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Shielding. Purpose of shielding is to reduce dose We are considering shielding in the early stages of the project to get a general idea for the amount of shielding needed to reduce the dose to a reasonable level Materials that shield neutrons over wide neutron energy range were sought

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MNE

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  1. Shielding • Purpose of shielding is to reduce dose • We are considering shielding in the early stages of the project to get a general idea for the amount of shielding needed to reduce the dose to a reasonable level • Materials that shield neutrons over wide neutron energy range were sought • Other materials were sought to lower the neutron flux at specific energy intervals MNE

  2. Hydrogenous Materials • Low Z materials are more effective in scattering and slowing down neutrons • Hydrogen is the best neutron moderator because its nucleus, a proton, has almost the same mass as a neutron • The neutron can theoretically give up all its energy in one scatter • Materials that have a high hydrogen density will therefore be the best for shielding high energy neutrons • Lithium hydride, polyethylene, paraffin wax, water, and ammonia all have a high hydrogen density. MNE

  3. Hydrogenous Materials MNE

  4. Water Base Case • Spherical water shielding was used as a “base” case to compare all later results against. • Simple MCNP configuration used for analysis of a point source centered in the spherical water medium. • The radius of the water sphere was increased from 5-100 cm in 5 cm increments. • The neutron flux data, binned by energy was then used to calculate dose at the outer surface of the water for different thicknesses. MNE

  5. The Effects of Water MNE

  6. Dose Considerations • Since the definition of a radiation area is 2 mR/hr, the thickness of water required to achieve this was sought. • An exponential curve was fit to the dose results, and the curve was projected for thicker shielding scenarios • Using our projected results this dose level is achieved for 10e11 n/s at a radius of 1.5 m • Dose at 0.75 m was 5760 mR/hr MNE

  7. Other Low Z Shielding Materials • Lithium • Boron • Cadmium MNE

  8. High Z Materials? • Neutrons give up a small fraction of their energy per collision in a medium of heavy nuclei and the heavy material shields neutrons poorly. • However, in order to reduce the high energy flux a material with higher cross sections at higher energies is needed. • Could a heavy material, like lead, be used to perform this task? MNE

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