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Homework Help: Radiation Equilibrium

  1. Oct 17, 2008 #1
    1. The problem statement, all variables and given/known data
    This problem comes from Frank Attix's book Intro to Radiological Physics and Rad Dosimetry.
    From ch. 4 - problem #1:

    Approximately what diameter for a sphere of water would be required to approach radiation equilibrium wihin 1% at its center, assuming it contains a uniform dilute solution of 60Cobalt (1.25 MeV gamma rays). Use u(en) and u as approximations to the effective gamma ray attenuation coefficient; this will over- and under-estimate the size respectively.

    2. Relevant equations

    u - is the attenuation coefficient for primary photons only
    u(en) - is the effective attenuation coefficient for ideal broad beam attenuation

    Attix gives the solutions as 312 cm and 144 cm

    3. The attempt at a solution

    I come close to one of the answers but I'm not sure how u and u(en) come into play here.

    (4/3) pi r^3 = .01(1.25MeV).

    This gives r = .144
  2. jcsd
  3. Oct 17, 2008 #2
    This thread can be disregarded. I was able to solve the problem.

    The solution to this problem is to use exponential attenuation:

    N = N(0) exp(-uL). The approach is to solve for L for given u and u(en) values when 99% of the photons are allowed to pass thru the sphere.
  4. Sep 19, 2009 #3

    hi..one of my classmates has asked about this problem too, i was thinking of this solution too, but i cant find the real values for u(en) and u...

    i would like to ask, where can i find those values, i keep searching the net but gave me none... hope you would help us.

    thanks :)

    andy from philippines

    God bless
  5. Sep 20, 2009 #4
  6. Sep 20, 2009 #5
    thank you so much kmoh111

    God bless :)
  7. Sep 24, 2009 #6
    hi me again :(

    ive tried to solve the problem using exponential attenuation but i always got a different answer...however i tried to use equation 3.14 (build up factor) in attix book... where B is approximately equal to 1.06... here's how it went..

    0.99 = (1.06) exp(-uL) and 0.99 = (1.06) exp(u(en)L)

    i used u = 0.46903 and u(en)= 0.216626 (this is base from the link that you've given me before)

    but i got, L = 0.145 cm and L = 0.313 cm :( Attix gives the solutions as 312 cm and 144 cm

    did i lack something in my equation? :(

    pls help us...any comments will be very much appreciated

    million thanks..

    andy :(
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