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Simple Atomic Mass Unit question

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  • 06-05-2012 4:10pm
    #1
    Smythe
    Registered Users Posts: 434 ✭✭


    The uranium isotope 235U captures a neutron and undergoes fission to produce 93Rb and 141Cs. Calculate the energy released in this process.
    The nuclear masses of the relevant isotopes are
    235U 235.0439u,
    93Rb 92.9217u,
    141Cs 140.9195u

    The answer is given as
    n + 235U → 93Rb + 141Cs + 2n
    LHS 235.0439 + 1.0087
    RHS 92.921712 + 140.91949 + 2.10087
    Difference is 0.1943u = 181 MeV

    I realise the quantity of 1.0087 is the mass of neutron divided by one unified atomic mass unit. So that's (1.67493 x 10^-27) / (1.66 x 10^-27).
    Also, the 2.10087 number is simply double this.
    How have the 235.0439, 92.921712 & 140.91949 values been calculated? I know it's to do with E=Mc^2
    Thank you.


Comments

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    #2
    Parsley
    Closed Accounts Posts: 3,144 ✭✭✭Parsley


    they're given to you, they're the masses of the Ur, Pb, and Cs.


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    #3
    Smythe
    Registered Users Posts: 434 ✭✭Smythe


    Oh yes, I see what you mean. Thanks Parsley.

    In the answer, how is it known that 2n should be added? Why 2?


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    #4
    Parsley
    Closed Accounts Posts: 3,144 ✭✭✭Parsley


    you've to balance the numbers, i don't think any neutrons get completely destroyed, they're released, and propagate the chain reaction. been a while since i covered it though.

    as for why it's 2, i think you either balance the number of neutrons on both sides, or, if you don't know the number of neutrons, round off the atomic masses to balance, make that difference up in neutrons, and anything left in mass difference is what's converted to energy. if you get me.


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