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  1. Evaluations • Please fill out evaluation and turn it in. • Written comments are very helpful! • Lecture will start 12:15 • Today, evaluate Prof. Rzchowski • If you weren’t here Tuesday, also evaluate Prof. Montaruli today. Phy208 Lect29

  2. Final Exam • Fri, Dec 21, at 7:45-9:45 am in Ch 2103 • 2 equation sheets allowed (HAND WRITTEN!) • About 40% on new material • Rest on topics of exam1, exam2, exam3. Study Tips: Download blank exams and take them. Download blank quizzes and take them. Look through group problems. Look through lab question sheets. Make up an exam question, explain solution to your study group. Phy208 Lect29

  3. From last time… • Radioactive decay: alpha, beta, gamma • Radioactive half-life • Today: • More about beta, gamma, decay • Medical uses of radiation • Nuclear fission Phy208 Lect29

  4. From last time: Alpha radiation Alpha particle: (2 protons, 2 neutrons) Piece of atom (alpha particle) ejected from heavy nucleus Phy208 Lect29

  5. But what are these? Decay sequence of 238U Number of protons  decay Number of neutrons Phy208 Lect29

  6. Beta decay Number of neutrons decreases by one Number of protons increases by one Electron (beta particle) emitted Number of protons Number of neutrons But nucleus has only neutrons & protons. Phy208 Lect29

  7. Beta decay • Nucleus emits an electron (negative charge) • Must be balanced by a positive charge appearing in the nucleus. This occurs as a neutron changing into a proton Phy208 Lect29

  8. Changing particles Neutron made up of quarks. One of the down quarks changed to an up quark. New combination of quarks is a proton. Phy208 Lect29

  9. 7 neutrons7 protons 8 neutrons6 protons beta decay example = + 14 nucleons 14 nucleons 1 electron 6 positive charges 7 positive charges = + 1 negative charge Used in radioactive carbon dating. Half-life 5,730 years. Phy208 Lect29

  10. 10 neutrons8 protons 9 neutrons9 protons This is antimatter Other carbon decays • Lightest isotopes of carbon emit positron • antiparticle of electron, has positive charge! e+ + Phy208 Lect29

  11. Phy208 Lect29

  12. Antimatter • Every particle has an antiparticle. • Antimatter (anti-atoms) has been formed. Matter and antimatter annihilate Photons are created, conserving energy, momentum. Phy208 Lect29

  13. Positron Emission Tomography - PET Short-lived radioactive tracer isotope emits positron Positron annihilates with nearby electron • e+ + e-→ 2γ • Two 511 keV gamma rays are produced • They fly in opposite directions (to conserve momentum) Gamma Photon #2 Phy208 Lect29

  14. Emission Detection • If detectors receive gamma rays at the approx. same time, we have a detection • Nuclear physics sensor and electronics Ring of detectors Phy208 Lect29

  15. “The coming Sunday in an unguarded moment I added some radioactive deposit [lead-212] to the freshly prepared pie and on the following Wednesday, with the aid of an electroscope, I demonstrated to the landlady the presence of the active deposit in the soufflé.” Radioactive tracers Worked on radioactivity as student with Ernest Rutherford. Lodged in nearby boarding home. Suspected his landlady was serving meals later in the week ‘recycled’ from the Sunday meat pie. His landlady denied this! deHevesy described his first foray into nuclear medicine: George de Hevesy Phy208 Lect29

  16. Detect ionizing radiation • Electroscope Geiger counter Phy208 Lect29

  17. Decay of 60Co • This is one of sources used in the lab. • Decays by electron emission, as predicted. • But decays to an excited state. • Photons emitted as excited state drops to its ground state. Phy208 Lect29

  18. 60 60 Ni Ni 28 28 Gamma decay Alpha decay (alpha particle emitted), Beta decay (electron or positron emitted), can leave nucleus in excited state • Nucleus has excited states just like hydrogen atom • Emits photon as it drops to lower state. Nucleus also emits photon as it drops to ground stateThis is gamma radiation Extremely high energy photons. Phy208 Lect29

  19. Cancer Radiation Therapy • 50-60% of cancer patients treated with radiation • Goal: disable cancerous cells without hurting healthy cells • Typically X and γ-rays (60Co) from 20 KV to 25 MV Phy208 Lect29

  20. Radiation Levels • rad (radiation absorbed dose) • amount of radiation to deposit 0.01 J of energy in 1 kg of absorbing material Ground 0.30 rem/yr Mercury 9 60.6 rem/yr Apollo 14 146.2 rem/yr MIR Station 34.9 rem/yr Space Station 36.5 rem/yr • RBE (relative biological effectiveness = # rads of x-rays that produces same biological damage as 1 rad of radiation being used • rem (roentgen equivalent in man) = dose in rem = dose in rad x RBE Radiation type RBE X-rays 1Gamma rays 1Beta particles 1-2Alpha particles 10-20 Safe limit ( US gov ) 0.50 rem/yr Phy208 Lect29

