1. 3: Medical Cyclotrons Nuclear Power Reactors Lasers
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 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)

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

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

24. Nuclear Power Reactor

25. Neutron Capture – Fission – more Neutronsnf
U-235
f.p. nf
nth nf
f.p.

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

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

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

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

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

31. 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.

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

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

34. Reactor Control

35. 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 !

36. CANDU Reactor

37. Radiation Hazards from Reactor
During Operation !

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

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

40. Fission Fragment Yield for U-235

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

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

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

44. Fission Product Release

45. 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.

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

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

48. Useful Activation Products

49. 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).

50. Reactor Cool-Off Period

52. Lasers Frequent application in health care institutions:
Patient positioning and alignment devices in scanning and radio-therapy equipment.

53. Lasers

54. Lasers Example: ECAT PET scanner Laser Radiation
Do not Stare into Beam
Class II Laser Product
(1 mW maximum output)
Wavelength: 600 to 700nm

55. Lasers Biological Effects: Principally temperature effects (burns).
Critical Organs: eye, skin

56. Classification of Lasers
Class I: not hazardous
Class II: continuous (but not momentary) intrabeam exposure damages eye
Class III: can damage eye during momentary intrabeam exposure
Class IV: damage to eye from momentary intrabeam exposure and from exposure to diffuse reflection

57. Lasers Protective Eye Wear:
Goggles should be adapted to the type (wavelength!) of laser being used.

58. Ultraviolet Radiation
Attention:
UV light (wavelength ~ 300nm) was (is?)
used in operating rooms to keep
equipment sterile.
Therefore, wear goggles, when working
in areas where UV light is being used, in
order to avoid eye irritation!

60. Cyclotron Principle