N5 Physics Waves and Radiation

N5 Physics Waves and Radiation Learning Outcomes and Summary Notes Page 1

Waves and Radiation Sound: Learning Outcomes 1. Carry out calculations involving the relationship between distance, time and speed in problems on sound. 2. Describe a method of measuring the speed of sound in air, using the relationship between distance, time and speed. Waves: Learning Outcomes 1. State that a wave transfers energy. 2. Carry out calculations involving the relationship between distance, time and speed in problems on water, sound, radio and light waves. 3. Use the following terms correctly in context: wave, frequency, wavelength, speed, amplitude. 4. State the difference between a transverse and longitudinal wave and give examples of each. 5. Carry out calculations involving the relationship between speed, wavelength and frequency for waves. Page 2

Sound Waves Type of Wave Sound is a longitudinal wave. Speed of Sound In air the speed of sound is about 340 ms -1. In solids and liquids the speed of sound is faster than this. Speed of Sound Compared to Light Light waves, like all electromagnetic waves, travel through air at 300 million m/s (3 x 10 8 ms -1 ). Thunderstorms The light from a thunderstorm is seen before the sound of the thunder since the speed of light is much greater than the speed of sound. Page 3

Measuring the Speed of Sound The speed of sound in air can be measured using the experiment below. timer d Method Measure the distance that the sound travels. This is distance d between the microphones in the diagram above. Strike the bottle and measure the time it takes for the sound to travel that distance using an electronic timer. Calculation Speed can be calculated using the equation: distance travelled in speed in metres per second (ms -1 ) v = d t metres (m) time in seconds (s) Page 4

Method 2 Low tech Hand Clap Stand a distance, d, from the side of a building. Make a loud noise e.g. hand clap or short hoot using an air horn. Use a stopwatch to time how long it takes between making the sound and hearing the echo. The time for the sound to reach the building, t, will be half of this. Calculate the speed using the equation s = d / t. Example A firework explodes in the air and observers hear the sound 0.5 seconds after they see the explosion. How high in the air is the firework. (The speed of sound is 340 ms -1 ). t = 0.5 s d = vt v = 340 ms -1 = 340 x 0.5 d =? = 170 m Page 5

Waves: Summary Notes A wave is moving energy e.g. water waves transfer energy across the water. They transfer this energy without changing the medium (substance) through which they travel. Transverse Waves A transverse wave is one in which the particles making up the wave vibrate at 90 0 to the direction of the energy flow. Particles vibrate in this direction. Examples of transverse waves are water waves, light, gamma rays, X-rays and all members of the electromagnetic spectrum. Page 6

Longitudinal Waves A longitudinal wave s particles vibrate to and fro along the same line as the direction of the energy flow. Movement of molecules Compression Rarefaction Sound travels as a longitudinal wave. Page 7

Wave Properties Any wave can be described in terms of some basic properties. These are shown in the diagram below. crest A trough Definitions crest = highest point on wave trough = lowest point on wave = wavelength (the distance till a wave repeats itself) A = amplitude (the height of the wave from the rest position) Page 8

Frequency (f) The frequency of a wave is the number of waves produced in one second. This is the same as the number of waves that pass a point in one second. It is measured in hertz (Hz). Number of waves Frequency in hertz (Hz) f = N t passing a point time in seconds (s) Period (T) This is the time it takes for one complete wave to pass a point. Equation The equation linking the period and frequency is: f = 1 T Page 9

Example In the diagram below the distance between X and Y is 9 m and 20 waves pass B in 5 s. Use this information to find: a) the wavelength b) the frequency c) the period of the wave B X Y Solution a) XY = 3 complete wavelengths 3 = 9 m = 9 / 3 = 3 m b) F = N / t = 20 / 5 = 4 Hz c) T = 1/f = ¼ = 0.25 s Page 10

Wave Equation The wave equation is : speed in m/s or ms -1 v = f wavelength in metres (m) frequency in hertz (Hz) Example A microwave has a wavelength of 0.06m and travels at the speed of light. Calculate its frequency. v = 3 x 10 8 ms -1 f = v / = 0.06 m = 3 x 10 8 / 0.06 = 5 x 10 9 Hz Page 11

Diffraction Diffraction is the bending of waves around an object. Diffraction through a wide gap Diffraction through a narrow gap Radio Waves and TV Waves Diffraction of long wavelengths (e.g. radio waves) Diffraction of short wavelengths (e.g. TV waves) Radio waves have a longer wavelength (lower frequency) than TV waves. Radio waves will therefore bend further round a hill than TV waves and they can sometimes be received in the shadow of hills. Page 12

