Intermediate 2 Physics. Radioactivity

Transcription

1 Intermediate 2 Physics Radioactivity neutron electron proton ROR Page 1

2 Ionising Radiation What is an atom? Scientists believe that an atom is made up of three types of particle. 1. protons: positively charged particles 2. neutrons: uncharged particles 3. electrons: negatively charged particles There are equal numbers of protons and electrons in a neutral atom, the charges cancel out. electron neutron proton Collect an atom diagram; stick it into your jotter. Copy and complete the table below Particle Proton Neutron electron charge Q. How do the numbers of electrons and protons compare in a neutral atom? ROR Page 2

3 Radioactivity Effects Radiation carries energy. We will investigate radiation that is emitted from the nucleus of atoms. The three types of radioactivity investigated are: 1. Alpha Helium nucleus 2. Beta electron 3. Gamma electromagnetic radiation When these radiations are absorbed by any medium energy is transferred to the medium. Construct a table to show the information regarding the three types of radioactivity. Copy: All types of radiation will transfer energy to the medium it passes through. ROR Page 3

4 Absorption of radioactivity Radioactivity is dangerous because it can kill living cells. It can also change cells in such a way to make them cancerous. This means that it is important to be very careful when working with radioactive materials. Copy: Safety Precautions 1. Reduce your time in contact with radioactivity to as short a time as possible. 2. Never touch the radioactive source with your hands, use tongs or forceps. 3. Never point the source at the body, especially the eyes. 4. Monitor your radiation levels at all times. We can reduce harm by shielding, reducing time of exposure or increasing distance from source. [vicious dog] We will now investigate the penetrating properties of alpha, beta and gamma radiations. ROR Page 4

5 Absorption of radioactivity (cont.) radioactive source GM tube Counter Absorber Your teacher will demonstrate an experiment that shows how far different types of radioactivity can pass through materials. If you have completed the previous activity collect a diagram sheet, stick it into your jotter then copy out the following two tables. First the background radioactivity level must be measured. Background average counts in 30 seconds The average background count was in 30 seconds. ROR Page 5

6 Absorption of radioactivity (cont.) The background radiation is always around us. When the absorbed count is close to the background level we can be confident the radioactivity has been absorbed. alpha beta gamma counts in 30 seconds no absorber paper few mm few cm aluminium lead Copy and complete. The alpha radiation was stopped by. Alpha can only pass through a few cm of air. The beta radiation needed to stop it passing. Beta can pass through a few metres of air. The gamma radiation would only be stopped by. Gamma radiation can pass through many metres of air. ROR Page 6

7 Background radiation Background radiation is mainly natural radioactivity, all around us. As you can see from the pie chart, the vast majority of our annual dose comes from radon gas, food & drink, the ground, and cosmic rays. others, 0.20% food & drink, 11.50% background radiation in UK cosmic rays, 10% nuclear power/weapons, 0.30% radon gas, 50% medical, 14% ground/buildings, 14% Unless you are having radiotherapy, your dose from medical sources is quite low. The chart also shows that the nuclear industry adds very little to the level of background radioactivity. Many people don't realise that your radiation dose from cosmic rays is increased considerably if you fly a great deal. This is because our atmosphere provides some protection against cosmic rays, so the higher you fly the more you get. However, don't worry - this only tends to be a problem if you're an airline pilot or an astronaut! ROR Page 7

8 Background radiation (cont.) Use the information on the previous page to produce an A4 poster. The poster should be an information poster to inform non scientists of the sources of background radiation. ROR Page 8

9 Ionisation An ion is an atom that has an overall charge. This normally happens because an electron has been knocked off or an electron has been captured. When a neutral atom becomes charged we say it has been ionised. Atoms can be ionised by radioactivity. Because alpha particles are biggest they cause the greatest amount of ionisation. alpha beta gamma ROR Page 9

10 Ionisation (cont.) Collect a laptop. Log on. Go to Int. 2. folder open radioactivity Answer the following questions in sentences. 1. What is the overall charge on a neutral atom? 2. How do the number of protons and electrons compare in a neutral atom? 3. What happens to an atom when it is ionised? 4. Which type of radiation causes the greatest amount of ionisation? Explain your answer. 5. Which of the radioactive sources in the cardboard boxes below would be safest to transfer from A to B? Explain your answer. A B Attempt questions 4.1 to 4.5 from the problems booklet. ROR Page 10

