Wallace Hall Academy Physics Department. Radiation. Pupil Notes Name:

Transcription

1 Wallace Hall Academy Physics Department Radiation Pupil Notes Name:

2 Learning intentions for this unit? Be able to draw and label a diagram of an atom Be able to state what alpha particles, beta particles and gamma rays are Be able to state what materials can be used to absorb alpha, beta or gamma Be able to state what ionisation is and describe how ionising alpha, beta and gamma are Be able to describe how to detect radiations with a Geiger Muller tube, a scintillation counter and a film badge Be able to describe the effect of radiation on living things Be able to state what absorbed dose is Be able to perform calculations using the D = E/m formula Be able to state what equivalent dose is Be able to perform calculations using the H = Dw R formula Be able to state what the risk of biological harm depends on Be able to state sources of background radiation Be able to state the safety precautions required when handling radioactive sources. 2

3 Be able to draw the radioactive hazard sign Be able to state methods of reducing exposure to radiation Be able to describe medical and industrial uses of radiation Be able to state what the activity of a source is Be able to perform calculations using the A = N/t formula Be able to state what the half-life of a source is Be able to determine the half-life of a source from a graph or by calculation Be able to describe what nuclear fission and nuclear fusion are 3

4 NUCLEAR RADIATION The structure of the atom All matter consists of atoms which are very, very small. Atoms themselves have even smaller particles within them. In a group of 2 or 3 discuss what you think an atom looks like. Once you have discussed it, draw a picture of what you think an atom looks like and share it with the rest of the class. Draw and label a diagram of an atom in the space below. Labelled diagram of an atom 4

5 Alpha, beta and gamma There are 3 types of particles which an atom could emit if it was radioactive. They are alpha, beta and gamma. Complete the table below to describe what alpha, beta and gamma are (leave the last 2 columns blank for now). Type of radiation Symbol Picture Description Charge Absorbed by Ability to ionise Alpha Beta Gamma Absorbing alpha, beta and gamma Although they are all very small alpha, beta and gamma can be absorbed by different materials. paper aluminium lead 5

6 Ionisation Ionisation is when an atom loses or gains and electron to become a charged ion. Alpha, beta and gamma are called ionising radiation as they can cause ionisation. In a group of 2 or 3 discuss which of the 3 ionising radiations are most or least likely to cause ionization. Think about the size of them and their charge. Once you have discussed it with the whole class complete the table on page 5. Detecting radiation There are 3 principle methods used for the detection of radiation; a Geiger Muller tube, a scintillation counter or a film badge. Each has their advantages and disadvantages. Geiger Muller tube 6

7 Scintillation counter Film badge 7

8 Absorbed dose Absorbed dose is the energy absorbed per unit mass. D = E = m = Example 1. A 60 kg man absorbs 40 mj of radiation. Calculate the absorbed dose he received. 8

9 Equivalent Dose The equivalent dose is a measure of biological harm. It takes account of the amount of radiation absorbed (absorbed dose) and the type of radiation absorbed (radiation weighting factor). This is important as different types of radiation cause different amounts of damage. H = D = w R = The radiation weighting factor of some common types of radiation is shown below. Radiation Radiation weighting factor Alpha 20 Fast neutrons 5 Slow neutrons 3 Beta 1 Gamma 1 Example 1. A 52 kg man absorbs 93 mj of Alpha radiation. a. Calculate the absorbed dose she received. b. Calculate the equivalent dose she received. 9

10 As well as being quoted in Sieverts (Sv) equivalent dose is often quoted as an equivalent dose rate in Sieverts per hour (Sv h -1 ). There is an equation to go with this but it overcomplicates things and it is better that you understand the problem rather than trying to apply the equation. Example 1. A 54 kg radiation worker works a 6 hour shift and receives and equivalent dose rate from alpha particles of 0.04 msv h -1 during her shift. a. Calculate the total equivalent dose she receives during one shift. b. Calculate her absorbed dose during one shift. c. Calculate how much energy she absorbs during one shift. 10

11 The effect of radiation on living things In a group of 2 or 3 discuss what you think the radiation does to living cells. Once you have discussed it with the whole class fill in the answer below. Radiation can have a very damaging effect on living things. It can cause damage to living cells in 3 ways, In a group of 2 or 3 discuss what you think the risk of biological harm depends on. Once you have discussed it with the whole class fill in the answer below. The risk of biological harm from radiation is dependent on 3 things,

12 Background Radiation There is a small amount of radiation (alpha, beta and gamma) that we are exposed to all of the time. It is known as background radiation. Some of these are natural sources and some are artificial. A list of background radiation sources is shown below. Plot this data in a bar graph. Background Radiation Demonstration Using a Geiger Müller tube record the background radiation in the lab over a period of 1 minute. Reading 1: Reading 2: Reading 3: Average background radiation level = counts per minute 12

13 Example Equivalent Doses Complete the table below showing the equivalent dose received in a variety of scenarios. Equivalent Dose Source/Effect Eating a banana Using a CRT monitor for a year Normal daily dose from background radiation Chest X Ray Head CT scan Normal yearly dose from background radiation Spending one hour at the Chernobyl plant (roughly) in 2010 Maximum yearly dose for radiation workers Minimum dose linked to cancer Minimum dose to cause radiation poisoning Severe radiation poisoning Extreme radiation poisoning usually fatal Fatal dose even with treatment 10 minutes next to the Chernobyl reactor after meltdown on 26 April

