2. A level Radioactivity 17/09/2015 Graham Perrin perring@edgehill.ac.uk

3. AQA Spec 17/09/2015

4. AQA Spec cont’d 17/09/2015

5. Safety 1.16+ only 2.RPO appointed 1.Use logged 1.Exposure time 2.Stored and labelled correctly 3.Adequate training – 1.Distance Many simulations available

6. Ionisation 17/09/2015 When radiation collides with neutral atoms or molecules it alters their structure by knocking off electrons. This will leave behind IONS – this is called IONISING RADIATION.  particle Electron

7. Radioactivity 17/09/2015 If a substance is capable of ALWAYS emitting radiation under any conditions we say it is ____________. There are three types of radiation: ALPHA, _____ and GAMMA. These types of radiation are always given off by rocks, _____, building materials, air and cosmic rays around us – this is called BACKGROUND RADIATION. Each type is capable of penetrating different materials:    Sheet of paper Few mm of _________ Few cm of lead Words – aluminium, beta, food, radioactive

8. Types of radiation 17/09/2015 1) Alpha (  ) – an atom decays into a new atom and emits an alpha particle (2 protons and 2 neutrons – the nucleus of a helium atom) 2) Beta (  ) – an atom decays into a new atom by changing a neutron into a proton and electron. The fast moving, high energy electron is called a beta particle. 3) Gamma – after  or  decay surplus energy is sometimes emitted. This is called gamma radiation and has a very high frequency with short wavelength. The atom is not changed. Unstable nucleus New nucleus Alpha particle Beta particle Gamma radiation

9. Inverse Square law for  17/09/2015 Uses light as a model for  radiation. (Both em waves)

10. The further the better! 17/09/2015

11. Background Radiation 17/09/2015 Radon gas Food Cosmic rays Gamma rays Medical Nuclear power 13% are man-made

12. Uses of radioactivity 17/09/2015 1) Medical uses – gamma rays can be used to destroy cancerous cells or to sterilise medical instruments 2) Tracers – a tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe: Gamma source Tracers can also be used to develop better plant fertilisers and in medicine to detect tumours: The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.

13. Uses of radioactivity 2 17/09/2015 Rollers Beta emitter Beta detector Paper

14. Dangers of radioactivity 17/09/2015 OUTSIDE the body  and  are more dangerous as  radiation is blocked by the skin. INSIDE the body an  source causes the most damage because it is the most ionising. Alpha Beta Gamma Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia.

15. Half life 17/09/2015 The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… At start there are 16 radioisotopes After 1 half life half have decayed (that’s 8) After 3 half lives another 2 have decayed (14 altogether) After 2 half lives another half have decayed (12 altogether) = radioisotope= new atom formed

16. 1 st law of radioactive decay 17/09/2015 If there are a lot of unstable nuclei in a sample, then it follows that many of them decay in a period of time. The fewer the nuclei, the fewer will decay.

17. A radioactive decay graph 17/09/2015 Time (Throw) Count = Number of dice 1 half life

18. Exponential decay 17/09/2015 A business is said to go EXPONENTIAL if it’s growth is proportional to the length of time its been open. The longer it’s been running, the faster it grows. Exponential decay is the inverse relationship. These types of relationship are governed by a natural ‘irrational’ number. Euler’s number (or Napier’s constant) e = 2.718…………

19. 2 nd Law of radioactive decay 17/09/2015

20. 3 rd law of radioactive decay 17/09/2015

21. Dating materials using half-lives 17/09/2015 Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. 8 8 Answer: The sample was originally completely uranium… …of the sample was uranium 4 8 2 8 1 8 Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old. 1 half life later…

22. Isotopes 17/09/2015 An isotope is an atom with a different number of neutrons: Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. Notice that the mass number is different. How many neutrons does each isotope have? A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

23. Neutrons and Protons 17/09/2015 Are there always equal numbers of protons and neutrons?

24. Transforming the nucleus 17/09/2015 As we have seen both  and  decay involve changes of the particles within the nucleus. This results in the radioactive isotope changing into a different element. 235 U 92 Loses 2 protons and 2 neutrons during  decay 235 U 92  4 2 231 Th 90 231 ? + 231 Th 90 Loses a neutron but gains a proton during  decay 231 ? + 91 00

25. Positrons 17/09/2015  decay occurs when the nucleus is unstable due to having too many protons.  decay occurs when the nucleus is unstable due to having too many neutrons. More recently a type of unstable nucleus that releases tiny positive particles (a positive version of an electron). This is called  + decay (or positron decay). This seems to be the opposite of normal  decay (from now on called  - decay) i.e. a proton changes into a neutron and a positron. 231 Th 90 Loses a proton but gains a neutron during  + decay 231 ? + 89 00 +1

26.  emission 17/09/2015 Almost all decays produce ‘over excited nuclei’ which ‘cool down’ by emitting gamma rays. Technetium 99m (meta-stable isotope of technetium) is a source that is widely used in medicine and has a half life of about 6 hours.

