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Nucleus

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Nucleus

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  1. Nucleus D.K.

  2. Leaving Certificate Physics: Topics • Mechanics • Temperature • Heat • Waves • Vibrations and Sound • Light • Electricity • Modern Physics • Option 1: Particle Physics • Option 2: Applied Electricity PDST Resources for Leaving Certificate Physics2

  3. 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 PDST Resources for Leaving Certificate Physics3

  4. 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 PDST Resources for Leaving Certificate Physics4

  5. 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. PDST Resources for Leaving Certificate Physics5

  6. 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. Three different types of radiation PDST Resources for Leaving Certificate Physics6

  7. 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” PDST Resources for Leaving Certificate Physics7

  8. 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 Half-life PDST Resources for Leaving Certificate Physics8

  9. 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 PDST Resources for Leaving Certificate Physics9

  10. 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”. PDST Resources for Leaving Certificate Physics10

  11. Recap questions • Who discovered radioactivity? • How was it recognised that there were 3 types of radiation? • What did Rutherford conclude from the alpha scattering experiment? • What is the principle on which most radiation detectors work? • What does the symbol mean? PDST Resources for Leaving Certificate Physics11

  12. Fission • A neutron collides with a uranium nucleus . . . . . . . . . it shatters into two large fragments and some neutrons are released. • A typical fission reaction is shown below: PDST Resources for Leaving Certificate Physics12

  13. Nuclear reactors • The fuel rods contain uranium, whose nuclei undergo fission, releasing energy. • The control rods absorb neutrons to reduce the rate of production of energy. • If water is used as the coolant, it can also serve as a moderator. • The moderator slows down the neutrons released by fission to the sort of speeds at which they are more likely to cause further fission. • The coolant removes energy to a heat exchanger, where steam is generated to turn turbines as in a conventional power station. PDST Resources for Leaving Certificate Physics13

  14. ADVANTAGES of FISSION • It releases a huge amount of energy per kilogram of fuel. It causes less pollution of the environment than fossil fuels PDST Resources for Leaving Certificate Physics14

  15. DISADVANTAGES of FISSION • Reactors produce waste which emit dangerous radiation, that is difficult to dispose of, store, or recycle, safely.If nuclear radiation escaped it could cause serious illness or even death. • A reactor meltdown could occur. • In a meltdown, the fission chain reaction goes out of control, leading to an explosion, releasing great amounts of radiation. • Three Mile Island (USA) 1979. • Chernobyl (Russia) 1986. PDST Resources for Leaving Certificate Physics15

  16. A chain reaction is not possible in natural uranium where 99% is U-238 and 1% is U-235. Most of the neutrons released in fission reactions are not fast enough to cause fission in U -238 and are too fast to cause fission in U-235. Slowing down the neutrons and increasing the percentage of U-235 improve the chances of a chain reaction. Chain reaction PDST Resources for Leaving Certificate Physics16

  17. Fusion is where two light atomic nuclei combine to form a heavier nucleus with the release of a large amount of energy. An example is set out here: Tritium and Deuteriumfuse to form an isotope of Helium, which then releases a neutron and energy Fusion is a desirable source of energy because:- It relies on fuel that is cheap and abundant and It produces virtually no radioactive waste. Fusion PDST Resources for Leaving Certificate Physics17

  18. Applications of Radioactive Isotopes • used in industry in quality control, for measuring thickness (by absorption). • used in the irradiation of food to kill parasites, bacteria. (The food itself does not become radioactive). • used to sterilize medicalequipment. • used as tracers in medicine, to locate blockages in the circulatory system. • used as tracers in water and gas pipes, to locate leaks. • used in smoke alarms • Carbon-14 dating used by archaelogists PDST Resources for Leaving Certificate Physics18

  19. Radioactivity is a natural phenomenon Nuclear radiation is ionising A Geiger counter is used to detect radiation Fission is the splitting of a heavy nucleus yielding energy Fusion is the combining of two light nuclei yielding energy The core of a nuclear reactor contains fuel rods, control rods, moderator and coolant. A chain reaction is self-sustaining if sufficient neutrons from early fission events result in successful fission later. There are many uses of radioactive isotopes in industry and medicine Carbon-14 dating is a valuable technique for archaeologists and historians Radon gas could be a hazard to health Summary PDST Resources for Leaving Certificate Physics19