© boardworks ltd 2001 ks4 radioactivity. © boardworks ltd 2001 radioactivity contents what is...

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Page 1: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

KS4 Radioactivity

Page 2: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Radioactivity

Contents

What is Radioactivity?

Radioactivity in the environment.

Types of Radioactivity.

Half-Life.

Effects of Radioactivity.

Uses of Radioactivity.

Questions.

Page 3: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 4: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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)

Page 5: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 6: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 7: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Mica window

Argon gas

Geiger-Muller Tube

counter

Collision & ionisation

radiation

124125

Page 8: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 9: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 10: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 11: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 12: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 13: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Thin mica Thin aluminiumstops BETA

Thick leadstops GAMMA

Skin or paper stops ALPHA

How to block the three types of radiation.

Page 14: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 15: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 16: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 17: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 18: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 19: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 20: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Uses of Radioactivity

Sterilisation

Radiotherapy

Leak detection in pipes

Thickness/level control

Tracers in medicine

Dating rocks

Nuclear Power

Page 21: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 22: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 23: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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

Page 24: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Hydraulicram

detector

Thickness Control Mill

Electronic instructionsto adjust rollers.

Beta Source

Page 25: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 26: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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?

click

click

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.

Page 27: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

click

click

Page 28: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

Page 29: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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.

click

Page 30: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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!

Page 31: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Nuclear Power - fission

Fast neutron from previous decay

Kr

Ba

n

n

n

Uranium

Page 32: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

Nuclear PowerKr

Ba

n

n

n

n

Fis

sion

Uranium

Page 33: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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).

click

Page 34: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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%.

click

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.

Page 35: © Boardworks Ltd 2001 KS4 Radioactivity. © Boardworks Ltd 2001 Radioactivity Contents What is Radioactivity? Radioactivity in the environment. Types of

© Boardworks Ltd 2001

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