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
Oxygen -15
(F-18 ~ H)
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) reactions
Therefore: 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
nf
U-235
f.p.
nf
nth
f.p.
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 Size
For 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 Reactor
During 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-90
Molybdenum-99
Iodine-131
Xenon-133
Cesium-137
Fission Product Build-Up
Short T1/2 : fast build-up
Equilibrium Activities for some
Fission Products
Iodine-131:
Iodine-133:
Xenon-133:
17’360 Ci
35’770 Ci
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) reactions
Therefore: target material and
radioactive product are the same
chemical species (Z does not change).
Reactor Cool-Off Period
Lasers
Frequent application in health
care institutions:
Patient positioning and
alignment devices in scanning
and radio-therapy equipment.
Lasers
Lasers
Example: ECAT PET scanner
Laser Radiation
Do not Stare into Beam
Class 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