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Nuclear Physics and Radioactivity

Nuclear Physics and Radioactivity. Online Introduction to Nuclear Physics. http://www.sciencejoywagon.com/physicszone/lesson/12nuclear/intronuc.htm Online lesson on nuclear decay http://207.10.97.102/chemzone/lessons/11nuclear/nuclear.htm

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Nuclear Physics and Radioactivity

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  1. Nuclear Physics and Radioactivity

  2. Online Introduction to Nuclear Physics • http://www.sciencejoywagon.com/physicszone/lesson/12nuclear/intronuc.htm • Online lesson on nuclear decay http://207.10.97.102/chemzone/lessons/11nuclear/nuclear.htm • Nuclear Fusion http://ippex.pppl.gov/ippex/About_fusion/INDEX.HTML

  3. Protons and Neutrons • Atomic nuclei are made of protons and neutrons • Proton is positive mp = 1.6726 x 10-27 kg • Neutron is neutral mn = 1.6749 x 10-27 kg • Both called nucleons

  4. Courtesy Lawrence Berkeley Laboratory

  5. Different types of Nuclei Are Called Nuclides • Protons and neutrons are nucleons • Atomic number Z is number of protons • Atomic mass number A is protons plus neutrons • Neutron number N = A – Z • Nuclide symbol ZXA

  6. Usually A and Z are on the left Courtesy Lawrence Berkeley Laboratory

  7. What is 7N15 ? • Chemical element? • Atomic number? • Atomic mass number? • Neutron number? • Pronounced? Nitrogen 7 15 8 Nitrogen Fifteen

  8. Properties • Atomic properties determined by number of electrons • Nuclei with certain atomic number but different neutron number are called • Most elements have many isotopes isotopes

  9. Nuclear Masses • 6C12 has mass 12.000000 u • Neutron 1.008665 u • Proton 1.007276 u • Neutral hydrogen atom 1.007825 u • By E = mc2 1 u = 1.6605 x 10-27 kg = 931.5 MeV/c2 Try this yourself

  10. Rest Masses in MeV/c2 • Electron 0.51100 • Proton 938.27 • Neutron 939.57 • 1H1 atom 938.78 • Is hydrogen more or less massive than proton and electron together? • How can you explain this?

  11. Binding Energy • Energy holding the nucleus together • Stable Nucleus called a bound state • Mass of stable nucleus less than sum of masses of protons and neutrons in it • It takes energy to break it apart • Binding energy is negative

  12. Example of 2He4 • 2 x mn = 2(1.008665 u) = 2.017330u • 2 x 1H1= 2(1.007825 u) = 2.015650u • Sum 4.032980u • Measured 2He4 mass = 4.002602u (With electrons) • Difference 0.030378u • Must use 1H1 instead of p to balance electrons

  13. 2He4 continued • 0.030378u x 931.5 MeV/c2/u =28.3 MeV • Total binding energy of nucleus • Energy that must go into nucleus to split it into separate nucleons Comparison: binding energy of electron in hydrogen atom is 13.6 eV. What does that tell you?

  14. Average Binding Energy per Nucleon

  15. Four Forces of Nature(in order of decreasing strength) • Strong • Electromagnetic • Weak • Gravity The strong force holds the nucleus together. It is very short range compared to electric and gravity

  16. Radioactivity • Some nuclei change disintegrate into pieces whose total mass is less than mass of nucleus • Called radioactive decay • Discovered by Bequerel in 1896 (U) • Curies found Ra and Po Pitchblende sample

  17. Marie and Pierre Curie • She coined term “radioactivity” • Both won Nobel prize • Pierre killed crossing street • Marie gets his teaching Job at Sorbonne-first Woman to teach there in 650 Years. Later she dies of anemia.

  18. Three Kinds of Radioactivity • Alpha (a) • Positively charged • Least penetrating. Paper stops it • Beta (b) • Negatively charged • ½ cm Aluminum stops it • Gamma (g) • Uncharged, released as photons of light • Most penetrating. Thick lead may not stop it.

  19. Neutron Emission A fourth type of Radioactivity- emits no charged particles, just releases a neutron from the nucleus. By far the most damaging of any type, takes 3 ft of lead or a massive amount of concrete to stop it.

  20. Which Way Will It Bend? Magnetic field in x x x Radium source Lead block

  21. Alpha Decay • Nucleus gives off a particle - 2He4 • Z decreases by 2 • A decreases by 4 • 88Ra226 -->86Rn222 + 2He4 • Rn is different element Graphics courtesy of Centennial of Discovery of radioactivity http://web.ccr.jussieu.fr/radioactivite/english/accueil.htm

  22. Energy in a Decay • Energy released is (Mp – Md – ma) c2 • (Mp – Md – ma) = mass defect • Mp is mass of parent 88Ra226 • Md is mass of daughter 86Rn222 • Energy appears as KE of a particle and daughter (recoil energy) Compare the energy of the a particle with that of the recoiling daughter. What is true about their momenta and directions?

