside they can be stopped with paper!)
How much ionisation do beta particles cause? Medium. Less than alpha, more than gamma.
How much ionisation do gamma particles cause? The least. Gamma particles are most dangerous when outside the body because they can easily travel into the body. But theyre least dangerous when inside because they can escape.
54

Can you?
Describe a simple model of the atom which
includes
protons, neutrons and electrons.
State that radiation energy may be absorbed in
the
medium through which is passes.
State the range through air and absorption of
alpha,
beta and gamma radiation.
Explain what is meant by an alpha particle, beta
particle
and gamma radiation.
Explain the term ionisation.
State that alpha particles produce much greater
ionisation density that beta particles or gamma
rays.

55
Quick Recap
Type of radiation Symbol What is this radiation? Charge and absorption Range in air
a
A few m
Uncharged. Absorbed by lead.
56

Key words atom, protons, neutrons, electrons,
radiation energy,
absorption, alpha, beta, gamma, ionisation
By the end of this lesson you will be able to
Describe how one of the effects of radiation is
used in
a detector of radiation.
State that radiation can kill living cells or
change
the nature of living cells.
Describe one medical use of radiation based on
the
fact that radiation can destroy cells.
Describe one use of radiation based on the fact
that
radiation is easy to detect.

57
Detecting Radiation

To protect those who work
with radiation it is important
to be able to detect
radiation. The detection of
radiation is also vital in its use in
many applications.

58
Geiger Muller Tube

The Geiger counter is commonly used
to detect radiation (demo).
The Geiger counter consists of a
Geiger Muller tube attached to a
counter.

59
Geiger Muller Tube
The tube is filled with argon gas. Where else
is argon gas used?
60
Geiger Muller Tube
Around 400 V is applied to the thin wire.
61
Geiger Muller Tube
Radiation causes ionisation of the gas what do
we mean by this?
The thin window alllows radiation to enter.
62
Geiger Muller Tube
Ions produce electrical pulses which are counted
and displayed.
63
Geiger Muller Tube
We can either display total counts and use a
timer to determine counts per second, or use a
rate meter, which displays counts per second.
64
Geiger Muller Tube
Radiation Ionisation in tube (lots of
electrons) Discharges central wire Counted as
a pulse

65
How the Geiger Muller tube works
66
Photographic Fogging

We know that photographic film can be
fogged or blackened by radiation.
Where is this commonly used in medicine?

67
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68
Photographic Fogging

This principle is used in film badges
worn by radiation workers.
The darker the film the more radiation
the person has received.

69
Photographic Fogging
Why are there different materials in the film
badge?
70
Photographic Fogging
Different radiations pass through or are absorbed
by different materials.

71
Radiation and the Human Body

When the source of radiation is outside the
body, alpha radiation may not be able to harm
the vital internal organs as it is easily stopped
by the air, layers of clothing or the skin.
If swallowed an alpha radiation source is
extremely dangerous. It causes large amounts
of ionisation (remember it has a high ionisation
density) it changes or kills a lot of living
cells.
It cant escape from the body.

72
Alexander Litvinenko
Poisoned using extremely rare radioactive
substance Polonium-210 which is 250000 more
toxic than hydrogen cyanide. Swallowing a dose
less than 1/10th the size of a Smartie is lethal
for a grown adult male.
73
Radiation and the Human Body

Beta radiation will penetrate the first 1cm or
skin and tissue though, and will damage that
tissue. A small amount can penetrate the body.
If the beta source is inside the body, then it
will cause damage internally, for example to
organs.

74
Radiation and the Human Body

Gamma radiation will penetrate the skin and
tissue, and will deposit its energy as it travels
further into the body. It is more dangerous
than alpha or beta radiation in this case.
Gamma radiation inside the body will also
damage tissue however it can escape and be
detected from outside the body, and this makes
it very useful.

75
Making Use of Radioactivity

Gamma radiations ability to travel through skin
and tissue is used in medical and non medical
applications of radioactivity.