The Electromagnetic Spectrum: Learning Outcomes 1. State in order of wavelength, the members of the electromagnetic spectrum: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, TV and radio. 2. State that all electromagnetic signals are transmitted through air at 300 million m/s and that light is also transmitted at this speed. 3. Describe medical and non-medical uses of these radiations. 4. State that a radio station can be identified by the frequency of the signal it transmits. 5. State that infrared radiation is called heat radiation. 6. State that excessive exposure to ultraviolet radiation may produce skin cancer. 7. State that photographic film may be used to detect X-rays. 8. State that X-rays and gamma rays are dangerous since they can damage or change the nature of living cells. Page 13

The Electromagnetic Spectrum This is a family of waves with similar properties. increasing frequency increasing wavelength Speed of electromagnetic waves In air all electromagnetic waves travel at 3 x 10 8 ms -1 Example Microwaves have a frequency of 9.4 GHz and a speed of 3 x 10 8 ms -1. Calculate their wavelength. Solution v = 3 x 10 8 ms -1 f = 9.4 x 10 9 Hz v = f = v / f =? = 3 x 10 8 / 9.4 x 10 9 = 3.2 x 10-2 m Page 14

Radio Waves Radio Wave Transmission When you listen to a radio station you hear information that is carried by radio waves. The radio carrier wave carries the information from the radio transmitter over long distances. Radio waves travel at a speed of 3 x 10 8 ms -1 (the same as the speed of light). Sound waves travel at only 340 ms -1. A radio (carrier) wave looks like this A sound wave looks like this radio wave to form an amplitude modulated (AM) wave To transmit the sound, it is added to the or a frequency modulated wave Microwaves Microwaves are used in cooking (microwave ovens) and in communication. They are used to transmit information such as mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones in their range. Some microwave wavelengths can pass through the Earth s atmosphere and can be used to transmit information to and from satellites in orbit. Page 15

Infrared Radiation Infrared radiation is absorbed by the skin and we feel it as heat. It is used in heaters, toasters and grills. It is also used for television remote controls and in optical fibre communications. Infrared cameras take pictures of the heat emitted from and object e.g. a thermogram. These pictures show different pictures as different colours and are used by doctors to detect tumours. Police helicopters have infrared cameras to find criminals in the dark. Ultraviolet Radiation Ultraviolet radiation is found naturally in sunlight. It causes the skin to become tanned or burned. Too much ultraviolet radiation can lead to skin cancer. Ultraviolet radiation is used for sun beds, security pens (to check for forged banknotes or concert tickets) and fluorescent lights. X-rays X-rays can pass through skin and soft tissue, but they do not easily pass through bone or metal. X-rays are used to detect broken bones. They are used in industry to find cracks or damage to metal components. X-rays can cause cancer so hospitals limit the dose of X-rays received by patients and staff. Page 16

Gamma Radiation Gamma radiation can pass through skin and soft tissue, but some of it is absorbed by cells. Gamma radiation can kill or damage cells and so can lead to cancer if too much is absorbed. Since gamma radiation can kill cells it can be used to sterilise surgical instruments, kill harmful bacteria in food and kill cancer cells. Page 17

Refraction of Light: Learning Outcomes 1. State what is meant by the refraction of light. 2. Draw diagrams to show the change in direction as light passes from air to glass and glass to air. 3. Use correctly in context the terms angle of incidence, angle of refraction and normal. 4. Describe the shapes of converging (convex) and diverging (concave) lenses. 5. Describe the effect of a converging and diverging lens on parallel rays of light. 6. State the meaning of long and short sight. 7. Explain the use of lenses to correct long and short sight. Page 18

Refraction of Light: Summary Notes When a light ray travels from glass to air or from air to glass, it will change direction, unless it is travelling along the normal. This happens because the speed of light is lower in glass than it is in air. Refraction is the change of speed of light as it passes from one material to another. air glass air 3 x 10 8 ms -1 2 x 10 8 ms -1 3 x 10 8 ms -1 r normal i r i The normal is a line drawn at right angles to a surface (interface). The angle of incidence i is measured between the incident ray and the normal. The angle of refraction r is measured between the refracted ray and the normal. When travelling from air to glass the ray of light slows down and bends towards the normal. When travelling from glass to air the ray of light speeds up and bends away from the normal. Page 19

