Shielding for Activated Components Produced during Cyclotron Irradiation (Part 2 of 2)

Dr. John Wilson | University of AlbertaEnoch ABC Ballroom

Neutron production ceases when beam is stopped however there are many secondary radionuclidic byproducts that result from interaction with the proton beam and from the generated neutron flux. Cyclotron targets can be in the gas phase (e.g.14N(p,α)11C), liquid phase (18O(p,n)18F) or solid phase (e.g.205Tl(p,3n)203Pb) and must be able to dissipate the tremendous heat caused by the beam on target. Gas and liquid targets operate at high-pressure and are sealed with a HAVAR metal foil through which the proton beam passes. HAVAR is exceptionally strong and flexible however is very prone to proton activation, generating long lived radionuclides byproducts. The interior of the cyclotron must have high electrical conductivity and is typically copper. Spurious protons from the beam on the copper result in secondary radionuclide formation and radiation field from the main tank at end of beam. The energy of the beam is degraded as it passes through target material and gas and liquid target are ‘thick’ targets as the entire beam is stopped in the target. Solid targets are used for elements of higher molecular number which can have multiple stable isotopes or can undergo various nuclear transformations depending on the beam energy. These are typically ‘thin’ targets as itis advantageous to stop only the desired energy fraction of the beam within the target material to reduce undesired radionuclide contaminants. Avoidance of cooling water activation from the lower energy exiting beam is important when irradiating thin targets. Neutrons emitted during irradiation have a high propensity to activate many materials in the vault. Moderation of the overall neutron field in the vault during irradiation is important to reduce activation of copper pipes, 63Cu(n,γ)64Cu, steel components, 58Fe(n,γ)59Fe and metal contaminants in the concrete.

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