© boardworks ltd 2001 ks4 radioactivity. © boardworks ltd 2001 radioactivity contents what is...
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© Boardworks Ltd 2001
KS4 Radioactivity
© Boardworks Ltd 2001
Radioactivity
Contents
What is Radioactivity?
Radioactivity in the environment.
Types of Radioactivity.
Half-Life.
Effects of Radioactivity.
Uses of Radioactivity.
Questions.
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Radioactivity
What is it?
Radioactivity occurs as a result of changes in the nucleus (the centre) of an atom.
Atoms which give out radioactivity are called radioactive isotopes.
The nucleus of a radioactive isotope is unstable.
It can become stable by emitting small particles and energy.
These particles and energy are called radioactivity.
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Radioactivity
Atoms can achieve stability in two ways: Alpha decay and beta decay.
ALPHA () DECAY
Alpha() decay
Uranium-238 Thorium-234
-particle(2 neutronsand 2 protons so 4 units of mass lost from the nucleus)
-ray
(an electromagnetic wave like light)
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Beta() decay-particle(an electron)
Carbon-14(Atomic number 6)
Nitrogen-14(Atomic number 7)
Radioactivity
BETA Decay ()
One proton is GAINED so the element changes to the next one in the Periodic table.
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Radioactivity
How do we detect it?
Photographic film
Cloud chamber
Gold-leaf electroscope
Spark counter
Geiger-Muller Tube - this is the detector we mostly use in the laboratory.
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Mica window
Argon gas
Geiger-Muller Tube
counter
Collision & ionisation
radiation
124125
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The Geiger Muller Tube
The detector is a metal tube filled with gas. The tube has a thin wire down the middle and a voltage between the wire and the casing.
When the radioactivity enters the tube, it ionises the gas in the tube. This produces a pulse of current which is amplified and passed to a counter.
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The Spark Detector
The spark detector consists of a metal grid and a metal strip. A high voltage is applied between the grid and the strip. The voltage is increased until electrical arcing (sparking) across the gap just occurs.
When ionising radiation ( and radiation, see later) is placed close to the detector there is a marked increasing in the amount of sparking.
High voltage supply
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Radioactivity and the environment -background radiation
Natural background radiation comes from several sources:
From the sun and space – this radioactivity is known as cosmic rays.
From naturally occurring unstable isotopes in rocks, building materials and food etc.
From human activity. For example, from nuclear waste and explosions. This is a very small proportion of the total background radiation.
granite
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12%
12%
14%
10%
52%
radon and thoron gas
food
medical x-rays
rocks and building materals
cosmic rays
The Relative Proportions of Background Radiation.
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Types of Radioactivity
Alpha particles - these are the big radioactive particles and are heavy and slow.
Alpha particles are helium nuclei (2 protons and 2 neutrons). They are positively charged with a charge of +2.
Beta particles - these are fast moving electrons. They are small and light and carry a negative charge of –1.
Gamma rays - these are very high frequency (and therefore high energy) electromagnetic waves. Being electromagnetic waves they are UNCHARGED.
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Thin mica Thin aluminiumstops BETA
Thick leadstops GAMMA
Skin or paper stops ALPHA
How to block the three types of radiation.
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Radioactivity
How can we block radiation?
particles are stopped by paper or skin.
particles are stopped by thin aluminium.
rays are stopped by thick lead.
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Penetrating and Ionising Power
particles are big and slow moving so they don’t penetrate very well and are stopped easily. Because they are big and heavy they are strongly ionising - they easily knock electrons off atoms.
particles are quite small and quite fast so they penetrate moderately. They are moderately ionising.
rays penetrate a long way without being stopped. However, they are only weakly ionising as they tend to pass through atoms rather than knocking off electrons.
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The half-life of a radioactive isotope is the time taken for half the atoms to decay.
The half-life of a radioactive isotope is the time taken for half the atoms to decay.
How many half lives would lead to 1/16 of the original atoms?
The graph on the next slide shows a decay curve for a radioactive material.
After 1 half life, half the atoms have disintegrated and only half remain. After another half-life, the activity has halved again so only a quarter of the original atoms survive.
Half Life
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activ
ity
time
Graph to show how the activity of a radioactive source changes with time.
One half-life, the original activity has halved.
One half-life, the original activity has halved.
Two half lives, the activity is now at a quarter of its original level.
Two half lives, the activity is now at a quarter of its original level.
The original activity.The original activity.100
50
25
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Effects of Radioactivity
Alpha beta and gamma cause ionisation in living cells. This can damage or kill the cell.
Low doses of this radiation can cause damage or create mutant cells.
Mutant cells can divide in an uncontrolled manner. This is cancer.
Higher doses of radiation kills cells, which causes radiation sickness.
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Alpha particles do not pass through the skin. However, inside the body they can damage cells in the local area. Therefore, substances that emit alpha particles are dangerous if inhaled or ingested.
Beta and gamma rays are dangerous when outside the body as they can penetrate the skin and damage the vital organs.
