AQA GCSE 1b-6
Radioactivity
AQA GCSE Physics pages 94 to 105
AQA GCSE Science pages 298 to 309
October 18th, 2010
AQA GCSE Specification
RADIOACTIVE DECAY
11.6 What are the uses and dangers of emissions from radioactive substances?
Using skills, knowledge and understanding of how science works:
• to evaluate the possible hazards associated with the use of different types of nuclear radiation
• to evaluate measures that can be taken to reduce exposure to nuclear radiations
• to evaluate the appropriateness of radioactive sources for particular uses, including as tracers, in terms of the type(s)
of radiation emitted and their half-lives.
Skills, knowledge and understanding of how science works set in the context of:
• The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons.
• The atoms of an element always have the same number of protons, but have a different number of neutrons for each
isotope.
• Some substances give out radiation from the nuclei of their atoms all the time, whatever is done to them. These
substances are said to be radioactive.
• Identification of an alpha particle as a helium nucleus, a beta particle as an electron from the nucleus and gamma
radiation as electromagnetic radiation.
• Properties of the alpha, beta and gamma radiations limited to their relative ionising power, their penetration through
materials and their range in air.
• Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not.
• The uses of and the dangers associated with each type of nuclear radiation.
• The half-life of a radioactive isotope is defined as the time it takes for the number of nuclei of the isotope in a sample
to halve or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.
Atomic structure
An atom consists of a small
central nucleus composed
of protons and neutrons
surrounded by electrons.
An atom will always have
the same number of
electrons as protons.
A Lithium atom
protons
neutrons
electrons
Atomic and mass number
The atomic number of
an atom is equal to the
number of protons in its
nucleus.
The mass number of
an atom is equal to the
number of protons plus
neutrons in its nucleus.
protons = 3
neutrons = 4
electrons = 3
This Lithium atom has:
atomic number = 3
mass number = 7
Properties of protons, neutrons and electrons
Position in
the atom
Relative
mass
Relative
electric
charge
PROTON
nucleus
1
+1
NEUTRON
nucleus
1
0
ELECTRON
outside
nucleus
0.005
-1
Isotopes
The atoms of an element always have the same number
of protons.
Isotopes are atoms of the same element with different
numbers of neutrons.
The three isotopes of hydrogen
hydrogen 1
hydrogen 2
(deuterium)
neutrons
hydrogen 3
(tritium)
Note: The number after ‘hydrogen’ is the mass number of the isotope.
Radioactivity
The atoms of some
substances are unstable and
they give out radiation from
their nuclei all the time,
whatever is done to them.
These substances are said to
be radioactive.
The first three types of
radiation discovered were
alpha particles, beta
particles and gamma rays.
Henri Becquerel discovered
radioactivity in 1896
Detecting radioactivity
Radioactivity can be
detected using a Geiger
counter.
This clicks each time a
particle of radiation from a
radioactive substance
enters the Geiger tube.
Hans
Geiger
Alpha, beta and gamma radiation
An alpha particle is the same as a helium
nucleus.
It consists of two protons and two neutrons.
A beta particle is a high speed electron.
It has come from the nucleus where a neutron
has decayed into an electron and proton.
Gamma rays are very high frequency
electromagnetic waves.
They are produced when an unstable nucleus
loses energy.
The penetrating power of
alpha, beta and gamma radiation
Paper or a few
cm of air stops
alpha particles
1cm or 1m of air
of aluminium
stops beta
particles
Several cm of lead or
1m of concrete is
needed to stop
gamma rays
Choose appropriate words to fill in the gaps below:
nucleus containing protons
Atoms consist of a very small _______,
and neutrons, surrounded by _______.
electrons Atoms of the same
protons but
element will always have the same number of _______
isotopes of the same element will have different
different ________
neutrons
numbers of _________.
radioactive
The atoms of some substances are unstable and _________.
beta particles or gamma rays.
They may give off alpha or ______
Gamma rays are the most penetrating type of radiation,
alpha is the least.
