17 Mar 31 Radioactivity For Slideshare

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soap bubble interference, radioactivity, alpha, betta, gamma radiation, cloud chamber

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17 Mar 31 Radioactivity For Slideshare

  1. 1. Today: Radioactivity Introduction <ul><li>Some of our goals for the next few lectures </li></ul><ul><ul><li>Learn about the physics of radioactivity and nuclear reactions. </li></ul></ul><ul><ul><li>Learn about the hazards and safety mechanisms </li></ul></ul><ul><ul><li>Be able to apply the physics to real-life debates related to nuclear physics </li></ul></ul>
  2. 2. A soap bubble or film is a thin layer of water stabilized by detergent molecules http://www.funsci.com/fun3_en/exper2/exper2.htm Water: speed of light slower Air: speed of light fast Air Incident light will reflect and pass through interfaces. Reflections off the top surface and bottom surface will interfere with each other KEY POINT: 180 degree phase shift when reflecting off interface going from fast to slow (top interface in this case) Detergent layer
  3. 3. 180 degree phase shift explains the black color in the thinnest part of the soap film http://www.funsci.com/fun3_en/exper2/exper2.htm Water: speed of light slower Air: speed of light fast Air When film is very thin, path length difference is negligible. So interference is destructive . Appears black because I used black mug. As film gets thicker, there is some path length difference, so interference is not completely destructive – white color. Wikipedia.org Detergent layer
  4. 4. Rachelle Regan
  5. 5. Jocelle B. <ul><li>Rachelle: “I do not agree with the book because when I look at my picture of my bubbles I do see a lot of green” </li></ul><ul><li>John Z.: “I will say that the predominant colors will be the green and magenta. Yellow and cyan dot appear as much.” </li></ul><ul><li>Laura F.: “I saw greens, blues, reds, and violets. I think the author should explain how varied angles of light striking the bubble can have an effect on color. “ </li></ul>(click on movie)
  6. 6. If we were to learn and apply the mathematical physics, we could very well predict the pattern of colors! You know which colors are missing / present (e.g. magenta = blue + red) It is a really nice way to apply your knowledge of physics!
  7. 7. Today: Radioactivity Introduction <ul><li>Some of our goals for the next few lectures </li></ul><ul><ul><li>Learn about the physics of radioactivity and nuclear reactions. </li></ul></ul><ul><ul><li>Learn about the hazards and safety mechanisms </li></ul></ul><ul><ul><li>Be able to apply the physics to real-life debates related to nuclear physics </li></ul></ul>
  8. 8. First! Cloud Chamber Demo <ul><li>We will see the trails of particles from radioactive decay! </li></ul>Andrew Foland / Cornell University / Wilson Lab / Slide Projector Alcohol-soaked side Dry Ice Super-saturated alcohol vapor Alpha particle source Let’s observe the demo first, and then learn more about what we saw. http://www.youtube.com/watch?v=chxv5G6UFl0&feature=related You can build one yourself!
  9. 9. One slide about charge (electrical charge) Particles in the universe can have positive (+), negative (-) or neutral (zero) charge Opposite charges attract each other Same charges repel each other “ is proportional to” <ul><li>Physics can exactly predict the force between two charged objects: </li></ul><ul><ul><li>Increase charge = increase force </li></ul></ul><ul><ul><li>Increase separation = decrease force </li></ul></ul>The main reason I wanted to show you this slide: Charge is conserved; Charge is quantized (+1, +2, etc. -1, -2, etc.) “ Conservation of Charge” … akin to “conservation of energy” Force  Charge1 * Charge2 distance 2
  10. 10. Clicker Question—Conservation of charge <ul><li>Only one of these hypothetical events is allowed by the law of “conservation of charge.” Which one? </li></ul><ul><li>Two electrons (-1 charge each) combine to form a neutron (0 charge) </li></ul><ul><li>Two protons (+1 charge each) annihilate with two neutrons to form four gamma ray photons (0 charge) </li></ul><ul><li>An electron and a positron (+1 charge) annihilate to form two gamma rays (0 charge) </li></ul>
  11. 11. Clicker Question—Conservation of charge <ul><li>Only one of these hypothetical events is allowed by the law of “conservation of charge.” Which one? </li></ul><ul><li>Two electrons (-1 charge each) combine to form a neutron (0 charge) </li></ul><ul><li>Two protons (+1 charge each) annihilate with two neutrons to form four gamma ray photons (0 charge) </li></ul><ul><li>An electron and a positron (+1 charge) annihilate to form two gamma rays (0 charge) </li></ul>This actually happens!
  12. 12. OK, one more thing about charge Charged particles move in curved paths when in a magnetic field Neutral particles move in straight paths Path of a charged particle “ B” is magnetic field
  13. 13. Summary of charge characteristics Particles in the universe can have positive (+), negative (-) or neutral (zero) charge Opposite charges attract each other Same charges repel each other “ is proportional to” Charge is conserved Charge is quantized (+1, +2, etc. -1, -2, etc.) Charged particles move in curved paths when in a magnetic field Neutral particles move in straight paths Path of a charged particle Force  Charge1 * Charge2 distance 2