27. Scattering experiments 17/09/2015 Rutherford scattering Electron Diffraction. At high enough energies, electrons can behave as waves.

28. Rutherford’s size of a nucleus 17/09/2015 When the  is brought momentarily to rest ( “ having climbed as far as it can up the electrostatic hill ” ) the work done in bringing it to rest will just equal its initial kinetic energy. When the speed and hence the kinetic energy is zero, all the energy is now electrostatic potential energy. If the  momentarily stops when at a distance d from the (centre of) the nucleus of charge Ze, its electrical potential energy is This equals the initial kinetic energy of the  particle. Rutherford used an  source given to him by Madame Curie. The  energy was ~ 7.7MeV. For gold, Z = 79. Solving gives d ~ 3 ×10 -14 m. Compare this with the diameter of gold atoms When the a is brought momentarily to rest (“having climbed as far as it can up the electrostatic hill”) the work done in bringing it to rest will just equal its initial kinetic energy. When the speed and hence the kinetic energy is zero, all the energy is now electrostatic potential energy. If the a momentarily stops when at a distance d from the (centre of) the nucleus of charge Ze, its electrical potential energy is For gold, Z = 79. Solving gives d ~ 3 ×10 -14 m

29. Electron diffraction (Scattering) 17/09/2015 Low energy electrons > crystal information High energy electrons > nuclear information

30. Wavelength of an electron? 17/09/2015 De Broglie wavelength shows the duality of energy and matter. Most wavelengths are too small for effects to be noticeable in the macroscopic world.

31. Nuclear Radius 17/09/2015 This gives a result of the radius being 2.65 × 10 -15 m. We need to note the following: To get an appreciable electron scattering effect, we need to have electrons with a de Broglie wavelength of about the nuclear diameter. This requires very high energies. The electron diffraction minima are not zero, indicating that the boundary of the nucleus is fuzzy, not sharp. Since the boundary is not sharp, various methods of determining the radius give rather variable results, from 1.2 fm to 1.5 fm.

32. Nuclear radius / atomic number 17/09/2015 By carrying out electron diffraction for different elements, we can investigate nuclear density. R = r 0 A 1/3 As the nucleon number increases, the volume increases. [The term A 1/3 means the cube root of A, the nucleon number. The term r 0 is a constant with the value 1.4 × 10 -15 m. R is the nuclear radius.]

33. Nuclear fission 17/09/2015 Uranium nucleus Unstable nucleus New nuclei (e.g. barium and krypton) More neutrons Neutron

34. E = mc 2 17/09/2015 This is possibly the most important equation in the history of Science. What does it mean? The ‘daughter products’ of fission and the neutrons together weigh less than the original neutron and the uranium. The mass difference times the speed of light squared = the energy given off.

35. Chain reactions 17/09/2015 Each fission reaction releases neutrons that are used in further reactions.

36. Fission reactions summary 17/09/2015 Each fission reaction releases energy in the form of _______. In a nuclear power plant this heat is used to boil _______, which is used to drive turbines etc. The energy from each reaction is very ______, but there are ________ of reactions every second. The waste products from these reactions are __________, which is why nuclear power plants are ___________. Words – radioactive, water, billions, controversial, heat, small

37. Fuel Rods 17/09/2015 In a nuclear bomb, the material is packed tightly together above ‘critical mass’. In a reactor the fissionable material is distributed evenly throughout the reactor. The uranium is packed into cylinders called fuel rods and suspended throughout the reactor.

38. Control Rods 17/09/2015 Control rods, made of materials that absorb neutrons, are held above the reactor. These can be raised and lowered into the reactor to absorb the neutrons released in the chain reaction. If all the neutrons are removed the chain reaction stops.

39. Cooling the reactor 17/09/2015 The important product of a reactor is the heat energy. This is removed to heat water for steam to drive turbines. The heat may be taken out by a gas (like a fridge). These are called AGRs. If the heat is removed by pressurised water, the reactor is called a PWR.

40. Nuclear power stations 17/09/2015 These work in a similar way to normal power stations: The main difference is that the nuclear fuel is NOT burnt – it is used to boil water in a “heat exchanger”

41. Disposing of radioactive waste 17/09/2015 High level waste is immobilised by mixing with ____ making ingredients, melting and pouring the glass into steel containers. Intermediate waste is set in cement in _____ drums. The key to dealing with radioactive waste is to IMMOBILISE it. There are a number of ways of doing this depending on how __________ the waste is: The containers are then kept in stores, often _________. Words – glass, steel, underground, radioactive