Effects of Radioactivity
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Uses of Radioactivity
Sterilisation
Radiotherapy
Leak detection in pipes
Thickness/level control
Tracers in medicine
Dating rocks
Nuclear Power
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SterilisationGamma rays are used to kill bacteria, mould and insects in food. This can be done even after the food has been packaged. It can affect the taste, but supermarkets like it because it lengthens the shelf life.
Gamma rays are also used to kill bacteria on hospital equipment. It is particularly useful with plastic equipment that would be damaged by heat sterilisation.
Gamma Sourceunsterilised sterilised
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Radiotherapy
A carefully controlled beam of gamma rays can be used to kill cancer cells. It must be directed carefully to minimise the damage to normal cells.
However, some damage is unavoidable and this can make the patient ill.
It is therefore a balancing act - getting the dose high enough to kill the cancerous cells, but as low as possible to minimise the harm to the patient.
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Leak detection in Pipes
The radioactive isotope is injected into the pipe. Then the outside of the pipe is checked with a Geiger-Muller detector, to find areas of high radioactivity. These are the points where the pipe is leaking. This is useful for underground pipes that are hard to get near.
The radioactive isotope must be a gamma emitter so that it can be detected through metal and earth. Alpha and beta rays would be blocked.
The isotope must have a short half life so the material does not become a long term problem.
GM tube
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Hydraulicram
detector
Thickness Control Mill
Electronic instructionsto adjust rollers.
Beta Source
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Thickness/level control - how it works
A radioactive source is on one side of the material and a detector on the other.
If too much radioactivity is getting through, then the material is too thin and the rollers open up a bit to make the material thicker.
If not enough radioactivity is detected then the rollers compress to make the material thinner.
This method is used in the manufacture of lots of sheet materials: plastics, paper, sheet steel.
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Thickness/level control
Why is a beta source used in paper mills and a gamma source in sheet steel ?
Why is it important for the source material to have a long half-life?
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The radiation must be able to pass through the material. Beta radiation will pass through paper but not steel.
A source with a long half-life will last a long time before it needs to be replaced.
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Tracers in Medicine
Certain radioactive isotopes at low concentrations are injected into people (or swallowed). The location of the isotope can then be observed in the body using a detector. This way any blockages or absorption rates can be checked.
Which type of radioactive source would you prefer in your body?
What sort of half-life would be best?
Gamma so that it does not cause too much cell damage.
Very short, a few hours so that the radioactivity decays quickly.
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Carbon Dating
All living things take in a little radioactive carbon-14 in photosynthesis, as well as the normal carbon-12. When living things die, they stop taking in carbon-14 and the carbon-14 present at death slowly decays to carbon-12 (half-life is 5600 years). The radioactivity due to the decay of carbon-14 can be used to date bones, wood, paper and cloth.
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Example
A fresh bone gives a radioactive count of 170 counts per minute. Another ancient bone of the same mass gives a count rate of 50 counts per minute. The background count is 10 counts per minute. How old is the bone?
Counts due to bones are 170 - 10 = 160 (fresh) and
50 - 10 =40 (ancient)
The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40.This is two half lives,
So the bone is 5600 x 2 =11200 years old.
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Nuclear Power
When a nucleus decays it gives out heat energy - that’s what keeps the centre of the earth hot!
In a nuclear power station, uranium-235 atoms decay and give out energy and neutrons.
Each time a uranium atom splits it produces 3 neutrons These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled.
In a nuclear bomb the reaction is not controlled, and the bomb explodes!
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Nuclear Power - fission
Fast neutron from previous decay
Kr
Ba
n
n
n
Uranium
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Nuclear PowerKr
Ba
n
n
n
n
Fis
sion
Uranium
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Questions1. A certain radioisotope has a half-life of 3 minutes.
What fraction of the original atoms are still unchanged after:a) 3 minutesb) 6 minutes and c) 9 minutes?
a) after 3 minutes (1 half life) half of the original atoms remain.
b) after 6 minutes (2 half lives) one quarter of the original atoms remain (1/2 x 1/2).
c) after 9 minutes one eighth of the original atoms remain (1/2 x1/2 x1/2).
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2. What percentage of the original activity of a radioactive substance remains after four half-lives have passed?
Four half lives, so fraction of original activity is
1/2 x1/2 x 1/2 x 1/2 =1/16
% = 1/16 x 100 =6.25%.
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3. A radioactive source has an activity of 1.5 MBq. How many decays would occur in 1 hour?
(Note: 1Bq= 1 decay per second).
1500000 decays every second. Therefore in one hour there will be:
60 x 60 x 1500000 decays
= 5.4 x 109 decays.
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4. In a laboratory where the background count is 25 c.p.m. the uncorrected count rate from a radioisotope falls from 960 c.p.m to 54 c.p.m over 1 hour 15 minutes. What is the half-life of the isotope?
Starting count due to radioisotope is 960-25 =935
After 75 minutes, end count rate is 54 -25 =29
29/935 approx 1/32 which is 5 half lives
75 mins is 5 half lives so 1 half-life is 75/5 =15 minutes
5. The half-life of a radioisotope is 3.7 days. How long would it take for the activity of a sample to fall to one sixty-fourth of its original value?
1/64 is 6 half-lives (1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2)
6 x 3.7 = 22.2 days.click