_____
WORD SELECTION:
alpha
beta
protons
electrons isotopes
nucleus neutrons radioactive
Simulations
Build an atom - eChalk
Atomic Structure Quiz - by KT - Microsoft WORD
Hidden Pairs Game on Atomic Structure - by KT - Microsoft WORD
Types of Radiation - S-Cool section on types of radiations including an
animation of absorption and a couple of decay equations to fill in on
screen.
Andy Darvill's Radioactivity Pages
Understanding Radiation - National Radiological Protection Board Useful starting point to get at useful areas of the site.
BBC Bitesize Revision:
Introduction Page to AQA Radioactive Substances
Atoms & Isotopes
Alpha, beta & gamma radiation - what they are
Penetrating power of radiations - includes applet - also see page on
detecting radiations (two after)
Observing nuclear radiation
Notes questions from pages 94/298 & 95/299
1.
2.
3.
4.
5.
6.
7.
(a) What is radioactivity? (b) How was it first discovered? (c) How
is it detected nowadays?
(a) How did Marie Curie advance our knowledge of radioactivity?
(b) What did it cost her?
List the three first types of radioactivity discovered and state how
they differ from each other.
(a) Why do some atoms decay? (b) Why is the word ‘random’
used in the context of radioactivity?
Copy and answer questions (a), (b) and (c) on pages 94/298 and
95/299.
Copy the Key Points on page 95/299.
Answer the summary questions on page 95/299.
Observing nuclear radiation
ANSWERS
In text questions:
(a) No, the salts give out
radiation all the time.
(b) Yes.
(c) Because it is emitted
from the nucleus of
an atom.
Summary questions:
1. (a) Nucleus, protons and
neutrons.
(b) Nucleus, radiation.
2. (a) Alpha radiation.
(b) Beta or gamma
radiation.
3. Because they have an
unstable nucleus that can
become more stable by
emitting radiation.
Deflection by magnetic fields
S
Magnetic south pole
placed behind the rays
Alpha and beta particles are
deflected in opposite
directions due to their
opposite charges.
Due to their much larger
mass alpha particles are
deflected far less than beta.
Gamma rays are not
deflected because they are
not charged.
Deflection by electric fields
-
-
-
Alpha and beta particles are
deflected in opposite
directions due to their
opposite charges.
Due to their much larger
mass alpha particles are
deflected far less than beta.
+ + +
Electric field produced by
positively and negatively
charged plates
Gamma rays are not
deflected because they are
not charged.
Ionisation
Ionisation occurs when an atom
loses one or more of its electrons.
The atom becomes a positive ion.
Alpha particles cause intense
ionisation due to their large mass
double positive charge.
Beta particles cause moderate
ionisation.
Gamma rays only cause weak
ionisation because they are
uncharged.
Lithium atom
(uncharged)
Lithium ion
(positively charged)
Hazards of nuclear radiation
The ionisation cause by radiation can
damage or kill living cells.
This can lead to genetic mutation or
cancerous growth.
Alpha particles cause the greatest
amount of ionisation and are therefore
potentially the most dangerous type of
radiation. They are, however, the
easiest to shield against.
Safety precautions
The main precaution is to reduce the
dosage received to the minimum
possible.
To achieve this radioactive sources
should:
•
•
•
•
•
•
be stored in a lead-lined container
be handled for the minimum possible
time
be handled only with tongs
never be pointed at anyone
never be put in pockets
be checked by looking at them in a
mirror
Choose appropriate words to fill in the gaps below:
electric fields deflect alpha and beta
Magnetic and ________
opposite directions due to their opposite
particles in ________
charges Beta particles deflect more because their ______
mass is
________.
about 8000 times ______
less than alpha particles. Gamma rays,
being _________,
uncharged are not deflected by either type of field.
ionisation which can cause living cells
Radioactivity causes __________
mutation leading on to possibly
to undergo genetic _________
cancerous growth. It is therefore important to minimalise
alpha particles which cause the most
exposure especially to ______
intense ionisation.