14 Safety precautions when using radioactive sources Wherever possible we should avoid contact with radioactive sources but there are many ways that radioactive sources have enriched our lives in medicine, industry and through nuclear power. When we need to handle radioactive sources it is essential that certain safety precautions are upheld. In a group of 2 or 3 discuss what you think some sensible safety precautions would be when using radioactive sources. You should also consider ways of reducing your exposure. Once you have discussed it complete the leaflet below. Safety precautions when using radioactive sources 14

15 Uses of radiation There are a number of medical and industrial uses of radiation which we use to enrich our lives. Type of radiation used and why - Explanation of how it works - Medical use Radiotherapy Type of radiation used and why - Explanation of how it works - Medical use Sterilisation of medical equipment 15

16 Type of radiation used and why - Explanation of how it works - Medical use Radioactive tracer Type of radiation used and why - Explanation of how it works - Industrial use Checking for leaks in pipes 16

17 Type of radiation used and why - Explanation of how it works - Industrial use Checking the thickness of materials Type of radiation used and why - Explanation of how it works - Industrial use Smoke detectors 17

18 Activity The activity of a radioactive source is the number of decays per second. A = N = t = Examples 1. A radioactive source decays 3 x 10 5 times in 3 minutes. Calculate the activity of the source. 2. A radioactive source has an activity of 52 kbq. Calculate how long it will take to decay 4.2 x 10 6 times. 18

19 Half-life Over time all sources lose their radioactivity and their activity decreases. The half-life is the time taken for the activity of a source to halve. The half-life of a radioactive source is constant and does not change over time. Radioactive decay is a random process and there is no way of predicting when an individual atom will decay but the decay curves for large groups of atoms are very predictable. Experiment to show the radioactive decay of fifty 1p coins Aim: To observe how fifty 1p coins decay Method: Results: Toss number Coins left 19

20 Conclusion: 20

21 Simulation to show the radioactive decay of 100 atoms Aim: To observe how 100 atoms decay Method: Results: Time (s) Atoms left 21

22 Conclusion: 22

23 Simulation to show the radioactive decay of Protactinium Aim: To observe how Protactinium decays Diagram: Method: Results: Time (s) Count rate (Bq) Corrected count rate (Bq)

24 Conclusion: 24

25 Activity (Bq) Activity (Bq) Determining the half-life of a source from a graph Examples - Determine the half-life of the sources shown below. Decay curve Time (days) Decay curve Time (hours) 25

26 Activity (MBq) Activity (Bq) Decay curve Time (seconds) Decay curve Time (minutes) 26

27 Determining half-life by calculation Determining the half-life of a source by calculation is a straight forward process if carried out properly as shown below. Construct a table with 2 headings; time and activity Fill in all the information you are given in the question under each heading Make sure the first entry in the time column is 0 The time column should increase by one half-life per step The activity column should decrease by half per step Examples 1. The initial activity of a radioactive isotope is 400 Bq. The sample has a half life of 2 minutes and is allowed to decay for 8 minutes. Calculate the final activity of the isotope. 2. Radioactive rocks emit radiation, which can be harmful, if exposure to them is not controlled. Some rocks have an activity of 160 Bq and emit radiation over a 3 day period. Calculate the final activity of these rocks given that their half life is 12 hours. 27

28 3. A sample of radioactive uranium has an initial activity of 600 kbq. After 10 days its activity has dropped to 150 kbq. Use this information to calculate the half life of the source. 4. Calculate the half life of a radium spray source, which emits alpha radiation, given that it takes 45 minutes for the activity to drop from 2400 counts per minute to 75 counts per minute. 5. Calculate the initial activity of a radioactive source whose activity falls to 20 kbq in 16 minutes given that it has a half life of 2 minutes. 28

29 Nuclear Fission Nuclear fission is when a neutron causes a Uranium nucleus to split into 2 smaller nuclei, extra neutrons and energy. The extra neutrons created in a fission reaction can go on to cause further fission reactions creating even more neutrons to cause yet further fission reactions in a process called a chain reaction. This chain reaction process is what happens in an atomic bomb and also what happens in a more controlled way in a nuclear power station. 29

30 Generating Electricity Using Nuclear Fission There are 5 main components in a nuclear fission reactor which is used to generate electricity. Concrete shield Nuclear fuel rods Control rods Graphite moderator Coolant 30

31 Nuclear Fusion Nuclear fusion occurs at the extremely high temperatures found inside a star. It is the process of joining two small nuclei (typically hydrogen isotopes) together. This releases very large amounts of energy as is easily observed by looking at our own Sun. There is a lot of effort being made to see if the process of nuclear fusion could be replicated under controlled conditions on Earth for the generation of electricity. Nuclear fusion theoretically offers substantial improvements over conventional fission nuclear power plants: Improved operational safety Far less harmful waste Far easier to acquire fuel No ability to create materials for nuclear weapons 31