76
The gamma camera
77
Radioactive Tracers

A radioactive tracer is a gamma emitting
substance (a radiopharmaceutical) which
can be injected into the body to allow
internal organs and functions to be
investigated without surgery.

78
Radioactive Tracers

Technetium-99 and Iodine-123 are
commonly used because they emit only
gamma, which can be detected outside
the body, and cause little ionisation.
However, different substances are
chosen for different organs.

79
Radioactive Tracers

A gamma camera is used to detect
radiation from outside the body.

80
This scan is produced after a few hours of the
patient being injected with an isotope that emits
gamma radiation. A detector is moved around the
body and a computer produces an image. Dark areas
show high concentrations of radiation coming from
those parts. This indicates increased blood flow
to these parts.
Tumour fast growing hence increased blood supply
81
If a radioisotope that emits alpha radiation is
used, no particles can be detected outside the
body why not? Alpha radiation will be stopped
within a few centimetres. Internal organs will be
seriously damaged.
82
Isotopes that emit gamma radiation must be used
why? Since gamma rays will pass through the body
(and out) while doing the least damage.
83
Radioactive Tracers in Industry

Leaks in underground pipes can be detected
using radioactive tracers and a Geiger Counter.
A rise in count rate detected would indicate
more radiation escaping the pipe and therefore
a leak or crack.
Oil companies also use radioactive tracers in
shared pipelines to identify their own oil.

84
Radiation Therapy

Radiotherapy is commonly used as part of
treatment for cancer. It might be used
instead of surgery, or after surgery, or
chemotherapy, to destroy any remaining
cancer cells.

85
Treating Cancer (Radiotherapy)

Ionising radiation kills living cells. Cancers
are simply growths of cells which are out
of control and have formed tumours.
By directing radiation at the tumour, the
living cells are damaged or killed, and this
shrinks the tumour. Unfortunately healthy cells
are also damaged or killed by the radiation.

86
Treating Cancer (Radiotherapy)

It is important
to ensure that
healthy tissue
does not receive
too much
radiation while
the tumour
receives enough
to damage it.

87
Treating Cancer (Radiotherapy)

Video clips. http//www.ccotrust.nhs.uk/about/site
map/access_map.htm
The machine rotates around the patient.
The tumour can be hit by radiation all of
the time while minimising the damage to
healthy tissue. Each section of healthy
tissue receives only a small dose.

88
Treating Cancer (Radiotherapy)

Why are alpha and beta sources unsuitable
for radiotherapy treatments?
Alpha and beta are absorbed by
air/skin/bone so would not reach the
diseased tissue within the body. Instead
high energy X-rays are used.

Gamma could be used but rarely now because of
half life, safety, transportation issues.
89
Radiation Sterilisation

The ability of radiation to kill living cells
makes it very useful for sterilising
equipment e.g. plastic syringes in hospital.
Previously expensive metal or glass
syringes had to be used and sterilised using
heat or chemicals.
Using heat to kill germs and bacteria would melt
the plastic syringes.

90
Paper Thickness Measurement in Industry

Virtual Int 2 Physics -gt Radioactivity -gt
Ionising Radiations -gt Uses of Ionising
Radiations
A beta source and detector is used. If the
paper is too thin then the reading on the
detector will increase. If it is too thick, the
reading will decrease.
Why is an alpha source no use for this
application?

Key words activity, radioactive source, decays,
decays per second,
becquerels, absorbed dose, grays, radiation
weighting factor,
equivalent dose, background radiation level
By the end of this lesson you will be able to
State that the activity of a radioactive source
is the number of
decays per second and is measured in becquerels
(Bq), where
one becquerel is one decay per second.
Carry out calculations involving the relationship
between activity,
number of decays and time.
State that the absorbed dose is the energy
absorbed per
unit mass of the absorbing material.
State that the gray (Gy) is the unit of absorbed
dose and
that one gray is one joule per kilogram.