More Refraction Diagrams air glass air Page 20

Refraction and Lenses Lenses are used to refract light. The ray diagrams below show the effect of converging (convex) and diverging (concave) lenses on parallel rays of light. convex or converging concave or diverging Convex lens focal length focal length The lens with more curvature has a shorter focal length. Page 21

Concave Lenses The concave lens diverges (spreads out) the light. Light and Sight Normal Vision cornea retina jelly lens Light enters the eye and is refracted by the cornea and the jelly lens. A sharp image is produced when the light is brought to a focus on the retina. A person with normal sight can bring the light from near and distant objects to a focus on the retina. Page 22

Long Sight In long sight the light entering the eye from objects near the eye is focussed long of the retina. This means that the person sees a blurred image of near objects. However, images of distant objects are sharp because the eye has enough refracting power to focus this light on the retina. Light from a near object enters as diverging rays. They are focussed behind the lens creating a blurred image. Short Sight In short sight the light entering the eye from distant objects is focussed short of the retina. This means the image of distant objects appears blurred to a person with short sight. However, a short sighted eye can focus the light from near objects on the retina producing a sharp image of these objects. Light from a distant object enters as parallel rays. The rays are focussed before the retina creating a blurred image. Page 23

Applications and Effects of Refraction Many optical illusions are due to the refraction of light. When viewed from the side a pencil appears to bend in water. In a swimming pool your feet appear closer to the surface than they actually are. This is because light changes speed (and direction) when travelling from water to air. Lenses are used in magnifying glasses, microscopes, telescopes and binoculars to magnify an image. Light is refracted as it passes through the lens. When the viewer looks through the lens the light rays appear to come from an enlarged object behind the lens. Page 24

Reflection of Waves: Learning Outcomes 1. State that light can be reflected. 2. Use correctly in context the terms: angle of incidence, angle of reflection and normal when a ray of light is reflected from a plane mirror. 3. State the principle of reversibility of a ray path. 4. Explain the action of curved reflectors on certain received signals. 5. Explain the action of curved reflectors on certain transmitted signals. 6. Describe the application of curved reflectors used in telecommunications. 7. Explain, with the aid of a diagram, what is meant by total internal reflection. 8. Explain, with the aid of a diagram, what is meant by the critical angle. 9. Describe the principle of operation of an optical fibre transmission system. Page 25

Reflection of Waves: Summary Notes Reflection from a Plane Mirror The diagram below shows the path of a ray of light when reflected from a mirror. The normal is a line drawn at 90 to the mirror. angle of incidence (i) angle of reflection (r) A B Law of Reflection This states that the angle of incidence is equal to the angle of reflection. i = r Principle of Reversibility of Light This states that a ray of light takes the same path in both directions. For example the light in the above diagram would take the same path if it started at A or B. Page 26

Curved Reflectors These can be used as transmitters and receivers of waves e.g. sound, infrared, microwaves, TV signals and satellite communication. Transmitting Dish Curved Dish Transmitting Aerial Transmitted Signal The aerial at the focal point of the dish transmits the outgoing signal towards the curved reflector. It is reflected and sent out in a directional parallel beam. Receiving Dish Curved Dish Transmitting Aerial Transmitted Signal The curved reflector collects the incoming signal and reflects it towards the aerial at the focal point of the dish. Larger area dishes can collect more signal energy and improve the strength of the signal received by the aerial. Page 27

Refraction and Critical Angle Refraction i The angle of incidence i is small, less than the critical angle C. The ray passes out of the glass and is refracted. Critical Angle weak reflection C The angle of incidence is now equal to the critical angle C. The angle of refraction is now 90 0. The ray emerges along the glass block. Some light is Page also 28 reflected.