11 Detectors of radiation The fact that radiation affects non-living material as well as living is very useful to us. It means we can detect the radiation without having to put someone into harm s way. Collect a laptop and log on. Visit the following web site and read through the materials carefully. Write a short note on the following radiation detectors. Your explanation should include how the radioactivity is detected by the radiation. Geiger Müller Tube Photographic film Scintillation detector When you have completed this task, form a small group and prepare a presentation on one type of detector. The detector does not have to be one of the three above. You can use the internet and any other resources in class or the library. ROR Page 11

12 Uses of radioactivity Radioactivity is useful in medicine because it can kill cells. The energy of the radioactivity is absorbed by the cells.this means it can be used to kill germs. Radioactivity is used to sterilise medical instruments such as syringes, scalpels, etc. The fact that radioactivity can kill cells is also of use in the treatment of cancer. If a concentrated beam of radiation is directed at a cancer site in a patient it may be possible to destroy the cancer. When a patient is being treated they need to lie very still so thet the beam of radiation does not miss the cancer. Also the beam is directed into the patient from many different angles so that healthy tissue recieves a small dose but the cancer recieves the full dose of radioactivity. ROR Page 12

13 The fact that gamma will pass through many materials makes it useful in medicine.gamma is used as a tracer, allowing the flow of a substance inside an object to be tracked. Detector Count Vein Blockage Distance along patients leg The radioactive tracer is injected into the patient. The detector is moved above the part of the patient being investigated. If there is a blockage then there will be an increase in count rate just before the blockage, then the count will fall to a low value after the blockage. Answer the following questions in sentences 1. Describe two uses of radioactivity in medicine that make use of the fact that radioactivity can kill cells. 2. What happens to the energy of the radioactivity in the above examples? 3. Give a use of radioactivity in medicine that makes use of the fact that radioactivity is easily detected. Answer question 4.4 from the problems booklet. ROR Page 13

14 Activity Dosimetry Copy The number of radioactive particles that are given off every second by a source is called its activity. Activity is measured in a unit called the becquerel (Bq). Example: radioactive source GM tube Counter The counter has been counting the radioactive decay of the source for four minutes. calculate the activity of the source. Activity = number of decays time = (4 x 60) = 50 Bq ROR Page 14

15 Effect on living things Copy Absorbed dose: This quantity gives a measurement of how much energy is absorbed by living material due to exposure to radioactivity. The absorbed dose is defined as the energy absorbed per unit mass. D = E m D absorbed dose gray Gy E energy absorbed joule J m mass of tissue kilogram kg The unit is the gray and 1 Gy is equivalent to 1 J/kg. Read: The absorbed dose does not give any indication of how much harm will be caused to living material. The main danger from radioactivity is the damage it does to the cells in your body. Most of this damage is due to ionisation when the radiation passes, although if levels of radiation are high there can be damage due to heating effects as your body absorbs the energy from the radiation, rather like heating food in a microwave oven. This is particularly true of gamma rays. ROR Page 15

16 Effect on living things (cont.) The harm to tissue depends then on three main factors 1. The absorbed dose a greater amount of energy absorbed will be likely to cause more harm. 2. The type of radiation absorbed if alpha gets into contact with tissue it will cause more damage because alpha ionises most. 3. The type of tissue exposed the eye is particularly susceptible to harm. Each type of radiation is assigned a weighting factor the radiation weighting factor W R which gives a measure of its ionisation potential. The harm is measured using the quantity the equivalent dose. [symbol H, unit sievert (Sv) ] H = D x W R 1. What is meant by the absorbed dose? 2. What quantity gives a measure of harm caused by radiation? 3. What factors affect the harm caused by radiation? Answer questions from the problems booklet. ROR Page 16

17 Half life and safety Half life If we measure the activity of any radioactive source we find that the activity gets smaller as time goes on. This is true for all radioactive sources. The time it takes for the activity to halve is called the half life of the source. This value is constant for each individual radioactive source. Examples Source Francium-221 Plutonium-241 Polonium-213 half life 4.8 minutes 13.2 years 4.2 x 10-6 seconds Half life is an important consideration in the choice of radioactive source for particular uses. The radioactive source for a smoke detector in the home needs a long half-life so that it does not need to be replaced too often. The radioactive source for a medical tracer needs a short half-life so that the patient is not radioactive for too long. ROR Page 17

18 Half life (cont.) Half life example If we had 16 atoms of francium-221 at the start then as time went on some would decay, leaving less francium-221 atoms behind. Every 4.8 minutes there would half of them decayed min 9.6 min 4.8 min 4.8 min 4.8 min Original 16 atoms One half-life 8 atoms One half-life 4 atoms One half-life 2 atoms It is impossible to predict which individual nucleus will decay. The process of decay is totally random. However we will always know that half of the total number of nuclei will decay in the time of the half life. ROR Page 18

