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Shielding simulation and nuclide vector

  1. Feb 9, 2015 #1
    I have to make a shielding calculation of some radioactive scrap metal in a cask, using Geant4 (hence Monte-Carlo).
    I will be given a nuclide vector and I'll have to calculate the ambient dose equivalent rate outside of the cask.
    The nuclide vector I will be given will be something like:

    Co60 X Bq
    Cs137 Y Bq

    The problem is that I've been told that I must not only consider the activity of the expressly listed nuclides but also of any eventual daughter in equilibrium with the parent, even if not expressly listed.
    The question is:
    how do professionals usually deal with this problem ? How do they calculate the daughters' activities ? By hand ?
    In principle I can try to calculate by hand all the equilibria related to each listed nuclide, but there are nuclides having many different decay branches, each with its own decay constant: it's going to be laborious.
    I've been asked to prepare a budget of working hours for the simulation. I'm afraid that such a preliminary work only to calculate all the possible equilibria is going to deeply affect the budget.
    Consider that just a nuclide like Co60 can decay through beta-minus to two possible excited states of Ni60, each in turn with its own gamma-decay-constant...

    Thank you in advance.
  2. jcsd
  3. Feb 9, 2015 #2
    Typically we would use programs that have been written to calculate isotopic concentrations. These programs perform exactly the sort of calculations you're talking about.
    The one that comes to mind is Origin/Scales (I've never used it) but it can be used for calculating the isotope mix due to a wide number of reactions. Depending on your application this may be overkill.

    With a hand calculation you might be able to simplify the problem significantly by eliminating some isotopes. For example, you might not need to consider alpha/beta emissions because they are so easy to shield. Or your accuracy might not require you to include small branching ratio decays. If a half-life is short enough (relative to your timescale) you could perhaps assume it is continuously in equilibrium and thus proportional to its parent isotope.
  4. Feb 9, 2015 #3


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    Staff Emeritus
    Science Advisor

    If it's just a few radionuclides, one can start with an initial vector and write analytical expressions for the decay chains.

    If one has to do shielding calcs, then one would use a program to solve the scattering problem numerically.

    As Hologram mentioned, there are already codes that handle large numbers of radionuclides and shielding calcs.
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