  21. Your exposure in the lab • 60Co has an activity of 1 µCurie • Each decay: 1.3MeV photon emitted • Assume half are absorbed by a 10kg section of your body for 2 hours. • Energy absorbed = 0.5 rem 0.3 rem 0.1 rem 0.05 rem 0.003 rem What is your dose in mrem? Phy208 Lect29

  22. Decay summary • Alpha decay • He nucleus (2 protons, 2 neutrons) • Nucleus loses 2 protons, 2 neutrons • Beta- decay • Electron • Neutron changes to proton in nucleus • Beta+ decay • Positron • Proton changes to neutron in nucleus • Gamma decay • Nucleus emits photon as it drops from excited state Phy208 Lect29

  23. Decay question 20Na decays in to 20Ne, a particle is emitted? What particle is it? Na atomic number = 11 Ne atomic number = 10 • Alpha • Electron beta • Positron beta • Gamma 20Na has 11 protons, 9 neutrons20Ne has 10 protons, 10 neutronsSo one a proton (+ charge ) changed to a neutron (0 charge) in this decay. A positive particle had to be emitted. Phy208 Lect29

  24. Nuclear fission • Nuclear binding energes • 56Fe is most stable • Move toward lower energies by fission or fusion. • Energy released related to difference in binding energy. Phy208 Lect29

  25. Fission • Fission occurs when a heavy nucleus breaks apart into smaller pieces. • Does not occur spontaneously, but is induced by capture of a neutron Phy208 Lect29

  26. Chain reaction • If neutrons produced by fission can be captured by other nuclei, fission chain reaction can proceed. Phy208 Lect29

  27. Neutron capture • When neutron is captured, 235U becomes 236U • Only neutron # changes, same number of protons. Nucleus distorts and oscillate,eventually breaking apart (fissioning) Phy208 Lect29

  28. 235Uranium Binding energy /nucleon Fission fragments Energy/nucleon released by fission Mass number How much energy? Binding energy/nucleon ~1 MeV less for fission fragments than for original nucleus This difference appears as energy. Phy208 Lect29

  29. Energy released • 235U has 235 total nucleons, so ~240 MeV released in one fusion event. • 235U has molar mass of ~235 gm/mole • So 1 kg is ~ 4 moles = 4x(6x1024)=2.5x1025 particles • Fission one kg of 235U • Produce ~6x1033 eV = 1015 Joules • 1 kilo-ton = 1,000 tons of TNT = 4.2x1012 Joules • This would release ~250 kilo-tons of energy!!! • Chain reaction suggests all this could be released almost instantaneously. Phy208 Lect29

  30. The critical mass • An important detail is the probability of neutron capture by the 235U. • If the neutrons escape before being captured, the reaction will not be self-sustaining. • Neutrons need to be slowed down to encourage capture by U nucleus • The mass of fissionable material must be large enough, and the 235U fraction high enough, to capture the neutrons before they escape. Phy208 Lect29

  31. The first chain reaction • Construction of CP-1, (Chicago Pile Number One) under the football stadium in an abandoned squash court. • A ‘pile’ of graphite, uranium, and uranium oxides. • Graphite = moderator,uranium for fission. • On December 2, 1942: chain reaction produced 1/2 watt of power. • 771,000 lbs graphite, 80,590 pounds of uranium oxide and 12,400 pounds of uranium metal, • Cost ~ $1 million. • Shape was flattened ellipsoid 25 feet wide and 20 feet high. Phy208 Lect29

  32. Level 3 Graphite layers form the base of the pile. Level 7 Uranium oxide pseudospheres start at level 7 Level 10 Tenth layer of graphite blocks containing pseudospheres of uranium oxide Level 19 The 19th layer of graphite covering layer 18 containing slugs of uranium oxide. Pile assembly Phy208 Lect29

  33. Hydroelectric plant • Uses 60,000 tons/sec waterto produce 1,000 MW • Coal-burning plant • 10,000 tons coal/dayto produce 1,000 MW • Fission reactor • Uses 100 tons uranium/yrto produce 1,000 MW Energy production Phy208 Lect29

  34. Another comparison How can we release this energy? Phy208 Lect29

  35. Energy of separated nucleons Mass difference / nucleon (MeV/c2) Question Suppose we could split the Iron (Fe) nucleus into two equal parts. Does this require energy input, or is energy released? ReleasedB. InputC. Same before & after Phy208 Lect29

  36. Neutrons • Neutrons may be captured by nuclei that do not undergo fission • Most commonly, neutrons are captured by 238U • The possibility of neutron capture by 238U is lower for slow neutrons. • The moderator helps minimize the capture of neutrons by 238U by slowing them down, making more available to initiate fission in 235U. Phy208 Lect29