By the end of this lesson you will
be able to
State that a radiation weighting factor is
given to each kind of radiation as a measure of
its biological effect.
State that the equivalent dose is the product
of absorbed dose and radiation weighting
factor and is measured in sieverts (Sv).
Carry out calculations involving the relationship
between equivalent dose, absorbed dose
and radiation weighting factors.

By the end of this lesson you will be able
to
State that the risk of biological harm from
an exposure to radiation depends on a) the
absorbed dose b) the kind of radiation,
e.g. a, ß, ?, slow neutron
c) the body organs or tissue exposed.
Describe factors affecting the background
radiation level.

94
How much exposure is safe?

It should be stressed that no minimum
amount of exposure to radiation is
completely safe.
In Physics we aim to understand how to
measure radiation and to estimate the
risk of exposure. In many cases the
benefit of exposure significantly
outweighs the risks.

95
Radioactive Decay

Radiation is caused by the unstable nucleii
of radioactive atoms splitting up.
This is called radioactive decay.
Virtual Int 2 Physics -gt Radioactivity -gt
Dosimetry -gt Activity

96
Activity

We talk about the activity of a
source.
What do we mean by this?
The activity of a radioactive source is a
measure of the number of decays per
second.

97
Units of Activity

The becquerel is used to measure
the activity of a source.
1 becquerel (Bq) is one decay per second.

98
Activity
Number of nuclei decaying
Time (s)
Activity (Bq)
99
The becquerel

In practice, particularly in medical
treatment, the Bq is too small. Larger
units such as kBq and MBq are commonly
used.

100
Dosimetry Absorbed Dose

When radiation reaches the body or
tissue it is absorbed.
This is called the absorbed dose (D).

101
Dosimetry Absorbed Dose
Energy (J)
Mass (kg)
Absorbed dose units?
102

Dosimetry

ABSORBED DOSE (D) is the energy absorbed PER UNIT
MASS of absorbing tissue.
Units are GRAYS (Gy)
1 Gy 1 J/kg
103
Dosimetry
Radiation Treatment Absorbed dose (Gy)
Chest X-ray 0.00015
CT Scan 0.05
Gamma rays which would just produce reddening of skin 3.0
Dose which if given to whole body in a short period would prove fatal in half the cases 5.0
Typical dose to a tumour over a six week period 60.0
104
Biological Harm from Radiation

Radiation can damage living cells through heat
or damage to molecule structure such as DNA.
The risk of biological harm from an
exposure to radiation depends on
the absorbed dose
the type of radiation (e.g. alpha, or other
nuclear particles such as neutrons)
the body organs or type of tissue

105
EQUIVALENT DOSE (H) is a quantity which takes
into account the TYPE OF RADIATION.
WR is the WEIGHTING FACTOR of the particular
radiation
Unit of equivalent dose is sieverts (Sv)
106
Typical Equivalent Dose
Investigation Equivalent dose (mSv)
Chest X-ray 0.1
Spine X-ray 2.0
Stomach X-ray 4.0
CT Scan 1 to 3.5
Bone Scan 2.0
Annual exposure of aircraft crew 2.0
Renogram 2.0
Astronaut in space for one month 15.0
107
How much is a sievert (Sv)? If 100 people
received a dose of 1 Sv, 4 would die as a result.
This is the type of dose youd receive after a
nuclear accident. We normally work in
millisieverts (mSv Sv ) or
microsieverts (µSv Sv)
108

1 mSv
One thousandth of a sievert
0.001 Sv
1 µSv
0.000001 Sv

109
Example

A 50kg person is exposed to radiation of
energy 0.25J. The weighting factor for
the radiation is 20.
(a) Calculate the absorbed dose for this
radiation
(b) What is the equivalent dose?

110
Example

(a) Calculate the absorbed dose for this
radiation

111
Example

(b) What is the equivalent dose?

