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Homework Help: Beta -decay converting u to MeV

  1. Jun 7, 2010 #1
    i'm new so i hope i am writing this in the correct part of the forums

    this is the question i have been given:

    What is the energy Q released when ^{131}_{\ 53}{\rm I}decays and ^{131}_{\ 54}{\rm Xe} is formed? The atomic mass of ^{131}_{\ 53}{\rm I} is 130.906118 \rm u and the atomic mass of ^{131}_{\ 54}{\rm Xe} is 130.90508 \rm u.
    Express your answer in megaelectron volts to three significant figures.
    Q =

    i have worked out that Delta m = 0.001038u
    so assumed i could use E= {\Delta m} c^2
    i am using 3.00*10^8 as c (if this is wrong please tell me)
    and received the answer 9.342*10^13

    could you please help me to convert this to MeV and let me know why i keep on getting this
    question wrong :(
  2. jcsd
  3. Jun 7, 2010 #2
    lol ok i worked it out now
    i didn't need to do the c^2 part because it canceled out
    so i just needed to go
    0.001038 x 931.5
    which gave me the right answer of
  4. Jun 12, 2010 #3
    Don't worry I've made little mistakes like this too! :wink:

    Just to advise that if you remember to check the units along the way, this can help to make sure you've done things like this right. But you'll probably always remember this now!

    So you had [itex]\Delta m = 0.001038u[/itex] which is obviously units of [itex]u[/itex].

    Then the conversion is [itex]1u = 931.494MeV[/itex]

    Which means indeed you can just multiply the two and the units work out fine. :smile:
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