Download Waves notes section 5 - Nuclear radiation

National 4
Unit 1: Section 5
• I can identify natural and artificial sources of nuclear
radiation and associated medical and industrial applications.
• I can explain some the pros and cons of generating
electricity using nuclear fuel.
• I can make comparisons of risk due to nuclear radiation and
other environmental hazards.
• I can describe how to manage the risks associated with
radiation.
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National 5
Unit 1: Section 5
• I can describe the nature of alpha, beta and gamma radiation in
terms of relative effect of ionization, absorption, shielding.
• I can identify sources of background radiation.
• I can calculate absorbed dose, equivalent dose and make
comparisons of equivalent dose due to a variety of natural and
artificial sources.
• I can describe some applications of nuclear radiation.
• I can state that activity is measured in Becquerel’s.
• I can give a definition of Half-life.
• I can make use of graphical or numerical data to determine the
half-life of a source.
• I can give a qualitative description of fission and fusion,
emphasising the importance of these processes in the generation
of energy.
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Section 5: Nuclear Radiation
Alpha, Beta and Gamma Radiation
The Atom
Atoms are the smallest possible particles of the elements. Atoms
make up everything around us. The three main particles which make
up atoms are
All atoms have a tiny central nucleus which has a _________ charge.
We can imagine the ____________ charged electrons to be circling
around this, rather like planets around the sun. The nucleus contains
the ___________ protons and the neutrons, which are
_______________.
Ionisation
Ionisation is the break up of a neutral
atom into a positive ion and an electron.
The electrons near the outside of the
atoms are very light, and can easily be
knocked away from the atom. This can
happen if radiation from a radioactive substance passes nearby.
Because radiations from radioactive substances make ions so easily,
they are often called ____________ ____________.
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Ionising Radiations
When the alpha or beta or gamma radiation passes
through a material they lose _________ by colliding with
the atoms of the material. Eventually the radiations lose so much
_________ that they cannot get through (____________) the
material and so are ____________.
Alpha particles: (α
α) are the nuclei of helium atoms. They have 2
neutrons and 2 protons in the nucleus and are therefore
___________ charged.
Alpha particles will travel about 5 cm through the air before they are
fully absorbed. They will be stopped by a sheet of paper. Alpha
particles produce the greatest ionisation.
Beta particles: (β
β) are fast moving electrons and so are
___________ charged.
Beta particles can travel several metres through air and will be
stopped by a few millimetres of aluminium. They cause less ionisation
than alpha particles.
Gamma rays: (γγ) have ____ mass or charge and carry energy from
the nucleus leaving the nucleus in a more stable state.
Gamma rays can only be stopped by a very thick piece of lead. They
travel at the speed of light and very little ionisation.
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Detecting Radiation
Geiger-Muller Tube
The Geiger-Muller tube is a detector which use the effects of
ionisation to measure the amount of radiation present.
The central wire inside the cylindrical tube is kept at a high voltage
of about +400 V compared with the outer case. When radiation
enters the tube and produces a few ions, these are accelerated
towards the central wire. As they pass through the low-pressure gas,
they bump into atoms at high speed and knock out many more
electrons off. When they reach the central wire they send a pulse of
current round the circuit. These pulses are counted electronically by
the scalar or ratemeter, and so the amount of radiation being
detected by the G-M tube is measured.
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Measuring Background Radiation
Experiment 8
What you need: A Geiger-Muller tube, a scalar meter
What to do:
Switch on the counter for one minute and measure the amount of
radiation detected in the lab during this time. Reset the counter and
repeat the experiment twice more. Complete the table using your
own results and seven more obtained by other groups.
Measurement
number
1
2
3
4
5
6
7
8
9
Counts in 1
minute
Are all the values the same?
What is the average background count in counts per minute?
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Background radiation
Everyone is exposed to background radiation from natural and from
man-made radioactive material. Background radiation is always
present. Some of the factors affecting background radiation levels
are:
• Rocks which contain radioactive material, exposing us to ionising
particles
• Cosmic rays from the sun and outer space which emit lots of protons
which cause ionisation in our atmosphere
• Building materials containing radioactive particles and radioactive
radon gas from the soil and which collects in buildings, mainly due to
lack of ventilation.
• The human body which contains radioactive potassium and carbon.
• A persons chosen occupation. Radiographers exposed to X-rays used
in hospitals and nuclear workers
from the reactor.
Natural radiation is by far the
greatest influence on our exposure
to background radiation.