WORD SELECTION:
mass uncharged opposite electric less ionisation mutation alpha charges
Simulations
Types of Radiation - S-Cool section on types of radiations including an
animation of absorption and a couple of decay equations to fill in on
screen.
Andy Darvill's Radioactivity Pages
Understanding Radiation - National Radiological Protection Board Useful starting point to get at useful areas of the site.
BBC Bitesize Revision:
Alpha, beta & gamma radiation - what they are
Penetrating power of radiations - includes applet - also see page on
detecting radiations (two after)
Deflecting radiations using electric and magnetic fields - includes
applets showing deflections
Detecting radiation using photographic film (badges) & GM tube includes applet testing penetrating power with GM tube detector
Hazards of radiation
Alpha, beta and gamma radiation
Notes questions from pages 96/300 & 97/301
1.
2.
3.
4.
5.
6.
Copy the table on page 96/300.
Draw both Figures 2 and 3 on page 97/301 and
describe how each of the three types of radiation is
affected by magnetic and electric fields.
(a) What is ionisation? (b) Why can ionisation be
dangerous? (c) Compare the ionisation caused by the
three types of radiation.
Copy and answer questions (a), (b) and (c) on pages
96/300 and 97/301.
Copy the Key Points on page 97/301.
Answer the summary questions on page 97/301.
Alpha, beta and gamma radiation
ANSWERS
In text questions:
(a) To stop the
radiation, so it can’t
effect objects or
people nearby.
(b) It is not deflected
by a magnetic or
an electric field.
(c) To keep the source
out of range.
Summary questions:
1. (a) Gamma
(b) Alpha and beta, gamma.
2. (a) Gamma (b) Alpha
(c) Beta
3. Radiation can knock
electrons from atoms. This
ionisation damages the
genes in a cell which can be
passed on if the cell
generates more cells.
Half-life
The half-life of a radioactive isotope is:
The time it takes for the number of nuclei of
the isotope in a sample to halve.
OR
The time it takes for the count rate from a
sample containing the isotope to fall to half its
initial level.
Examples of half-life
Uranium 238 = 4500 million years
Uranium 235 = 704 million years
Plutonium 239 = 24 100 years
Carbon 14 = 5600 years
Strontium 90 = 29 years
Hydrogen 3 (Tritium) = 12 years
Cobalt 60 = 5.2 years
Technetium 99m = 6 hours
Radon 224 = 60 seconds
Helium 5 = 1 x 10-20 seconds
Example – The decay of substance X
Substance X decays
to substance Y with a
half-life of 2 hours.
At 2 pm there are
6400 nuclei of
substance X.
Time
Nuclei of Nuclei of
X
Y
2 pm
6400
0
4 pm
3200
3200
6 pm
1600
4800
8 pm
800
5600
10 pm
400
6000
midnight
200
6200
When will the nuclei of substance X fallen to 50? 4 am
Question 1 – The decay of substance P
Substance P decays
to substance Q with
a half-life of 15
minutes. At 9 am
there are 1280 nuclei
of substance P.
Complete the table.
Time
Nuclei of Nuclei of
X
Y
9 am
1280
0
9:15
640
640
9:30
320
960
9:45
160
1120
10 am
80
1200
10:15
40
1240
How many nuclei of substance X will be left at 11 am?
5
Question 2
Substance E has a half-life of 3 hours. If at 8 am it has
a count rate of 600 per second, what will be its count
rate at 2 pm?
at 8 am count rate = 600 per second
2 pm is 6 hours later
this is 2 half-lives later
therefore the count rate will halve twice
that is: 600  300  150
count rate at 2 pm = 150 per second
Finding half-life from a graph
600
The half-life in this
example is about
30 seconds.
number of nuclei
500
400
300
200
100
half-life
0
0
20
40
60
80
time (seconds)
100
120
A more accurate
value can be
obtained be
repeating this
method for a other
initial nuclei
numbers and then
taking an average.
Question 1
The half-life is
approximately 20
seconds
900
800
700
activity (Bq)
Estimate the half-life of
the substance whose
decay graph is shown
opposite.