112
1 mSv is about 100 times the radiation you
experience when you travel by aircraft on
holiday. If you are part of the aircrew,
you will experience larger amounts due to the
amount of travel. There are regulations about
total flying times which take into account
exposure to radiation.
113
In the UK people receive an average of 2 mSv each
year from background sources (cosmic rays, radon
gas etc).
Legal limits have been set on the additional dose
equivalent which people can receive
Members of the public an additional 5 mSv each
year
Workers exposed to radioactivity - an additional
50 mSv each year
114
Background Radiation
Life on Earth has evolved to cope with this. Your
cells have self-repairing mechanisms which allow
them to survive relatively unscathed. The amount
of background radiation varies considerably
around Britain, as shown on the map. You can see
that it is particularly high in Cornwall, because
of the types of rock there.
115
Background Radiation

Background radiation is present all around us
from natural and artificial sources.
Sources which contribute to background
radiation are
radon from rocks and soil
Chernobyl and fall out from weapons
testing
medical uses of radiation
gamma rays from building materials
cosmic radiation from outer space
industrial use
nuclear industry

116
Chernobyl (April 1986)

Failure in safety
procedures meant
nuclear reaction
became out of
control
30 people died
immediately, a
Further 19 within
four months.
135000 were
evacuated from their
homes in a 20 mile
radius.

117
Long term consequences

Thyroid cancer increased ten fold with
biggest increases in children under 15.
Difficult to assess and much
controversy.

118

Key words activity, radioactive source, half
life, shielding, safety precautions
By the end of this lesson you will be able to
State that the activity of a radioactive source
decreases with time.
State the meaning of the term half-life.
Describe the principles of a method for measuring
the half-life of a
radioactive source.
Carry out calculations to find the half-life of a
radioactive isotope from
appropriate data
Describe the safety procedures necessary when
handling radioactive
substances.
State that the dose equivalent is reduced by
shielding, by limiting the time
of exposure or by increasing the distance from a
source.

119
Half-Life

Each radioactive substance has a different
half-life.
The half life is the time taken for
half the radioactive nuclei to
disintegrate OR the time taken for
the activity of a source to fall by one
half.

120
Radioactive Decay and Half Life

The activity of a
radioactive source
decreases with time.
Virtual Int 2 Physics Radioactivity Half Life

121
Radioactive Decay and Half Life

The graph of activity
(measured in counts per
second) against time has a
distinct shape.
Virtual Int 2 Physics Radioactivity Half Life

122
Radioactive Decay Half Life
Sketch a graph of activity against time
123
Finding the half life of a source

We can find the half life of a radioactive
source but we must remember to correct
for background radiation.

124
If we are measuring the activity of a source we
must always take off the background
radiation For example We measure background
radiation at 2 counts each second. We then
introduce a source and find that there are 47
counts each second. What is the radiation due to
the source? Source radiation total radiation
background radiation Source radiation 47 2
45 counts each second.
Background Radiation
125
Draw out this table
Time (s) Counts per second Corrected count rate
0
10
20
30
40
Continue to 250 seconds Continue to 250 seconds
126
Measuring Background Radiation

Counts in 60 seconds
Counts per second

127
Tasks

Use the data to plot a graph of corrected
count rate against time. Remember to
label axes and include units. Calculate 2 or
3 half life values from the graph and find
the average half life.

128
What makes a good graph?
129
Measuring the Half-Life of a radioactive source
Read the time taken for the activity to half.
You can choose any starting point.
The half life is found by calculating T2-T1.
T2
T1
130
Measuring the Half-Life of a radioactive source
T2
T1
131
Construct a table like this
1st activity 2nd activity T1 (s) T2 (s) Half-life (s)
80 40
70 35
60 30
Average half-life . s
132
Radioactive Decay and Half Life
133
Half Life Calculations

Below is a graph of corrected count rate
plotted against time

134
Time elapsed (mins) Count rate (counts/sec) Fraction of initial count rate
0 1200 1
10 600 ½
20 300 ¼
30 150 1/8
40 75 1/16
50 37.5 1/32
135
Half Life Calculations

A freshly prepared radioactive substance
has an initial activity of 60kBq. What will its
activity be after 1 hour if the half life is 15
minutes?
1 hour 4 x 15 minutes
So the substance has been through ? half lives

4
136
Half Life Calculations

After 1 half life the activity falls by half
From 60 kBq to ? kBq.
After 2 half lives, the activity halves again
From 30 kBq to ? kBq.