Total Internal Reflection i r When the angle of incidence (i) is greater than the critical angle all light is reflected. This is called total internal reflection. Applications of Total Internal Reflection Information is sent in light pulses along the optical fibre made up of thin, flexible strands of glass. The diagram shows how light passes through an optical fibre. The light is repeatedly totally internally reflected. The signal passes along the fibre at a speed of almost 200 000 000 m/s. This is slower than the speed that light travels through air. Page 29

Radioactivity Ionising Radiations: Learning Outcomes 1. Describe a simple model of the atom which includes protons, neutrons and electrons. 2. State that radiation energy may be absorbed in the medium through which it passes. 3. State the range through air and absorption of alpha, beta and gamma radiation. 4. Explain what is meant by an alpha particle, beta particle and gamma ray. 5. Explain the term ionisation. 6. State that alpha particles produce much greater ionisation density than beta particles or gamma rays. 7. Describe how one of the effects of radiation is used in a detector of radiation. 8. State that radiation can kill living cells or change the nature of living cells. 9. Describe one medical use of radiation based on the fact that radiation can destroy cells. 10. Describe one use of radiation based on the fact that radiation is easy to detect. Page 30

Ionising Radiations: Summary Notes Atoms Every substance is made up of atoms. Nucleus Inside each atom there is a central part called the nucleus. The nucleus contains two particles: protons: these have a positive charge neutrons: these have no charge. Overall the nucleus has a positive charge Electrons Surrounding the nucleus are negatively charged electrons that orbit the nucleus. Diagram of an Atom Page 31

Neutral Atoms An uncharged atom will have the same number of protons and electrons. Consider the element helium which as two neutrons and two protons in the nucleus, and two electrons surrounding the nucleus. This can be represented as: Neutron Proton Electron atom of helium Page 32

Ions Atoms are normally electrically neutral because the number of negative electrons is the same as the number of positive protons. However, it is possible to add electrons to an atom or take them away. When an electron is removed a positive ion is formed. Positive helium ion (helium atom with one less electron). Ionisation This is the addition or removal of an electron or electrons from a neutral atom. The nucleus of the atom remains unchanged during this process. Page 33

Radioactive Atoms There are some atoms which have unstable nuclei which throw out particles to make the nucleus more stable. These atoms are called radioactive. The particles thrown out cause ionisation and are called ionising radiations. Radioactive hazard symbol Ionising Radiations There are three types of ionising radiation: beta alpha gamma Alpha particles These are the nuclei of helium atoms. They have 2 neutrons and 2 protons in the nucleus and are therefore positively charged. Symbol: 4 2 +2 Page 34

Beta Particles These are fast moving electrons. They are special electrons because they come from within the nucleus of an atom. They are cause by the break up of a neutron into a positively charged proton and a negatively charged electron. Symbol: 0-1 Gamma rays These are caused by energy changes in a nucleus. Often the gamma rays are sent out at the same time as alpha or beta particles. Gamma rays have no mass or charge and carry energy from the nucleus leaving the nucleus in a more stable state. Symbol: Summary Radiation Nature Symbol Alpha particle Helium nucleus Beta particle Fast electron Gamma ray High frequency electromagnetic wave 4 2 0-1 Notice: Gamma radiation has zero mass and zero charge. Page 35

Absorption of Radiation Alpha particles These will travel about 5 cm through the air before they are fully absorbed. They will also be stopped by a sheet of paper. They are absorbed by the surface of the skin. Beta particles These can travel several metres through air before being absorbed. They are also absorbed by a sheet of aluminium a few millimetres thick. They are absorbed by a few centimetres of skin and muscle. Gamma Radiation This type of radiation can travel several hundred metres in air. Small levels of gamma radiation can be stopped by a few centimetres of lead or about 1 metre of concrete. Higher levels of radiation can only be stopped by many centimetres of lead or many metres of concrete. This type of radiation is the most penetrating and most passes through the body. Page 36

Other Properties of, and Radation Alpha Particles These cause more ionisation than beta particles or gamma rays. They move more slowly than beta or gamma radiation. They travel through the air with an initial speed of about 15 million m/s which is about 5% the speed of light. Beta Particles These cause less ionisation than alpha particles. They travel through the air with an initial speed of about 180 million m/s which is about 60% the speed of light. Gamma Radiation These rays travel at the speed of light. They cause less ionisation than alpha or beta particles. Page 37

Detection of Radiation A Geiger-Muller (GM) tube is used to detect alpha, beta and gamma radiation. If any of these enter the tube, ions are produced resulting in a small current flow. The current is amplified and a counter counts the number of events giving an indication of the level of radioactivity. Geiger-Muller (GM) tube source counter Photographic Film Badge Photographic film badges are worn by workers who use radioactive materials e.g. in hospitals and in nuclear power stations. Photographic film blackens or darkens when it is exposed to radiation. The more radiation that is absorbed, the darker the film becomes when it is developed. The badge is covered by different absorbing materials. This allows the dose to be calculated accurately since different radiations penetrate the materials by different amounts. Page 38

