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Homework Help: Finding the Equilibrium Temperature

  1. Apr 23, 2008 #1
    1. The problem statement, all variables and given/known data

    following on from my thread in the Nuclear physics subforum, I am trying to work out the equilibrium temperature of three isotopes (Neon) in a state of equilibrium established by thermal contact with a bath of neutrons at a temperature T.

    2. Relevant equations

    [tex]\frac{[20Ne][n]}{[21Ne]}=exp(\frac{B(21)}{kT})[/tex]

    [tex]\frac{[21Ne][n]}{[22Ne]}=exp(\frac{B(22)}{kT})[/tex]

    where for example,
    B(21Ne)=[Mass(20Ne) + Mass(neutron) - Mass(21Ne)]c[tex]^{2}[/tex]
    square brackets [..] represent concentrations

    This is the relavent data for Neon:

    A Ab mass (amu)
    20 90.92 19.9924
    21 0.26 20.9939
    22 8.82 21.9914

    3. The attempt at a solution

    Using the equations to find T gives:

    [tex]T=\frac{B(21Ne)-B(22Ne)}{kln\frac{[20Ne][22Ne]}{[21Ne]^2}}[/tex] (*)

    Now, B(21Ne)-B(22Ne)
    ={[Mass(20Ne)+Mass(neutron)-Mass(21Ne)]}-{[Mass(21Ne)+Mass(neutron)-Mass(22Ne)]
    =Mass(20Ne) + Mass(22Ne) - 2*Mass(21Ne)

    which is NEGATIVE (for neon, as well as for other isotope triplets)!

    Because the denominator of (*) is positive, the equilbrium temperature is NEGATIVE! Weisacker who calculated this temperature worked it out to be 2.9x10^9K. What am I doing wrong? (I work it out to be -4.6x10^9)
     
  2. jcsd
  3. Apr 26, 2008 #2
    I don't see any data about the number of each izotopes. Maybe you used higher number for higher energy state?
     
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