  37. Radiation Poisoning Killed Ex-Russian Spy The British authorities said today that A. V. Litvinenko, a former Russian Federal Security Service liutenant-colonel, and later dissident, died of radiation poisoning due to a rare and highly radioactive isotope known as Polonium 210. Highly radioactive metalloid discovered by M. Curie A N Isotopic T1/2 Activity mass (u) (d) (uCi) 210Po 84 126 209.98 140 0.1 Produced by bombarding bismuth-209 with neutrons in nuclear reactors. In the decay 210P creates 140 W/g so 1/2 a gram reaches 500 °C. Considered to power spacecrafts. Used in many daily applications: eg anti-static brushes in photographic shops Dangerous only if ingested because it is an α emitter. Phy208 Lect29

  38. Uranium isotopes • 235U will fission. • does not. When it absorbs neutron, it becomes , beta decays (neutron changes to proton) to , t1/2=23 min • This quickly beta decays to , t1/2=2.3 days Uranium, Neptunium, Plutonium 1941: discovered that Pu will fission. Fission limited to 235U, 239Pu Phy208 Lect29

  39. Uranium isotopes • Only the less abundant 235U will fission. • Natural abundance is less than 1%, most is 238U Phy208 Lect29

  40. Where does uranium come from? • Uranium is one most abundant elements, but in low concentration • E.g. uranium is mixed with granite, covering 60% of the Earth’s crust. • But only four parts of uranium per million million parts of granite. Phy208 Lect29

  41. Uranium ore processing • Mechanically crush the ore • Acid treatment to separate the uranium metal from rock. • Purified with chemicals to leach out (dissolve) the uranium. • Chemically precipitate uranium-rich fraction. • Dry the uranium-rich solution, giving U3O8, (yellowcake) • Yellowcake packaged into special steel drums, 400 kg when full. • Hauled by truck to uranium refinery. Barrel of yellowcake Phy208 Lect29

  42. Uranium Hexafluoride • Yellowcake converted to Uranium Hexafluouride • Main use is separation of two main isotopes of uranium;235U has only 0.71% natural abundance. • What is UF6 like? • White crystalline solid at room temperature Sublimes (turns to gas) at 56.5°C (133.8°F) Liquid only under pressures > 1.5 atmospheres T > 64°C Ampule of UF6 Phy208 Lect29

  43. Isotope separation • How is the separation achieved? • Several methods have been used:- • Gaseous diffusion • Electromagnetic isotope separation • Gas centrifugation • Electromagnetic isotope separation, using UCl4, was the original method used in the Manhattan project in a plant at Oak Ridge Tennessee. • Gas diffusion and Gas centrifuge now produce more than 90% of the world's enriched uranium. Phy208 Lect29

  44. person Gas diffusion enrichment • UF6 gas diffuses thru foil. • Lighter 235UF6 molecules diffuse slightly faster than the 238UF6 molecules. As the gas moves, the two isotopes are separated. • Over 1000 stages are required to produce a UF6 product with even 3-4% enrichment! • US (Paducah, KT) Phy208 Lect29

  45. What about the 238UF6? • As of June 1998, the US Department of Energy (DOE) owned approximately 57,800 steel cylinders of depleted UF6. • The total radioactivity of depleted UF6 is approximately 8.6 Ci/cylinder. • Depleted UF6 continues to be produced at the rate about 150 cylinders (2,100 short tons or 1,900 metric tons) per year. Phy208 Lect29

  46. Gas centrifuge enrichment • Gaseous UF6 is placed in a centrifuge. • Rapid spinning flings heavier U-238 atoms to the outside of the centrifuge, leaving enriched UF6 in the center • Single centrifuge insufficient to obtain required U-235 enrichment. • Many centrifuges connected in a ‘cascade’. • U-235 concentration gradually increased to 3 – 5% throughmany stages. Phy208 Lect29

  47. Reactor vs bomb • 3-5% enrichment ok for reactor. • Bomb needs much higher fraction of 235U • Oppenheimer suggested needed as much as 90% 235U vs 238U Phy208 Lect29

  48. Electromagnetic separation • Original separation method used in Manhattan project • Uses essentially a mass spectrometer. Phy208 Lect29

  49. Oak Ridge EM separation • Control panels and operators for calutrons at Oak Ridge. Operators, worked in shifts covering 24 hrs/day • Produced 10 kilograms of 90 percent enriched U-235 • Thought by Oppenheimer sufficient for bomb. Phy208 Lect29

  50. Uranium fission bomb • Uranium ‘bullet’ fired into Uranium target • Critical mass formed, resulting in uncontrolled fission chain reaction Phy208 Lect29