19 Measuring Half-life radioactive source GM tube Counter Measure the background radiation. 2. Measure the activity of the source every 10 seconds. 3. Subtract the background value from the measured activity. This is called the corrected count. Half Life 4. Plot a graph of corrected count against time. Activity(MBq) Time (min) Collect a Measuring half life record sheet. Collect a laptop and conduct the half life experiment. Take five background readings and calculate an average background count rate. Use the graph you obtain to calculate the half life of Protactinium 234 ROR Page 19

20 Calculating Half life From a graph 1. Identify the starting activity. 2. Find the activity that has half this value. 3. Find the time from the graph for this change to happen. 1 2 Half Life Activity(MBq) half life Time (min) In the example above the half life would be 10 minutes. How long does it take for the activity to fall from 4000MBq to 2000MBq? It takes 10 minutes again for the activity to halve from 4000MBq to 2000MBq. Now attempt the half-life graphs sheet ROR Page 20

21 Calculating Half life (cont.) In most problems that you are set you will be given numerical rather than graphical information From numerical information: Example 1: A radioactive source has an activity of 3200 Bq. The activity is measured again 16 days later. The activity is now found to be 200 Bq. Calculate the half-life of the source. 1. First you must find how many half-lives have passed. Halve the value and draw an arrow. Continue this until you reach the required value Four half lives. 2. Use this information to calculate the half-life. 4 half-lives = 16 days. 1 half-life = 16 = 4 days 4 The half-life of the source is 4 days. ROR Page 21

22 Half life calculation (cont.) Example 2: A source has a half-life of 10 minutes. The activity is measured and found to be 6400 MBq. What would the activity be 1 hour later? 1. First you must find how many half-lives have passed. Number of half-lives = total time half life = 60 = Halve your activity this number of times(use the arrows) The final activity will be 100 MBq. Now attempt questions from the problems booklet. Now attempt the odd number questions up to q71 from page in the yellow book. ROR Page 22

23 Nuclear Reactors Nuclear Power Station Torness Nuclear power can be used for the generation of electricity. There are advantages of using nuclear power. In the normal working of the power station there will be no pollution emitted into the atmosphere. The power station requires a much smaller mass of fuel to produce the same energy output as a fossil fuel power station. It is not all good news though, the small amount of waste produced is highly radioactive and has a very long half life [ hundreds of thousands of years]. This means that the radioactive waste must be kept in some well shielded, secure place essentially for ever. ROR Page 23

24 Nuclear fission Nuclear fission is the process where a large nucleus splits into smaller [usually two] parts. This split causes energy and neutrons to be released. The energy can be used to produce electricity. The neutrons can be used to cause other nuclei to split and keep the reaction going. When a nuclear fission reaction is self sustaining it is called a chain reaction. A diagram representing a chain reaction is shown below. neutron nucleus fission fragments Neutron collides with nucleus Nucleus fissions releasing energy and more neutrons These neutrons hit other nuclei causing them to fission Process repeats ROR Page 24

25 Nuclear fission (cont.) This process could continue to repeat producing vast amounts of energy. This is what happens in an atomic bomb. If two new neutrons are released causing new fissions each time then after 100 fissions the total number of nuclei splitting will be: = !! Obviously if we want to make some practical use of the energy released then the reaction has to be controlled in some way. Collect a chain reaction diagram and label it. Copy the steps explaining the process of a chain reaction. 1. Why would it be unwise to start a chain reaction with no means of controlling it? 2. Uranium is the fuel used in chain reactions. Do uncontrolled chain reactions happen in nature? Explain your answer. 3. Suggest a method of controlling a chain reaction. ROR Page 25

26 Nuclear reactors Nuclear energy produces about one fifth of the UK electricity supplies. It is an important part of our energy profile. We will examine the structure of a nuclear reactor and learn how the parts work together. moderator control rods containment vessel heat exchanger fuel rods coolant Parts of reactor Fuel rods: made of enriched uranium. Source of energy for reactor. Moderator: made of graphite. Slows neutrons. Neutrons will cause fission only if they are slowed down. Control rods: made of boron. Absorbs neutrons. Removing neutrons will slow or stop reaction. Coolant: High pressure CO 2. Removes heat from the core to change water into steam. Containment vessel: Thick concrete. Needed to trap radioactive particles released during the fission process. ROR Page 26