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Fission chamber

  1. Nov 14, 2009 #1
    why does fission chamber start to work from neutron fluxes more than source range in spite of being a kind of ionization chamber that is sensitive to gamma rays?
     
  2. jcsd
  3. Nov 18, 2009 #2

    Astronuc

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    Well, the purpose of a fission chamber is to measure neutron radiation (by virtue of induced fissions), and not gamma or beta radiation. If a detector measures the radiation fields of different types when one is interested in a specific type, the one has to have another detector sensitive to the type which one must exclude.

    The goal of a fission chamber is to have it be more sensitive to neutrons than gamma or beta particles. One way would be to shield it with a thick high Z material (which would absorb betas (electrons) and scatter (absorb) gammas (by Compton and photoelectric effects), while having a low absorption cross-section for neutrons, such that it responds primarily to the neutron radiation. I believe such a detector also has a lower voltage than a gamma or beta detector.
     
  4. Nov 18, 2009 #3

    QuantumPion

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    If you are talking about a LWR incore detector, there are 10 orders of magnitude more neutrons at full power than shut down. It would be hard to make a fission chamber detector capable of working over such a large range. That is why there are different kinds of detectors to use while at low power or shut down.
     
  5. Nov 18, 2009 #4

    Astronuc

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    Excore detectors possibly would be ion-chambers. In-core detectors are solid state.

    http://www.freepatentsonline.com/3872311.pdf (1973)

    http://www.freepatentsonline.com/4123658.pdf (1977)


    In the power range, one has a large signal-to-noise ratio as compared to the source range where the signal-to-noise ratio is very low due to the low level of neutrons compared to gammas.

    See also - V and Rh detectors are most common.
    http://www.tpub.com/content/doe/h1013v2/css/h1013v2_75.htm
    http://www.tpub.com/content/doe/h1013v2/css/h1013v2_76.htm (see discussion on wide range fission detectors)
     
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