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Notes 4

  1. 1. Sources of Radiation Professor Jasmina Vujic Lecture 4 Nuclear Engineering 162 Department of Nuclear Engineering University of California, Berkeley
  2. 2. SOURCES OF RADIATION • NATURAL RADIATION SOURCES – Radioactive Sources in the Ground – Cosmic Ray Interactions in the Atmosphere – Natural Radioactive Sources within the Body • MAN-MADE RADIATION SOURCES – Medical and Dental X-Ray Machines – Industrial X-Ray Machines – Accelerators – Man-made Radioisotopes – Fallout from Past Bomb Tests – Nuclear Power Reactors
  3. 3. Radioactive Sources in the Ground • Primordial Radionuclides: 238 U, 232 Th and 235 U radioac-tive decay chains; 40 K and 87 Rb 22Na • Cosmogenic radionuclides: 14 C, tritium (3 H), 7 Be, and 22 Na
  4. 4. Cosmic Ray Interactions in the Atmosphere
  5. 5. Cosmic Ray Interactions in the Atmosphere
  6. 6. • Radon as a source of internal exposure: 238 U chain, 226 Ra decays into 222 Rn. The significant dose is from the decay products of radon:
  7. 7. Natural Radioactive Sources within the Body • Potassium-40: 140 g of potassium in a man of 70 kg (i.e. 0.1 /μCi) • Carbon and hydrogen in the biosphere contain 14 C and 3 H • Radon and its decay products • 137 Cs, 131 I, 90 Sr
  8. 8. The Contribution of Natural Radiation Sources
  9. 9. SOURCES OF X-RAYS • Two principal methods for generating X- rays: – The rearrangement of atomic electron configurations • "Characteristic X-Rays" – The deflection of charged particles in the vicinity of the atomic nucleus • "Continuous X-Rays or Bremsstrahlung
  10. 10. CHARACTERISTIC X-RAYS • Characteristic X-rays are emitted from the atomic shells, when electrons jump from the shells at higher energy levels (with Iower binding energy) to the vacancies in the shells at lower energy lev-els (with higher binding energies). • The binding energy of the K-shell electron is the largest in an atom (for example, it is 13.6eV for H and up to 115keV for U) • The energy of the emitted X-ray is determined by where Em is the upper energy level and En is the lower energy level. m nhv E E= −
  11. 11. CONTINUOUS X-RAYS • In addition to loosing its kinetic energy in collisions with the atomic electrons causing ionization or excitation of the atoms along its path, a charged particle (in our case an electron) gives up its kinetic energy by a photon emission as it is deflected (or accel-erated) in the electric field of nuclei. • The emitted EM radiation has a continuous energy spectrum from 0 to Ek, where Ek is the kinetic energy of a charged particle. • For Ek < 100 keV, radiation is emitted at 900 to the direction of the charged particle. For higher Ek the direction of the emitted radiation shifts toward the forward-peaked direction.
  12. 12. X-RAY MACHINES • Made of a glass vacuum tube with two electrodes: cathode and anode: • Cathode with tungsten wire (filament) is heated and electrons are emitted • Electrons are accelerated by a large potential difference (high voltage) • The focusing cup concentrates electrons into the target oh the anode • After hitting the target, electrons are abruptly brought to rest with the lost of their kinetic energy • Only about 1% of electron kinetic energy is emitted as EM radiation, 99% is lost in electronic collisions and converted to heat (anode must be cooled)
  13. 13. Medical and Dental X-Ray Machines
  14. 14. Medical and Dental X-Ray Machines
  15. 15. Fallout from Past Bomb Tests
  16. 16. Fallout from Past Bomb Tests
  17. 17. Fallout from Past Bomb Tests
  18. 18. Nuclear Power Reactors
  19. 19. Nuclear Power Reactors
  20. 20. Radionuclides used in Diagnostics and Therapy
  21. 21. Choice of Radioisotopes for Imaging • The physical characteristics that are desirable for nuclear medicine imaging include: – A suitable physical half-life – Decay via photon emission – Photon energy high enough to penetrate the body tissue with minimal tissue attenuation – Photon energy low enough for minimal thickness of collimator speta – Absence of particulate emission
  22. 22. Important Nuclides of Biomedical Uses
  23. 23. Production of Radionuclides • There are several ways by which radionuclides are produced – Neutron capture (neutron activation) – Nuclear fission – Charged-particle bombardment – Parent decay (radionuclide generator)
  24. 24. Radionuclides produced by neutron absorption
  25. 25. Radionuclides produced by nuclear fission
  26. 26. Radionuclides produced by charged particle bombardment
  27. 27. Generator-produced radionuclides