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!
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 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).
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)
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).
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 !
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!
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 • 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!
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).