600
500
400
300
200
half-life
100
0
0
10
20
30
40
50
60
70
time (seconds)
80
90 100
Question 2
The count rate of a radioactive substance over a 8 hour
period is shown in the table below.
Draw a graph of count rate against time and use it to
determine the half-life of the substance.
Time
(hours)
Counts per
minute
0
1
2
3
4
5
6
7
8
650
493
373
283
214
163
123
93
71
The half-life should be about:
2½ hours
Choose appropriate words or numbers to fill in the gaps below:
half-life
The ________
of a radioactive substance is the time taken for
half of the _______of
the substance to decay. It is also equal
nuclei
to the time taken for the _____
count rate of the substance to halve.
5600 years. If today a
The half-life of carbon 14 is about _______
sample of carbon 14 has a count rate of 3400 counts per
minute then in 5600 years time this should have fallen to
1700
425
______.
11200 years later the rate should have fallen to ____.
The number of carbon 14 nuclei would have also decreased
eight times.
by ______
WORD & NUMBER SELECTION:
5600 nuclei eight half-life
425 1700 count
Simulations
Andy Darvill's Radioactivity Pages
Radioactive decay law - half-life graph - NTNU
Radioactive decay and half-life - eChalk
Half-life with graph - Fend
Half-life with graph - 7stones
Half-Life - S-Cool section on half-life and uses of
radioactivity including an on-screen half-life calculation
and an animation showing thickness control.
Hidden Pairs Game on
Half Life - by KT - Microsoft WORD
Understanding Radiation - National Radiological
Protection Board - Useful starting point to get at useful
areas of the site.
BBC Bitesize Revision:
Half-life
Half-life
Notes questions from pages 98/302 & 99/303
1.
2.
3.
4.
5.
6.
7.
8.
What are isotopes?
Copy both parts of Figure 1 on page 98/302.
What is meant by ‘count rate’?
Copy both ways of defining half-life found in bold type at the
bottom of page 98/302.
Copy Figure 2 on page 98/302 and explain how this type of
graph can be used to find the half-life of an isotope.
Copy and answer questions (a) and (b) on pages 98/302
and 99/303.
Copy the Key Points on page 99/303.
Answer the summary questions on page 99/303.
Half-life
ANSWERS
In text questions:
(a) 75 counts per
minute
(b) 6.5 hours
Summary questions:
1. (a) Unstable, stable
(b) Half-life, unstable
2. (a) 4 milligrams
(b) 1 milligram
Automatic thickness monitoring
The amount of radiation received by the
detector depends on the thickness of the
aluminium foil.
If the thickness increases then the
detector reading falls.
This will cause the computer to bring the
rollers closer together and so decrease
the foil thickness.
Beta radiation must be used.
- alpha would not pass through the
thinnest aluminium
- gamma would not be affected by any
thickness change.
A long half-life source must be used.
- or else a false thickness increase will be
detected.
Radioactive tracers (medical)
Radioactive tracers are used to follow the flow
of a substance through a system.
The gamma camera shown opposite can show
where a patient has absorbed a tiny amount of
radioactive substance.
Doctors can tell from the image obtained how
well particular organs are functioning.
Gamma radiation must be used.
- alpha or beta would not be able to pass out of
the patient’s body to the camera.
A short half-life source must be used.
- or else the source will irradiate the patient’s
body for a longer than needed time.
The radioactive substance must not be toxic nor
decay into a substance that is toxic or
radioactive.
Smoke detectors
A radioactive source inside the alarm
ionises an air gap so that it conducts
electricity.
In a fire, smoke prevents the radiation
causing ionisation.
The drop in electric current caused sets off
the alarm.
Alpha radiation must be used.
- beta or gamma would not cause sufficient
ionisation nor would they be affected by
smoke.
A long half-life source must be used.
