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Do shielding materials get excited and start reactions?

  1. Feb 16, 2010 #1
    In neutron activations, target materials are being beamed by high energy flux, thus making the target materials excited and undergo radioactive decay. Neutron analysis then can be performed after the materials are "cool" enough by placing inside the hood with enough shielding.

    My question is: This may sound silly, but when shielding materials like iron, lead, depleted uranium (<---uranium can be used as shield????)....etc being hit by radio particles from radiations, will there be a reaction occurring at any chance? Like if a shielding material is placed in space and being hit by high energy GCE, will reaction occur? I am confuse about the neutron activation and shielding concept...
  2. jcsd
  3. Feb 16, 2010 #2


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    Some atoms, when bombarded with neutrons, can absorb one, thus turning into a new isotope. Usually, these isotopes are unstable, and thus radioactive. These radioactive isotopes usually decay by beta emission until they reach a stable isotope. When you put these neutron-activated materials within a shielded container, there can be secondary radiations from the shielding material itself, depending on what you are dealing with. High energy betas can cause additional x-rays to be produced when they knock around the atomic electrons of the shielding material, but these are not nuclear reactions. Most of the time, gamma, beta and alpha radiation do not cause atoms they interact with to become radioactive. Likewise, materials activated by neutron radiation do not themselves emit more neutrons. There are exceptions to these rules though.
    Last edited: Feb 16, 2010
  4. Feb 16, 2010 #3


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    Adding to what QuantumPion mentioned, ordinarily alpha, beta and gamma radiation does not 'activate' a material, i.e. does not engage in a nuclear reaction. They generally interact with atomic electrons through ionization.

    Gamma rays of sufficient energy can induce neutron emission, but that usually requires gamma rays above 1 MeV.

    Neutrons can certainly be captured by the nuclei of shield material, and that material will become radioactive.

    For galactic cosmic rays, where the particles are in the MeV range, they can cause spallation reactions, and some of the products can be radioactive. Spallation reactions are problematic for space craft and high altitude aircraft because of the secondary ionizing radiation produced.
  5. Feb 16, 2010 #4
    I think I get most of it, but according to Astronuc "Neutrons can certainly be captured by the nuclei of shield material, and that material will become radioactive.". May you give me an example of a shield material that can absorb a neutron? (what magnitude of energy or flux?) Thanks,
  6. Feb 16, 2010 #5


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    Well you probably wouldn't want to use depleted uranium to shield against neutrons, as you would end up getting some fissions (more neutrons and radioactivity) as well as accumulating plutonium and higher transuranics. Steel alloys contain Nickle or Cobolt isotopes which can absorb a neutron to become Cobolt-60 which is highly radioactive. I don't think lead does anything with neutrons off the top of my head.
  7. Feb 16, 2010 #6


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    I believe the answer must be nearly all, if not all, of the elements will absorb a thermal (slow) neutron - some much more readily than others.
    You'll note that Boron, for instance has a cross section of 767 barnes, roughly a thousand times greater than Tin. What the nucleus does after it absorbs the neutron is yet another issue - as QP points out.
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