Learning outcomes (1)

2. Learning outcomes (1) • Discovery of radioactivity and that three kinds of radiation exist. Know the nature and properties of each type of radiation • Applications: of alpha, beta and gamma rays • Rutherford’s alpha scattering experiment …the significance of the nuclear structure of atoms. • Application: enhanced understanding of chemical bonding • The principle of operation of an ionization chamber and other radiation detectors as well as appreciation of units like the Curie and Becquerel • Application:monitoring levels to ensure safety

3. Learning outcomes (2) • Concept of half-life, decay constant as measurable quantities as well as the random and uncontrolled nature of radioactivity • Application: Radiocarbon dating • Understand how energy may be obtained from nuclear reactions like fission and fusion • Application:Nuclear reactors

4. Radioactivity • In 1896 Becquerel found that some materials emitted radiation that blackened photographic plates.It was noticed that the radiation caused ionization and so could be detected by an electroscope • By 1900 it was realised there were three types of radiation.

5. Three different types of radiation • Deflection in magnetic fields revealed that α was positiveβ was negative andγ had no charge • α, β and γ each penetrate different distances…….have different energies of emission………and cause different levels of ionization.

6. Radioactive decay • The rate of decay of a radioactive source is proportional to the number of radioactive atoms (N) present: • λ is the decay constant. It is constant for a given isotope. • The solution of this equation is an exponential one, where N0 is the initial number of atoms present: • While radioactivity is a random and uncontrolled phenomenon, it is found that the time for half the nuclei in a sample to decay is constant. This is known as the “half life”

7. Half-life • Half-life is the time taken for half the radioactive nuclei present to decay. • A practical application of the idea of Half life is Carbon 14 dating • Half life of C-14 is 5730 years

8. Rutherford’s alpha scattering experiment Rutherford’s insight: • The atom must have a central core of positive charge ……with the electrons a good distance outside

9. Ionization chamber • When radiation enters the detector it ionises the gas inside. • A voltage exists between the central axis and the cylinder wall. • Negative ions are attracted towards the positive central axis and travel through the meter, registering a “count”.

13. Using a G-M tube to investigate the range of Alpha, Beta and Gamma radiation in air • Get the background count. • This is done by first setting the counter to zero without any radiation source nearby and then recording the number of counts over a 5-minute period. • From this calculate the number of counts per second. • Place the alpha source in front of the detector. • Find the average count rate per second. • Move the detector away from the source in small steps and calculate the average count rate at each step. • Continue until count rate equals background count rate. • Repeat for Beta source and Gamma source.

14. The Gamma radiation will be detected at the greatest distance (from source to detector), and Alpha radiation the least.

15. To demonstrate the ionizing affect of radioactivity • Procedure: Bring a radioactive source close to the cap of a charged Gold Leaf Electroscope • Observation: Leaves collapse • Conclusion: The charge on the G.L.E. became neutralised by the ionised air.

16. The effect of Ionising Radiation on humans depends on: • The type of radiation (whether it’s alpha, beta or gamma) • The activity of the source (in Bq) • The time of exposure • The type of tissue irradiated

17. Human Radiation Experiments in US 1944

18. Precautions when dealing with ionising radiation: • Make sure sources are properly shielded. • Keep sources as distant as possible from human contact, eg use a pair of tongs • Use protective clothing.

19. Background Radiation • Radiation from enviromental sources, mainly the Earths crust and atmosphere.

20. 1. Radon Gas – from granite rock • Responsible for almost all the raditaion we get exposed to in our lifetime. • Increases the risk of Lung cancer

22. The most common methods of reducing or preventing high radon concentrations in your home are: 1. Installing an active radon sump (sub-floor depressurisation) 2. Increasing ventilation 3. Passive sumps

23. 2. Cosmic rays (Space)

24. 3. Man-made radioactive materials

25. 4. Food Eg bananas 14Bq

26. What is Nuclear Energy? • Nuclear energy is the energy that comes from the core or the nucleus of an atom. • Massive amounts of it are available to make electricity. • It is released in 2 ways: • Fission • Fusion

27. Nuclear Fission • Nuclear fissionis the splitting of a nucleus into two smaller (roughly equal sized) nuclei, with the release of a large amount of energy and neutrons. Usually caused by the bombardment of neutrons. • It is usually accompanied by the release of large amounts of energy if the mass after the split is less than the mass before. • Energy is calculated through

28. Uranium235 • If bombarded with fast or slow moving neutrons (usually relatively slow moving) then fission occurs • U235/92 + N1/0 = Ba144/56 + Kr90/36 + 2N1/0 + KE

29. To Note: • Neutrons produced are fast moving (may trigger further fission • Loss in mass is known as mass defeat (KE) as identified by E=mc2 • eV used instead of Joules • Atomic mass units instead of Kg • (1amu = 1.67 X 10-34 Kg)

30. Sellafield Power Plant

31. Chain Reaction • During fission, a nucleus is split into two roughly equal fragments producing 2 or 3 neutrons. • If these neutrons are slowed down, they may produce further fissions producing more neutrons. • This is a chain reaction . It only occurs if a critical mass of material is available. video

32. Chain reaction definition • A chain reaction is said to occur if a least one neutron from each atom that undergoes fission triggers another fission reaction.

33. Critical mass • A chain reaction only occurs, if the mass of the fission material is such that one neutron is produced per fission. This is called critical mass.

35. Nuclear Reactor The reactor consists of • Fuel rods – enriched Uranium • Moderator - slow down the neutrons (Graphite) to enable further fission. • Control rods - to absorb neutrons. (slow/stop reactions) steel with cadmium or boron. • Shielding – thick concrete walls surround it. Lead is also used. Protect enviroment from radiation • Coolant – the nuclear energy is converted to heat. The reactor core is cooled using water or CO2 . • Heat exchanger – takes the heat to a turbine which drives an electrical generator

36. Enviromental Impact of Fission Reactors Advantages: • No co2 emissions • No green house gases • No gas released that would contrib to acid rain • Reduces dependence on fossil fuels • Vast amts of energy produced

37. Disadvantages • Radioactive waste produced • Storage of waste is difficult (T1/2 = 1000’s years) • Very expensive to reuse waste • Potential for disaster accident (eg Chernobyl 1986)

39. Nuclear Bomb • If a chain reaction continues uncontrolled, a nuclear explosion (video)occurs. • A certain mass (critical mass) of fissile material is required. • In the nuclear bomb, two masses of fissile material, each less than the critical mass, are brought quickly together to produce a nuclear explosion.

40. Nuclear Fusion • Nuclear fusion is the joining together of two small nuclei to form a larger nuclei with the release of large amt of energy. Two such reactions are the following

42. Tritium and Deuterium (Isotopes of Hydrogen) fuse to form an isotope of Helium, which then releases a neutron and energy

43. To overcome Coulomb repulsion (as both particles are +charged) , the nuclei must hit each other with enormous velocities. • Nuclei reach these velocities when heated to temperatures of around 100,000,000oC • Nuclear fusion in the interior of the sun is the principal source of the sun’s energy.

46. Hydrogen bomb • Uncontrollable fusion reaction and initial temperature for the H bomb is obtained from a fission reaction • USSR – Tsar Bomba 1961

47. Atomic bomb • Dropped in Hiroshima 1945 “little boy” • Uranium 235 • PLUTOIUM 239