- or else a drop in current would set off the
alarm
Radioactive dating
Radioactive Carbon 14 has
been used to try to find the
age of the Turin Shroud
Uranium in rocks can be
used to date formations
such as the Grand Canyon
Radiocarbon dating
Living material (for example a plant)
contains a known tiny proportion of
radioactive carbon. This has a half-life
of about 5600 years.
When the material dies it no longer
absorbs any more carbon. Therefore
the amount of radioactive carbon
decreases.
The age of the once living material can
be estimated by comparing its residual
radioactive carbon content with that of
living material.
Radiocarbon dating
has estimated that
the age of the Turin
Shroud is only about
one thousand years
– but this is disputed.
Uranium dating
Igneous rocks contain
radioactive uranium, which has
a half-life of 4500 million years.
Each uranium atom eventually
decays into a lead atom.
The age of a rock sample can
be worked out by comparing the
amount of lead to that of
uranium.
Uranium dating is
one of the methods
used to estimate he
age of the Earth
Choose appropriate words to fill in the gaps below:
tracer
A radioactive _______
can be used to detect leakages in
gamma source is added to the liquid
underground pipes. A _______
being transported by the pipe. The ground around the leak
radioactive
will therefore become ___________.
The source must produce gamma radiation because neither
beta
penetrate the
alpha nor ______
radiation would be able to _________
ground above the pipe to be detected.
half-life of the source must be long enough for it to
The ________
detectable but not too long as to cause long term
remain _________
radioactivity in the ground.
WORD SELECTION:
half-life detectable radioactive gamma beta penetrate tracer
Simulations
Various Radioactive Materials in the Home - 'Whys Guy'
Video Clip (4:30mins)
Andy Darvill's Radioactivity Pages
Understanding Radiation - National Radiological
Protection Board - Useful starting point to get at useful
areas of the site.
Radon Gas - National Radiological Protection Board
BBC Bitesize Revision:
Using radiation - tracers & thickness measurement includes applet showing sheet rolling application
Test bite on Radioactive Sources
Radioactivity at work
Notes questions from pages 100/304 & 101/305
1.
2.
3.
4.
5.
6.
7.
8.
Copy Figure 1 on page 100/304 and explain how radioactivity is
used in thickness control.
Copy and answer questions (a) and (b) on page 100/304.
(a) Describe one way of using a radioactive tracer for medical
treatment. (b) Why should such a tracer be a gamma emitter with
a half-life of a few days. (c) What other properties should the tracer
isotope have?
Explain what is meant by (a) carbon and (b) uranium dating.
Describe how a smoke alarm works.
Copy and answer questions (c) and (d) on page 101/305.
Copy the Key Points on page 101/305.
Answer the summary questions on page 101/305.
Radioactivity at work
ANSWERS
In text questions:
(a) The detector reading
increases and the
pressure from the
rollers is decreased.
(b) Alpha radiation would
be stopped by the foil.
Gamma radiation would
pass through it without
any absorption.
(c) B
(d) It was formed recently
(in geological terms).
Summary questions:
1. (a) Beta
(b) Beta or gamma
(c) Beta
2. (a) It needs to be
detectable outside the
body, non-toxic, have a
short half-life (1 to 24
hours) and decay into a
stable product.
(b) 11 200 years old.
Radioactivity issues
Notes questions from pages 102/306 & 103/307
1. Answer questions 1 and 2 on page
102/306.
Radioactivity issues
ANSWERS
1. The alpha particles are very ionising and
so cause a lot of damage to living cells. If
they get into the lungs they will do a lot of
harm. Our skin has a layer of dead cells
that prevent the particle reaching living
cells from the outside.
2. See page 97 figure 4.
How Science Works
ANSWERS
(a) Scores can vary. Have you considered
bias/impartiality/vested interests?
(b) Ethical: Should people make decisions about other
citizens that could affect their health?
Social: Lost jobs, poorer health services versus
environmental protection.
Economic: Are workers willing to take the increased
risk or order to earn more money? Are people willing to
pay more for their water to have it 100% safe?
Environmental: Mining uranium increases
radioactivity, but it is used to produce electricity in a
way that does not increase global warming.