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Half-life
In any radioactive source, the activity decreases with
time because the number of unstable atoms gradually decreases
leaving fewer atoms to decay.
The half-life of a radioactive source is the time for the activity
to fall to half its original value.
To Find the Half Life of a Radioactive Source
1. Without the source being present measure the background count
rate with the Geiger counter.
2. Place the radioactive source in front of the Geiger Muller tube
and measure the total count rate (this is at t = 0).
3. Measure the count rate at regular intervals.
4. Correct all of the count rates for background radiation to find
the source count rate.
Source count rate = total count rate – background count rate
5. Draw a graph of source count rate against time.
6. Use the graph to find at least two values for the half-life of the
source (the time it takes for the count rate to half). Find the
average value for the half-life of the source.
(Instead of using the count rate of the source, the activity of the
source in Becquerels could be used)
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Example
1. A Geiger-Muller tube and ratemeter were used to measure the
half-life of radioactive caesium-140. The activity of the source
was noted every 60 s. The results are shown in the table. By
plotting a suitable graph, find the half-life of caesium-140.
Time (s)
0
Count rate 70
(corrected)
(count/s)
60
120
180
240
300
360
50
35
25
20
15
10
From the graph the time taken to fall from 70 counts/s to 35
counts/s = 120 s
35 counts/s to 17.5 counts/s = 120 s
Average half life of caesium-140 = 120 s.
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Dosimetry
Activity
The activity, A, of a radioactive source is the number of decays, N,
per second. It is measured
in Becquerel where:
1 Bq = 1 decay per second.
Number of Decays
Activity (Bq) = --------------------------Time (s)
N
A
t
Absorbed dose
The greater the transfer of radiation energy to the body the greater
the chance of damage to
the body. The absorbed dose, D, is the energy absorbed per unit mass
of the absorbing
material and is measured in grays, Gy.
1 Gy = 1 Joule per kilogram
Energy absorbed (J)
Absorbed dose (Gy) = ------------------------Mass (kg)
E
D
m
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Questions
1. A radioactive tracer has an activity of 160 Bq. The tracer has a
half life of 5 hours and decays for 15 hours.
What is its final activity?
2. A radioactive source with a half life of 2.5 minutes decays for 10
minutes. The source has an initial activity of 64 kBq.
Calculate the final activity of the source.
3. A sample of radioactive uranium has an initial activity of 600
kBq. After 10 days its activity has dropped to 150 kBq.
Use this information to calculate the half life of the source.
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4. The activity of an isotope varies with time as shown below. The
count rate is uncorrected for background radiation.
Count rate
230
190
160
130
110
95
80
70
0
1
2
3
4
5
6
7
(per minute)
Time
(hours)
The background count is 30 counts per minute.
a. Collect some graph paper from your teacher and lot a
corrected graph of activity against time for the isotope.
b. Calculate the half life of the isotope.
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The biological effects of radiation
All ionising radiation can cause damage to the body. The
risk of biological harm from an exposure to radiation depends on:
• the absorbed dose
• the kind of radiation
• the body organs or tissue exposed.
The body tissue or organs may receive the same absorbed dose from
alpha or gamma
WR
Type of Radiation
1
Beta particles/Gamma rays
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Protons and Fast Neutrons
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Alpha particles
Effects of radiation on living things
All living things are made of cells. Ionising radiation can kill or change
the nature of healthy
cells. This can lead to different types of cancer.
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Equivalent Dose
When scientists try to work out the effect on our bodies of a dose of
radiation they prefer to
talk in terms of equivalent dose. The equivalent dose H is the product
of D and WR.
equivalent dose = absorbed dose x radiation weighting factor
H
D
WR
Example
A worker in the nuclear industry receives the following absorbed
does in a year:
30mGy from Gamma radiation (WR =1)
300mGy from fast neutrons (WR = 10)
Calculate the equivalent dose for the year.
for Gamma
H = 30 x 10-3 x 1 = 30 x 10-3 Sv
for neutrons
H = 300 x 10-6 x 10 = 3.0 x 10-3 Sv
total
H = 30 x 10-3 + 3.0 x 10-3 = 33 x 10-3 Sv
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Question
A technician in a nuclear power station is exposed to several
types of radiation over a 150-hour working month.
She receives a dose 0.2 mGy due to exposure to fast
neutrons, 15 µGy due to α-particles, and an absorbed dose of
1mGy from gamma rays.