  23. Conservation Laws in Nuclear Processes • Total energy is conserved • Momentum is conserved • Charge is conserved • Angular momentum is conserved • Number of nucleons (plus anti-nucleons) is conserved

  24. You Find Out • What does Americium 241 decays into • Use your periodic table at back of text Answer 93Np237 Neptunium • Application • 95Am241is used in smoke detectors

  25. Smoke Detector Ionization Chamber Americium source inside Smoke particles decrease flow of ionization current Courtesy “How Stuff Works”

  26. Beta Decay • 6C14 --> 7N14 + -1e0 + (anti)neutrino • -1e0 is electron (same as b-) • Z increases by 1; electron from nucleus • A does not change • Occurs for neutron heavy isotopes

  27. Wolfgang Pauli What is a Neutrino? • Massless*, neutral particle that travels with the speed of light (hypothesized by Pauli in 1930) • Incredibly penetrating - passes through Earth • Required to be emitted in beta decay in order that momentum and energy be conserved(beta energies are not unique) • Observed in 1956 by Reines and Cowan • Symbol is n(nu) with bar over it - antineutrino *There is some evidence that the neutrino has a tiny non-zero mass

  28. Positron (Beta+) Decay • 10Ne19 --> 9F19 + e+ + n • e+ is positron(anti-electron) • Z of nucleus decreases by 1 • A does not change • Occurs for neutron light isotopes

  29. Courtesy Stanford Linear Accelerator Lab Question: What is true about the directions of the daughter nucleus, beta, and neutrino?

  30. Electron Capture • Occurs when nucleus absorbs an orbital electron. Example: • 4Be7 + e- --> 3Li7 + n • Z of nucleus decreases by 1 • A does not change • Electron disappears and one proton becomes a neutron • X-rays are given off as electrons jump down

  31. Fermi’s Theory • Explained beta decay and EC in terms of a new “weak” force • Fermi was last “double threat physicist; great theorist and experimenter.

  32. Gamma Decay • Emitted when excited nucleus jumps down to a lower energy state • ZNA*  ZNA + g • Gamma and x-ray are same, high energy photon

  33. What is true about the momenta of the daughter nucleus and the gamma ray? Courtesy Lawrence Berkeley Laboratory

  34. Review • There are stable nuclides (isotopes) and unstable (radioactive ones) • Stable means mass of pieces is more than that of whole nucleus. • Unstable means opposite • MOST isotopes are NOT stable; they undergo one form of decay or another

  35. Radioactive Decay Law • Decay is random process • no. decays in short time DN = -lNDt • N = N0e-lt by integration • Decay constant is called l (rate of decay) • Number of decays per second also proportional to e-lt “exponential function” • Half life = time for half of original sample to decay = 0.693/l • Link for decay simulation

  36. Exponential Decay Curve N = N0e-lt

  37. Question • A sample contains about 1000 nuclei of a certain radioisotope. The half life is four minutes. About how many nuclei will remain after 16 minutes? • Hint: make a table Answer: about 62 nuclei

  38. Decay Table

  39. Randomness of Decay • No way to tell which nucleus will decay when • Actual number that decay varies around a most probable number • Uncertainty is proportional to

  40. Decay Series • A chain of successive decays Starting with U 238

  41. Radioactive Dating • n + 7N14 6C14 + p provides continual supply of carbon 14 at about rate of decay • 6C14 --> 7N14 + -1e0 + antineutrino • When organism dies no more supply so ratio of carbon 14 to 12 decreases – with 5730 yr half life • Useful for dating objects up to 60,000 years old

  42. Nuclear Reactions • Transformation of one element into another is called transmutation. • Sought unsuccessfully by Alchemists • Usually happens in collision • Rutherford(1919) discovered in 2He4 + 7N14 8O17 + 1H1

  43. Conservation Laws in Nuclear Reactions • Momentum • Energy • Charge • Nucleon(Baryon) Number – heavy particles • Lepton Number – light particles

  44. Example: slow neutron reaction • 0n1 + 5B10  3Li7 + ? • Answer 2He4 which is also called an • Alpha particle • Challenge: Given speed of helium atom 9.30 x 106 m/s find the • Velocity and KE of the lithium atom • Hint: what is initial momentum of the system?

  45. Nuclear Fission and Fusion • In fission a large nucleus breaks apart releasing energy • In fusion light nuclei merge to form a heavier nucleus and energy is released.

  46. Nuclear Fission • Uranium nucleus absorbs neutron and splits in two • Easier to do with 92U235 than common 92U238 • Discovered Germany 1938 • Dangerous time

  47. Courtesy students at Illinois Math and Science Academy

  48. Nuclear Chain Reaction Courtesy Nuclear Energy/Nuclear Waste. Chelsea House Publications: New York, 1992.

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