30
15
137
Half Life Calculations

After 3 half lives the activity halves again
From 15 kBq to ? kBq.
After 4 half lives, the activity halves again
From 7.5 kBq to ? kBq.

7.5
3.75
138
Half Life Calculations

A radioactive sample has an initial activity of
800 Bq.
What is the substances half-life if the activity
takes
24 years to decrease to 100 Bq?
Initial activity 800 Bq
After 1 half life 400 Bq
After 2 half lives 200 Bq
After 3 half lives 100 Bq
so in 24 years the substance has gone through 3
half
lives.
3 half lives in 24 years
1 half life in 24/3 8 years.

139
Radiation Safety
140
Protection when using radiation

There are three methods by which
radiation exposure can be reduced
Shielding a source with an appropriate thickness
of absorber
e.g. a radiographer wears a lead lined apron
e.g. radioactive sources are stored in lead
containers.

141
Protection when using radiation

There are three methods by which
radiation exposure can be reduced
2. Limiting the time of exposure
e.g. sources should be moved and used as quickly
as possible

142
Protection when using radiation

There are three methods by which
radiation exposure can be reduced
3. Distance from source
The further you are from the source the less
radiation you will receive. In fact, if you
double the distance you will receive only a
quarter of the radiation.

143
Radiation Safety
What safety precautions should be taken when
working with radioactive sources?
144
Radiation Safety

Use forceps or a lifting tool to remove a source
never bare hands.
Keep radiation window away from the body.
Never bring a source close to your eyes.
After any experiment with radioactivity, wash
hands thoroughly.

145
Radiation Safety

The symbol for radiation sources being stored
must be displayed where radiation is being used
or stored. It is an international symbol which
can be seen in hospitals, schools, colleges and
in
industry.

146
The Biological Effects of Radiation

The amount of damage caused depends
on
the absorbed dose
the kind of radiation
the body organs or tissue exposed
to the radiation.

147

The biological risk caused
by radiation is represented
by the equivalent dose
measured in
sieverts (Sv).

148
Questions

What is meant by ionisation?
2. (a) Why is ionising radiation dangerous.
(b) When is ionising radiation produced?
(c) Which is the most ionising of the three
types of radiation?
(d) Why is alpha radiation not dangerous if
the source is outside the body?
(e) Why is alpha radiation the most dangerous
if the source is inside the body?
3. (a) Why is it possible to use photographic
film to detect ionising radiation?
(b) Explain how a film badge works.
(c) How can fluorescent materials be used to
detect ionising radiation?
4. A radioactive source gives out one type of
radiation. A Geiger-Muller
tube and counter are used in an experiment to
determine the radiation present.
The detector is placed directly above the
source and the count rate measured
with different substances between the
detector and the source.
(a) What correction must be made to the count
rate before it can be used to
determine the type of radiation present
?
(b) The corrected count rate does not fall
significantly when a sheet of paper

149

Key words nuclear reactors, chain reaction,
fission, fuel
rods, moderator, control rods, containment
vessel, coolant,
nuclear waste.
By the end of this lesson you will be able to
State the advantages and disadvantages of using
nuclear
power for the generation of electricity.
Describe in simple terms the process of fission.
Explain in simple terms a chain reaction.
Describe the principles of the operation of a
nuclear
reactor in terms of fuel rods, moderator, control
rods,
coolant and containment vessel.
Describe the problems associated with the
disposal and
storage of radioactive waste.

