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