AQA GCSE 1b-6

Radioactivity

AQA GCSE Physics pages 94 to 105AQA GCSE Science pages 298 to 309

October 18th, 2010

AQA GCSE SpecificationRADIOACTIVE DECAY11.6 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 structureAn 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 numberThe 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.

This Lithium atom has:

atomic number = 3

mass number = 7

protons = 3

neutrons = 4

electrons = 3

Properties of protons, neutrons and electrons

Position in the atom

Relative mass

Relative electric charge

PROTON

NEUTRON

ELECTRON

nucleus

nucleus

outside nucleus

1

1

0.005

+ 1

- 1

0

IsotopesThe 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 neutrons

hydrogen 1 hydrogen 3 (tritium)

hydrogen 2 (deuterium)

Note: The number after ‘hydrogen’ is the mass number of the isotope.

RadioactivityThe 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 radioactivityRadioactivity 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 radiationAn 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:

Atoms consist of a very small _______, containing protons and neutrons, surrounded by _______. Atoms of the same element will always have the same number of _______ but different ________ of the same element will have different numbers of _________.

The atoms of some substances are unstable and _________. They may give off alpha or ______ particles or gamma rays.

Gamma rays are the most penetrating type of radiation, _____ is the least.

isotopes

radioactivenucleus

protons electronsalpha

neutrons

WORD SELECTION:

beta

isotopes

radioactive

nucleus

protonselectrons

alpha

neutrons

beta

SimulationsBuild 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 SubstancesAtoms & Isotopes Alpha, beta & gamma radiation - what they are Penetrating power of radiations - includes applet - also see page on detecting radiations (two after)

Observing nuclear radiationNotes questions from pages 94/298 & 95/299

1. (a) What is radioactivity? (b) How was it first discovered? (c) How is it detected nowadays?

2. (a) How did Marie Curie advance our knowledge of radioactivity? (b) What did it cost her?

3. List the three first types of radioactivity discovered and state how they differ from each other.

4. (a) Why do some atoms decay? (b) Why is the word ‘random’ used in the context of radioactivity?

5. Copy and answer questions (a), (b) and (c) on pages 94/298 and 95/299.

6. Copy the Key Points on page 95/299.7. 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.

S

Deflection by magnetic 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.

Gamma rays are not deflected because they are not charged.

Magnetic south pole placed behind the rays

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.

Gamma rays are not deflected because they are not charged.Electric field produced by

positively and negatively charged plates

+ + +

- - -

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 radiationThe 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 precautionsThe 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:

Magnetic and ________ fields deflect alpha and beta particles in ________ directions due to their opposite ________. Beta particles deflect more because their ______ is about 8000 times ______ than alpha particles. Gamma rays, being _________, are not deflected by either type of field.

Radioactivity causes __________ which can cause living cells to undergo genetic _________ leading on to possibly cancerous growth. It is therefore important to minimalise exposure especially to ______ particles which cause the most intense ionisation.

mass uncharged electricopposite chargesmutationionisation

WORD SELECTION:

less alpha

mass

uncharged

electric

opposite

charges

mutation

ionisation

less

alpha

SimulationsTypes 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. Copy the table on page 96/300.2. 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.

3. (a) What is ionisation? (b) Why can ionisation be dangerous? (c) Compare the ionisation caused by the three types of radiation.

4. Copy and answer questions (a), (b) and (c) on pages 96/300 and 97/301.

5. Copy the Key Points on page 97/301.6. 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 yearsUranium 235 = 704 million yearsPlutonium 239 = 24 100 yearsCarbon 14 = 5600 yearsStrontium 90 = 29 yearsHydrogen 3 (Tritium) = 12 yearsCobalt 60 = 5.2 yearsTechnetium 99m = 6 hoursRadon 224 = 60 secondsHelium 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 X

Nuclei of Y

2 pm

4 pm

6 pm

8 pm

10 pm

midnight 200

400

800

1600

3200

6400

6200

6000

5600

4800

3200

0

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 X

Nuclei of Y

9 am

9:15

9:30

9:45

10 am

10:15 40

80

160

320

640

1280

1240

1200

1120

960

640

0

How many nuclei of substance X will be left at 11 am? 5

Question 2Substance 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

0

100

200

300

400

500

600

0 20 40 60 80 100 120

time (seconds)

num

ber

of n

ucle

i

half-life

The half-life in this example is about 30 seconds.

A more accurate value can be obtained be repeating this method for a other initial nuclei numbers and then taking an average.

Question 1

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 80 90 100

time (seconds)

acti

vity

(B

q)

Estimate the half-life of the substance whose decay graph is shown opposite.

The half-life is approximately 20 seconds half-life

Question 2The 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)

0 1 2 3 4 5 6 7 8

Counts per minute

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:

The ________ of a radioactive substance is the time taken for half of the _______of the substance to decay. It is also equal to the time taken for the _____ rate of the substance to halve.

The half-life of carbon 14 is about _______ years. If today a sample of carbon 14 has a count rate of 3400 counts per minute then in 5600 years time this should have fallen to ______. 11200 years later the rate should have fallen to ____.

The number of carbon 14 nuclei would have also decreased by ______ times.

eight half-life5600 425 count1700

WORD & NUMBER SELECTION:

nuclei

eight

half-life

5600

425

count

1700

nuclei

SimulationsAndy 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. What are isotopes?2. Copy both parts of Figure 1 on page 98/302.3. What is meant by ‘count rate’?4. Copy both ways of defining half-life found in bold type at the

bottom of page 98/302.5. 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.6. Copy and answer questions (a) and (b) on pages 98/302

and 99/303.7. Copy the Key Points on page 99/303.8. 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 monitoringThe 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 detectorsA 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

Uranium in rocks can be used to date formations such as the Grand Canyon

Radioactive Carbon 14 has been used to try to find the age of the Turin Shroud

Radiocarbon datingLiving 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 datingIgneous 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:

A radioactive _______ can be used to detect leakages in underground pipes. A _______ source is added to the liquid being transported by the pipe. The ground around the leak will therefore become ___________.

The source must produce gamma radiation because neither alpha nor ______ radiation would be able to _________ the ground above the pipe to be detected.

The ________ of the source must be long enough for it to remain _________ but not too long as to cause long term radioactivity in the ground.

detectable radioactivehalf-life penetrategamma beta

WORD SELECTION:

tracer

detectable

radioactive

half-life

penetrate

gamma

beta

tracer

SimulationsVarious 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 workNotes questions from pages 100/304 & 101/305

1. Copy Figure 1 on page 100/304 and explain how radioactivity is used in thickness control.

2. Copy and answer questions (a) and (b) on page 100/304.3. (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?

4. Explain what is meant by (a) carbon and (b) uranium dating.5. Describe how a smoke alarm works.6. Copy and answer questions (c) and (d) on page 101/305.7. Copy the Key Points on page 101/305.8. 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.