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Homework Help: Exposure rate from thermal neutrons through water

  1. May 26, 2015 #1
    1. The problem statement, all variables and given/known data
    A beam of thermal neutrons (10^12 neutrons per cm^2 second) strikes a 1cm thick water target normal to its surface. The target is a round disk with diameter 20cm. Find the exposure rate (R/second) 100cm beyond the water target (the middle of the disk) from only the 1H(n,gamma)2H reaction. Ignore attenuation in water and air. Atomic density is 6e23 atoms/gram.

    2. Relevant equations
    The only real equation I know if for dose calculation, not exposure.

    D(n,gamma) = 1.6e-13 (J/MeV) * Flux * Number atoms (given) * cross section of hydrogen * Energy gamma * AF

    3. The attempt at a solution

    I can get a value for the dose rate created from the (n, gamma) reaction, but am unsure how to get exposure 100cm past that point. The above formula gives a Dose rate absorbed in the water shield (we often used water as a substitute for tissue anyway). Would it be logical to create a secondary equation with the water disk as a disk source of radiation that is irradiating the point 100 cm away from its center? If so, how would a number in Gy/s be converted back to something usable to calculate exposure.
    Last edited: May 26, 2015
  2. jcsd
  3. May 28, 2015 #2


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    Staff: Mentor

    I guess so. A point-like source might be a good approximation as 100 cm >> 10 cm.
    Don't calculate energy values - the formula you posted multiplies gamma photons per time with their energy, you can remove the energy part to get photons per time. Then you just have to take the distance into account.
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