  14. 14. Refresher on the atom <ul><li>Atoms are composed of electrons (-1), protons (+1) and neutrons (zero) </li></ul><ul><li>The type of element is determined by the number of protons . </li></ul><ul><li>6 proton = carbon; 7 protons = nitrogen; 8 protons = oxygen </li></ul><ul><li>Two atoms with same number of protons , but with differing number of neutrons are different isotopes of the same element </li></ul>Helium Atom Proton Neutron Electron “Cloud”
  15. 15. Refresher on the atom <ul><li>Because the electron “clouds” surround the nucleus, most interactions are governed by the physics of electron-electron interactions i.e. the electrical force … “Electronic reactions” = Chemistry </li></ul><ul><li>The electrons have been involved in most of what we’ve been talking about: atomic spectra; fluorescence; interaction of matter and electromagnetic waves </li></ul><ul><li>When an atom loses or gains an electron, it is still the same element ! </li></ul><ul><li>(It is called an ion ) </li></ul>Helium Atom Proton Neutron Electron “Cloud”
  16. 16. X-rays discovered in 1895, Wilhelm Röntgen Cathode Ray Tube (CRT) Röntgen noticed fluorescence across the room (through books and papers) While experimenting with putting objects in the unknown beam, he saw images of his (wife’s) bones appear!
  17. 17. Shortly thereafter, radioactivity discovered by Henri Becquerel (1896) (1903 Nobel prize with the Curies) Uranium Salt Photographic film Expose to sunlight Cloudy day / Dark drawer for a couple days Much, much larger effect! The uranium itself was emitting radiation! This sparked the exciting age of nuclear physics, led by Rutherford
  18. 18. Clicker question—Invention of Radioactivity <ul><li>When did Physicists invent radioactivity? </li></ul><ul><li>~1900 </li></ul><ul><li>World War I </li></ul><ul><li>World War II </li></ul><ul><li>None of the above </li></ul>
  19. 19. Clicker question—Invention of Radioactivity <ul><li>When did Physicists invent radioactivity? </li></ul><ul><li>~1900 </li></ul><ul><li>World War I </li></ul><ul><li>World War II </li></ul><ul><li>None of the above </li></ul>Radioactivity has existed since the infancy of the universe! For example, the uranium that Roentgen used was naturally occurring. Uranium is about as abundant as arsenic. Molecules in the atmosphere are radioactive. *** The interior of the Earth is heated by radioactive decay!
  20. 20. Three kinds of radiation emitted via radioactive decay named before they were understood…alpha, beta, gamma Alpha particles stopped by paper alpha particles are helium nuclei Beta particles stopped by aluminum beta particles are electrons (or positrons) Gamma rays stopped by lead gamma rays are high energy photons x-rays similar, but lower energy than gamma
  21. 21. Clicker question…charge of radiation <ul><li>Let’s say by examination, physicists and chemists discovered that one type of the radiation was not deflected by magnetic fields. Which of the following would be true? </li></ul><ul><li>The un-deflected radiation must be charged. </li></ul><ul><li>The un-deflected radiation must be uncharged. </li></ul>
  22. 22. Clicker question…charge of radiation <ul><li>Let’s say by examination, physicists and chemists discovered that one type of the radiation was not deflected by magnetic fields. Which of the following would be true? </li></ul><ul><li>The un-deflected radiation must be charged. </li></ul><ul><li>The un-deflected radiation must be uncharged. </li></ul>Charged particles travel in curved paths when in magnetic fields! Path of a charged particle
  23. 23. Alpha, beta and gamma radiation also distinguished by charge behavior Alpha particles deflected by magnet Gamma rays unperturbed by magnet Beta particles deflected opposite alpha It was noticed early on that beta particles behaved exactly like cathode rays (electrons) Alpha particles were discovered to be helium nuclei Gamma rays were very high energy x-rays (photons)
  24. 24. Explanation of Geiger counter Depending on the design: Maybe detect alpha-radiation CAN detect beta-radiation Maybe detect gamma-radiation Alpha particles difficulty passing through window But can make it through mica Ionizing radiation (beta, gamma, some alpha) enter through window and create ions inside. These events are detected via the electric current and turned into audible “clicks.”
  25. 25. OK, Let’s try to reinforce some learning about alpha, beta, gamma radiation http://www.youtube.com/watch?v=NP9Woxbkr_M
  26. 26. The “Inverse-Square Law” First, an introduction to the concept using bread and a butter gun http://www.youtube.com/watch?v=JW3tT0L2gpc
  27. 27. The intensity of radiation follows the inverse square law Force  Charge1 * Charge2 distance 2 Intensity of radiation  Source intensity distance 2 Remember for electric charge: If you double the distance, Intensity decreases by a factor of 4
  28. 28. Let’s explore the inverse square law with a radiation demo <ul><li>We should be able to make predictions for how intensity of radiation will change. </li></ul><ul><li>Also can investigate properties of shielding </li></ul>Geiger counter Moveable shielding Radioactive source?

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