Calculate the technician’s total equivalent dose.
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Safety with Radioactivity
• Always use forceps or a lifting tool to remove a
source. Never use bare hands.
• Arrange a source so that its radiation window points away from
the body.
• Never bring a source close to your eyes for examination.
• After any experiment with radioactive materials, wash your
hands thoroughly before you eat.
Reducing the dose equivalent
• Use shielding, by keeping all radioactive materials in sealed
containers made of thick lead. Wear protective lead aprons to
protect the trunk of the body.
• Keep as far away from the radioactive materials as possible.
• Keep the times for which you are exposed to the material as
short and as few as possible
Radioactive hazard warning sign
The sign should be displayed on all doors where radioactive
materials are stored.
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Fission and fusion
Advantages of using nuclear power to produce electricity
• Fossil fuels are running out, so nuclear power provides a
convenient way of producing electricity.
• A nuclear power station needs very little fuel compared with a
coal or oil-fired power station. A tonne of uranium gives as much
energy as 25000 tonnes of coal. • Unlike fossil fuels, nuclear fuel
does not release large quantities of carbon dioxide and sulphur
dioxide into the atmosphere,which are a cause of acid rain.
Disadvantages of using nuclear power to produce electricity
• A serious accident in a nuclear power station is a major disaster.
British nuclear reactors cannot blow up like a nuclear bomb but
even a conventional explosion can possibly release tonnes of
radioactive materials into the atmosphere. (The Chernobyl
disaster was an example of a serious accident.)
• Nuclear power stations produce radioactive waste, some of
which is very difficult to deal with.
• After a few decades nuclear power stations themselves will have
to be disposed of.
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Nuclear fission
An atom of uranium can be split by a
neutron. This can produce two new
nuclei plus the emission of neutrons
and the release of energy.
Chain reaction
Once a nucleus has divided by fission, the neutrons that are emitted
can strike other neighbouring nuclei and cause them to split releasing
energy each time. This results in what is called a chain reaction as
shown below.
In a controlled chain
reaction, on average only
one neutron from each
fission will strike another
nucleus and cause it to
divide. This is what
happens in a nuclear power
station. In an uncontrolled
chain reaction all the
neutrons from each fission
strike other nuclei
producing a large surge of
energy. This occurs in
atomic bombs.
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The nuclear reactor
There are five main parts of a reactor as shown in the diagram below:
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1. The fuel rods are made of uranium which produces
energy by fission.
2. The moderator, normally made of graphite slows down neutrons
that are produced in fission, since a nucleus is split more easily
by slow moving neutrons.
3. The control rods are made of boron, and absorb neutrons when
lowered into the reactor, so that the reaction can be slowed
down. In the event of an emergency they are pushed right into
the core of the reactor and the chain reaction stops completely.
4. A cooling system is needed to cool the reactor and to transfer
heat to the boilers in order to generate electricity. British gascooled reactors use carbon dioxide gas as a coolant.
5. The containment vessel is made of thick concrete which acts as
a shield to absorb neutrons and other radiations.
Radioactive waste
Nuclear power stations produce radioactive waste materials, some of
which have half-lives of hundreds of years. These waste products are
first set in concrete and steel containers then buried deep under
ground or dropped to the bottom of the sea. These types of disposals
are very controversial.
Some scientists believe the containers will
keep the radioactive material safe for a
long time, other scientists are worried that
the containers will not remain intact for a
sufficient time.
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Fusion
Nuclear fusion is the process by which two or more atomic nuclei join
together, or "fuse", to form a single heavier nucleus. This is usually
accompanied by the release of large quantities of energy. Fusion is
the process that powers active stars, the hydrogen bomb and some
experimental devices examining fusion power for electrical
generation.
Many scientists are working to produce controlled nuclear fusion
reactions, but have not yet been successful. If they do succeed the
fuel and end products, hydrogen and helium, will not be dangerously
radioactive.
Nuclear fusion reactions require very high temperatures, such as
found in the core of the Sun. A few scientists have claimed to have
produced cold fusion reactions where they fused hydrogen into
helium at approximately room temperature. If they were possible,
such reactions would immensely help the world's energy problems.
However other scientists have been unable to duplicate the work and
the cold fusion claims have been discredited.
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Unit 1: Section 5 - Additional notes
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Unit 1: Section 5 - Additional notes
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Unit 1: Section 5 - Additional notes
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Unit 1: Section 5 - Additional notes
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