health physics 3: medical cyclotrons nuclear power reactors lasers
Post on 24-Dec-2015
221 Views
Preview:
TRANSCRIPT
Health Physics
3:
Medical Cyclotrons
Nuclear Power Reactors
Lasers
Medical Cyclotron
Why a medical cyclotron ?
Ref. : Sorenson & Phelps: Physics in
Nuclear Medicine
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
PET Tracers
Carbon -11
Nitrogen -13 (F-18 ~ H)
Oxygen -15
Are all natural constituents of
human body (unlike Tc-99m).
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!
A Cyclotron as Seen by …
A Cyclotron as Seen by …
A Cyclotron as Seen by …
A Cyclotron as Seen by …
A Cyclotron as Seen by …
A Cyclotron as Seen by …
Cyclotron Principle
Cyclotron Principle
Positive Ion Cyclotron
Beam Extraction on Negative Ion Machine
Proton Beam Extraction: 2 electrons are removed from proton-ion beam by carbon stripping foil
Negative Ion Cyclotron
Advantages:•more efficient beam extraction•less internal component activation•dual beam extraction possibility•simple mechanism
Cyclotron Produced Radio-Nuclides
Cyclotron Produced Radio-Nuclides
Are usually carrier-free:
Reason: mostly (p,n), (d,n) reactionsTherefore: target material and radioactive product are not the same chemical species (Z changes).
RDS System (Radioisotope Delivery System)
Nuclear Power Reactor
References:
1) Health Physics (Dr. Robert Corns) Chapter 11
2) Sorenson & Phelps: Physics in
Nuclear Medicine
Nuclear Power Reactor
Produces important “by-products” for medical care:
Examples:• Fission products (e.g. 99Mo, 131I)
• Activation products (e.g. 60Co, 32P)
Nuclear Power Reactor
Neutron Capture – Fission – more Neutrons
U-235
nth
nf
f.p.
f.p.
nf
nf
Chain Reaction ?
Neutrons from first fission may induce other fissions.
Natural Uranium
Composition:0.71% of 235U
99.28% of 238U
Natural Uranium by itself will not sustain a chain reaction.
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.
Neutron Moderator
Thermal neutrons (E~0.025eV) are more likely to induce fissions.
Chain Reaction: The Fission Cycle
Reproduction factor k=1
start here
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.
Critical SizeFor small reactor: S/V = 6
For large reactor: S/V = 3
Reactor Control
Insert neutron-absorbing control rods into reactor (B, Cd).
In CANDU reactor, change level of heavy water moderator (D2 O).
Reactor Control
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 !
CANDU Reactor
Radiation Hazards from ReactorDuring Operation !
Reactor Shielding
Avoid streaming of radiation (neutrons!)
Compare with maze in cyclotron vault
Radiation Hazards from Reactor
When reactor is shut down :
Fission Products and
Activation Products
Continue to be a source of radiation hazard!
Fission Fragment Yield for U-235
Important Fission Products
Strontium-90Molybdenum-99Iodine-131Xenon-133Cesium-137
Fission Product Build-Up
Short T1/2 : fast build-up
Equilibrium Activities for some Fission Products
Iodine-131: 17’360 CiIodine-133: 35’770 CiXenon-133: 35’780 Ci
Fission Product Release
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.
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
Unwanted Activation Products
Tritium: (from D2O), vapor!
Co-60: (from Co-59, corrosion) contaminates cooling
system, pipes!
Useful Activation Products
Reactor Produced Radio-Nuclides
Are usually not carrier-free:
Reason: mostly (n,gamma) reactionsTherefore: target material and radioactive product are the same chemical species (Z does not change).
Reactor Cool-Off Period
Lasers
Frequent application in healthcare institutions:
Patient positioning and alignment devices in scanning and radio-therapy equipment.
Lasers
Lasers
Example: ECAT PET scanner
Laser RadiationDo not Stare into BeamClass II Laser Product
(1 mW maximum output)Wavelength: 600 to 700nm
Lasers
Biological Effects:Principally temperature effects (burns).
Critical Organs: eye, skin
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
Lasers
Protective Eye Wear:
Goggles should be adapted to the type (wavelength!) of laser being used.
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!
Cyclotron Principle
top related