Effects of Radiation on Living Things Danger 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. Medical Uses of Radiation 1. Treating cancer Radiation can be used in the treatment of cancer. The radioactive source, cobalt-60 emits gamma radiation that kills cancer cells. The source is rotated around the body, centred on the cancerous tissue, so the cancerous cells receive radiation all the time. However, as the source is moving the healthy tissue only receives the radiation for a short time and is therefore damaged less and can recover. Gamma rays Normal cells Cancer cells Page 39

Tracers (1) Radioactive tracers that emit gamma radiation help doctors to examine the insides of our bodies. A tracer can be injected into the bloodstream and monitored with a detector outside the body. This can identify blockages where the blood is not flowing as expected. Industrial Tracers In industry radioactive tracers can be used to find leaks or blockages in underground pipes, find the route of underground pipes and track the dispersal of waste. Page 40

Dosimetry: Learning Outcomes 1. State that the activity of a radioactive source is measured in becquerels, where one becquerel is one decay per second. 2. State that the absorbed dose D is the energy absorbed per unit mass of the absorbing material. 3. State that the gray Gy is the unit of absorbed dose and that one gray is one joule per kilogram. 4. State that the risk of biological harm from an exposure to radiation depends on: a) the absorbed dose b) the kind of radiation e.g. slow neutron c) the body organs or tissue exposed 5. State that a radiation weighting factor WR is given to each kind of radiation as a measure of its biological effect. 6. State that the dose equivalent H is the product of D and WR is measured in sieverts, Sv. 7. Carry out calculations involving the relationship H = DWR 8. Describe factors affecting the background radiation level. Page 41

Dosimetry: Summary Notes Activity The activity, A, of a radioactive source is the number of nuclear decays, N, per second. It is measured in becquerels. Activity in becquerels (Bq) A = N t Number of decays time in seconds (s) 1 becquerel (Bq) = 1 decay per second. Radioactive Decay As time passes the activity of a radioactive source decreases. To calculate the activity after a certain time the half life time of the radioactive substance is used. Page 42

Absorbed dose This measures the transfer of radiation energy to the body. The greater the energy transfer the higher the change there is 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. Absorbed dose is measured in grays (Gy) D = E m Energy absorbed in joules (J) Absorbing mass in kilograms (kg) 1 Gray (Gy) = 1 joule per kilogram (Jkg -1 ) Factors affecting Absorption of Energy The nature and thickness of the absorbing substance The type of radiation The total energy of the particles or photons absorbed Alpha radiation is absorbed within a fraction of a mm of tissue. This gives a very high absorbed dose because of he small absorbing mass. Page 43

The biological effects of radiation All ionising radiation can cause damage to the body. There is no minimum amount of radiation which is safe. 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 radiation, but the biological effects will be different. To solve this problem a radiation weighting factor WR is used which is simply a number given to each kind of radiation as a measure of its biological effect. Some examples are given below. WR 1 10 20 Type of radiation Beta particles / gamma rays Protons and fast neutrons Alpha particles Equivalent Dose The effective equivalent dose takes into account the relative risks arising from exposure of organs of the body to different types of radiation. It indicates the risk to health from exposure to ionising radiations. Equivalent dose measured in sieverts (Sv) Page 44 H = D x WR Absorbed dose measured in grays (Gy) Radiation weighting factor

Example A worker in the nuclear industry receives the following absorbed doses in a year. 30 mgy from gamma radiation, WR = 1 300 Gy from fast neutrons, WR = 10 Calculate the dose equivalent for the year. Solution Gamma radiation H = D x WR H = 30 x 10-3 x 1 = 30 x 10-3 Sv Neutrons H = D x WR H = 300 x 10-6 x 10 = 3 x 10-3 Sv Total = 30 x 10-3 + 3 x 10-3 = 33 x 10-3 Sv = 33 msv Page 45

