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  1. Health Physics 3: Medical Cyclotrons Nuclear Power Reactors Lasers

  2. Medical Cyclotron Why a medical cyclotron ? Ref. : Sorenson & Phelps: Physics in Nuclear Medicine

  3. Medical Cyclotron On-site Production of Short-lived PET Tracers: C-11 T1/2~ 20 min N-13 T1/2~ 10 min O-15 T1/2~ 2 min F-18 T1/2~ 2 hr

  4. PET Tracers Carbon -11 Nitrogen -13 (F-18 ~ H) Oxygen -15 Are all natural constituents of human body (unlike Tc-99m).

  5. Medical Cyclotron Relatively low-energy particles: ~20 MeV protons ~10 MeV deuterons But relatively large beam current: ~ 500 micro-A to yield large activities For quality medical imaging!

  6. A Cyclotron as Seen by …

  7. A Cyclotron as Seen by …

  8. A Cyclotron as Seen by …

  9. A Cyclotron as Seen by …

  10. A Cyclotron as Seen by …

  11. A Cyclotron as Seen by …

  12. Cyclotron Principle

  13. Cyclotron Principle

  14. Positive Ion Cyclotron

  15. Beam Extraction on Negative Ion Machine

  16. Proton Beam Extraction: 2 electrons are removed from proton-ion beam by carbon stripping foil

  17. Negative Ion Cyclotron • Advantages: • more efficient beam extraction • less internal component activation • dual beam extraction possibility • simple mechanism

  18. Cyclotron Produced Radio-Nuclides

  19. Cyclotron Produced Radio-Nuclides Are usually carrier-free: Reason: mostly (p,n), (d,n) reactions Therefore: target material and radioactive product are not the same chemical species (Z changes).

  20. RDS System (Radioisotope Delivery System)

  21. Nuclear Power Reactor References: 1) Health Physics (Dr. Robert Corns) Chapter 11 2) Sorenson & Phelps: Physics in Nuclear Medicine

  22. Nuclear Power Reactor • Produces important “by-products” for medical care: • Examples: • Fission products (e.g. 99Mo, 131I) • Activation products (e.g. 60Co, 32P)

  23. Nuclear Power Reactor

  24. Neutron Capture – Fission – more Neutrons nf U-235 f.p. nf nth nf f.p.

  25. Chain Reaction ? Neutrons from first fission may induce other fissions.

  26. Natural Uranium Composition: 0.71% of 235U 99.28% of 238U Natural Uranium by itself will not sustain a chain reaction.

  27. Neutron Moderator A substance containing light nuclei (e.g. D2 O) is most effective in slowing down neutrons to thermal energies (~ 0.025 eV) through elastic scattering.

  28. Neutron Moderator Thermal neutrons (E~0.025eV) are more likely to induce fissions.

  29. Chain Reaction: The Fission Cycle Reproduction factor k=1 start here

  30. Critical Size The more neutrons in the reactor core, the more likely a chain reaction will occur. Neutrons are lost through: 1) absorption in the core and 2) leakage from surface of reactor Therefore: A small reactor surface over reactor volume ratio (S/V) will favor a chain reaction.

  31. Critical Size For small reactor: S/V = 6 For large reactor: S/V = 3

  32. Reactor Control Insert neutron-absorbing control rods into reactor (B, Cd). In CANDU reactor, change level of heavy water moderator (D2 O).

  33. Reactor Control

  34. The CANDU Power Reactor CANadian Deuterium Uranium Uses Natural Uranium as fuel and Heavy Water (D2O) as a moderator This makes the CANDU very safe !

  35. CANDU Reactor

  36. Radiation Hazards from Reactor During Operation !

  37. Reactor Shielding Avoid streaming of radiation (neutrons!) Compare with maze in cyclotron vault

  38. Radiation Hazards from Reactor When reactor is shut down : Fission Products and Activation Products Continue to be a source of radiation hazard!

  39. Fission Fragment Yield for U-235

  40. Important Fission Products Strontium-90 Molybdenum-99 Iodine-131 Xenon-133 Cesium-137

  41. Fission Product Build-Up Short T1/2 : fast build-up

  42. Equilibrium Activities for some Fission Products Iodine-131: 17’360 Ci Iodine-133: 35’770 Ci Xenon-133: 35’780 Ci

  43. Fission Product Release

  44. Fission Product Release • Release of radio-iodines (e.g. I-131): • Half-Life: ~ 8 days • Volatile, vapor form • Contaminate grass, milk • Target the thyroid gland • Prevention: Pre-loading of thyroid with • “cold” iodine.

  45. Neutron Activation • Neutrons activate elements • present in reactor, mainly via • (n,gamma) and (n,p) reactions): • moderator, coolant • corrosion elements in coolant • commercial production ports

  46. Unwanted Activation Products Tritium: (from D2O), vapor! Co-60: (from Co-59, corrosion) contaminates cooling system, pipes!

  47. Useful Activation Products

  48. Reactor Produced Radio-Nuclides Are usually not carrier-free: Reason: mostly (n,gamma) reactions Therefore: target material and radioactive product are the same chemical species (Z does not change).

  49. Reactor Cool-Off Period