3: Medical Cyclotrons Nuclear Power Reactors Lasers 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) reactions Therefore: target material and radioactive product are not the same chemical species (Z changes).

RDS System (Radioisotope Delivery System)

Nuclear Power Reactor Nuclear Medicine 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 nf f.p.

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: 17’360 Ci Iodine-133: 35’770 Ci Xenon-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 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

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