Background Radiation Everyone is exposed to background radiation from natural and from man-made radioactive material. Background radiation is always present adn originates from the sources listed below: Rocks which contain radioactive material. Cosmic rays from the sun and outer space emit lots of protons which cause ionisation in our atmosphere. Building material contains radioactive particles and radioactive radon gas seeps up from the soil and collects in buildings, mainly due to lack of ventilation. The human body contains radioactive potassium and carbon. In some jobs people are at greater risk. Radiographers exposed to X- rays used in hospitals and nuclear workers from the reactor. Examples Natural Sources Annual Dose (msv) Man Made Sources Annual Dose (msv) From Earth 0.4 Medical 0.25 Cosmic 0.3 Weapons (fall out) 0.01 Food 0.37 Occupational 0.01 Buildings (Radon) 0.8 Nuclear Discharges 0.002 Total 1.87 Total 0.272 Typical Annual Equivalent Dose The annual dose equivalent per year is about 2 msv. Page 46

Half Life and Safety: Learning Outcomes 1. State that the activity of a radioactive source decreases with time. 2. State the meaning of the term half life 3. Describe the principles of the method for measuring the half life of a radioactive source. 4. Carry out calculations to find the half life of a radioactive isotope from appropriate data. 5. Describe the safety procedures necessary when handling radioactive substances. 6. State that the dose equivalent is reduced by shielding, limiting the time of exposure or by increasing the distance from the source. 7. Understand the process of nuclear fission. 8. Understand the process of nuclear fusion. Page 47

Half-Life and Safety: Summary Notes Radioactive decay is a random process. This means that for a radioactive source, it can never be predicted when an atom is about to decay. 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. Measuring half-life Firstly the background count is measured using a Geiger-Muller tube connected to a counter. The count rate is recorded several times (with no source present) and the average calculated. This is the background count and it is subtracted from all readings taken from the source. The source is placed a fixed distance from the Geiger counter and the count rate is recorded at regular time intervals. A graph is then plotted of count rate against time on a graph. GM tube Caesium 137 ratemeter Measure the time taken from any initial value of the count rate on the graph to half this value. The time for the count rate to keep halving stays the same. This is the half-life. Page 48

Example A Geiger-Muller tube and ratemeter were used to measure the half-life of a radioactive substance. The activity of the source was noted every 3 days. The results are shown in the table. Time (days) 0 3 6 9 12 15 18 21 24 27 30 Count Rate (counts/s) 80 55 40 28 20 14 10 7.9 5 3.8 2.5 By plotting a suitable graph, find the half-life of the source. Solution From the graph: The time taken to fall from : 80 counts/s to 40 counts/s = 6 days 40 counts/s to 20 counts/s = 6 days Half-life = 6 days Page 49

Example 1 A patient is to be given and injection of iodine-131 which has a half-life of 8 days in an investigation of her blood. The sample has an initial activity of 8.0 kbq. Calculate the activity of the sample after 24 days. Solution Number of half-lives = total time = 24 / 8 = 3 half-life 1 2 3 8.0 kbq 4.0 kbq 2.0 kbq 1kBq Each arrow represent one half-life time. Final activity = 1 kbq Example 2 The activity of a source falls from 80 MBq to 5 MBq in 8 days. Calculate its half-life. 1 2 3 4 80 MBq 40 MBq 20 MBq 10 MBq 5 MBq This takes 4 half-lifes (count the arrows) = 8 days One half-life = 8/4 = 2 days Page 50

Reducing Exposure to Radiation Radiation workers are at a higher risk of receiving large equivalent doses of radiation. It is therefore important that they take steps to reduce this risk. This is achieved by the following: Shielding 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. Any window used for viewing radioactive material should be made of lead glass. Distance Keep as far away from the radioactive materials as possible. Time Keep the times for which you are exposed to the material as short and as few as possible. Page 51

Safe Practice when Using Radioactive Sources 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. It should be identified by a colour or number. When in use, a source must be attended by an authorised person and it must be returned to a locked and labelled store in its special shielded box immediately after use. After any experiment with radioactive materials, wash your hands thoroughly before you eat. (This applies particularly to the handling of radioactive rock samples and all open sources.) In the U.K. students under 16 may not handle radioactive sources. Page 52

Nuclear Fission and Nuclear Fusion Nuclear Fission Nuclear power reactors use a reaction called nuclear fission. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors, because their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons. When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens: 1. the nucleus splits into two smaller nuclei, which are radioactive 2. two or three more neutrons are released 3. some energy is released The neutrons which are released can go on to split other nuclei in what is known as a chain reaction. Page 53

Nuclear Fusion Nuclear fusion involves two atomic nuclei joining to make a larger nucleus. Energy is released when this happens. The Sun and other stars use nuclear fusion to produce energy. The main reaction which occurs in stars is that hydrogen nuclei combine to form helium nuclei. Page 54