150
Nuclear Power
Is nuclear power renewable or non-renewable? Stri
ctly non-renewable because the uranium fuel is a
finite resource. At the current rate of use the
existing reserves will last a long time. The
spent fuel can be re-processed and used again.
151
Nuclear Power What are the advantages?
A lot of energy is produced per kilogram of
uranium. - 1 kilogram of coal produces 30
million Joules 30 x 106 J or 30 MJ - A
kilogram of uranium produces 5 million million
Joules 5 x 1012 J of energy.
152
Nuclear Power What are the advantages?
Nuclear power plants generate relatively little
carbon dioxide so contribute little to global
warming. Technology is readily available and
well established. It is reliable. Large amount
of electricity can be generated by one plant.
Produces small amount of waste.
153
Nuclear Power do we rely on it?
In the UK, about 50 of energy is created from
nuclear sources. In France it is about 70.
154
Nuclear Power What are the disadvantages?
Nuclear power stations produce radioactive waste
which can be harmful to us and the
environment. The waste must be stored safely
for many years sealed and buried.
155
Nuclear Power What are the disadvantages?
Chernobyl demonstrated the risks of this type of
technology. Nuclear power is reliable, but a lot
of money has to be spent on safety - if it does
go wrong, a nuclear accident can be a major
disaster. People are increasingly concerned
about this - in the 1990s nuclear power was the
fastest growing source of power in much of the
world. In 2005 it was the second slowest-growing.

156
There are 3 main types of power station
THERMAL POWER STATIONNUCLEAR POWER
STATIONHYDROELECTRIC POWER STATION
157
Each type has the same basic plan
Thermal
Hydro-electric
Nuclear
Coal is burnedchemical energy to heat
Water behind dampotentialenergy to kinetic
Nuclear reactionnuclearenergy to heat
Turbinekinetic energy
Generatorkinetic to electrical energy
158

Coal stockpile
Pulveriser which breaks the coal down why is
the coal broken up before use?

159
3. Boiler coal is burnt to produce heat energy,
the heat boils the water to produce steam. The
steam is used to turn the turbines
160
4. Turbines these have hundreds of blades.
The steam from the boiler hits the blades
and turns the turbine. The turbine has a
shaft attached to it. As the turbine turns so
does the shaft. The shaft from the turbine is
connected to the generator.
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162
5. Generator
163
5. Generator the generator is made up of large
electromagnet and coils of wire. The
electromagnet is attached to the shaft from
the turbine and turns inside the wire
coils. As the electromagnet turns an
electrical current is produced in the coil of
wire.
164
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165
6. Transformer
166
6. Transformer the transformer increases the
voltage of the electricity from 20 000 V to
275 000 V. This allows the electricity to
be transported efficiently through the
electrical transmission system.
167
7. Cooling Tower after the steam has turned the
turbine it is piped to the condenser. Cold
water is pumped from the cooling towers where
it is used to cool the steam. After circulating
round the condenser the cooling water which
is now about 10 ºC warmer, flows back to the
cooling tower. The water is cooled by air and
then falls back down to the bottom of the
cooling tower to be recycled through the
condenser (8) again. Some of the heat from the
water is released into the air in the form of
water vapour which you can see coming out of
the top of the tower.
168
7. Cooling Tower after the steam has turned the
turbine it is piped to the condenser. Cold
water is pumped from the cooling towers
where it is used to cool the steam. After
circulating round the condenser the cooling
water which is now about 10 ºC warmer, flows
back to the cooling tower. The water is
cooled by air and then falls back down to
the bottom of the cooling tower to be
recycled through the condenser (8) again. Some
of the heat from the water is
released into the air in the form of
water vapour which you can see coming out of the
top of the tower.
169
Stages In A Coal-Fired Power Station

Coal stockpile
Pulveriser
Furnace Boiler
Turbines
Generator
Transformer National Grid
7. Cooling Tower
8. Condenser

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171
A Conventional (Fossil Fuel) Power Station
Energy Changes
172
Energy Efficiency

What do we mean by the efficiency of
a machine?
How can we write this as an equation?

173
Energy Efficiency
Units?
Why is the useful energy out always less than the
total energy input?
174
Efficiency
It can be useful to consider the energy each
second rather than total energy. What would the
equation be for efficiency using energy each
second?
175
Efficiency (as a percentage)
The efficiency of a power station (or any
machine) tells us how much of the input energy is
converted to useful output energy. Energy that
is LOST has been converted to less useful forms
such as heat.
176
Efficiency
177
Fuel Consumption
To determine the amount of fuel required
Note that power is energy each second so for a
given power output we can find the fuel needed
each second.
178
Nuclear Power
Like fossil fuels, uranium is mined. A lengthy
(and expensive) process is required to extract
the uranium from the ore.
179
Inside the Nuclear Power Station
http//science.howstuffworks.com/nuclear-power2.ht
m
180
Inside the Nuclear Power Station
In place of the boiler found in a conventional
power station, there is a reactor. Heat energy
produced during nuclear fission is carried
by carbon dioxide gas to a heat exchanger where
it heats water, turning it into steam. The
steam drives a generator to produce electrical
energy. The steam is cooled (turned back into
water) and pumped round for reuse.
181
Inside the Reactor

To obtain energy from uranium-235 nuclei, they
are
bombarded with neutrons. (What is a neutron?)
The neutron is absorbed by the uranium-235
nucleus making is unstable it splits into two
pieces
releasing a large amount of (heat) energy and two
further neutrons. This process is called fission.
http//library.thinkquest.org/26285/english/animat
ion.html

182
Chain Reaction

The two neutrons released then strike two
further uranium nuclei. This time four new
neutrons are produced which cause further
fissions, producing more neutrons and so on.
This continuous reaction of fissions is called
a chain reaction.
http//www.npp.hu/mukodes/anim/Uuu13-e.htm
http//www.npp.hu/mukodes/anim/div2a-e.htm

183
Nuclear Fission

The total mass at the end is less than the mass
at the
start. The lost mass has changed into energy
E mc2 m the loss in mass and c speed
of light

184
A Chain Reaction
185
A Chain Reaction
The 2 neutrons released during the nuclear
fission can go on to bombard further uranium
nucleii which causes further nuclear fission
releasing even more neutrons which can in turn go
on to produce more fission An uncontrolled chain
reaction is used in a nuclear bomb In a nuclear
power station the rate of reaction is controlled
using boron control rods which can be lowered
into the reactor and absorb the neutrons
that induce the fission process.
186
The fuel rods

These rods contain natural uranium which
is enriched so that fission can occur.
The amount of uranium in a fuel rod is well
below critical mass so that an explosion cannot
naturally occur.
Fuel rods have to be replaced every few years.

187
The Graphite Moderator
When the neutrons are emitted after fission
they are moving very fast. They will not be
able to be captured by other nuclei so fission
will not occur. If they are slowed down there
is a greater chance that fission will
occur. This is done using a graphite moderator
collisions with graphite atoms slow the neutrons
down.
188
Keeping the Chain Reaction under control

http//www.npp.hu/mukodes/anim/sta1-e.htm

189
The Control Rods
The amount of electrical power required will
vary with peak demand during the day and lower
demand at night. Rods of boron absorb additional
neutrons and control the number available for
fission. They can be raised and lowered as
necessary, and provide an important safety
feature. In the event of an accident, all rods
are lowered to absorb neutrons and stop the
chain reaction.
190
Coolant
The heat produced during the reaction must be
removed from the reactor. This is done using the
coolant normally carbon dioxide. The carbon
dioxide is continually heat, then passes the heat
to water via the heat exchanger. The water turns
to steam, which drives the turbine.
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192
Calculating amount of fuel required for power
output

To determine the amount of fuel required
to produce a given power output
Number of kg of fuel

total energy required
energy stored in each kg of fuel
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194
Energy Changes in a Nuclear Power Station
Note that in conventional fossil fuel power
stations AND in nuclear power stations the
energy source is used to raise steam to drive
turbines to drive the electricity generator.
195
Containment Vessel