Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Nuclear decay

  1. Mar 2, 2007 #1
    complete data of question:
    a nitrogen nucleus N(A=14,Z=7) bombared with an alpha particle of a certain energy transmutes to an oxygen nucleus(A=17,Z=8) and a proton
    a:write a equation for this nuclear reaction
    b:find the minimum energy of alpha particle to make this reactin occur
    (mass of nitrogen=2.32530*10^-26kg,mass of oxygen=2.82282*10^-26kg,mass of proton=0.16735*10^-26kg and mass of alpha particle=0.66466*10^-26kg)
    attempt on this questin:
    i have written the reaction
    in second part we have to calculate mas defect for finding its energy by e=mc^2
    in fission reaction i learnt that mass of reaction will always be greater than product,but here by calculation mass of product is greater ,why is this so.and plz tell me why energy equivalent to mass defect is always alpha particle energy why not other elements like nitrogen and oxygen
    answer of this question is1.89*10^-13j,i have calculated this answer but that comes in negative as i subtracted reactants minus product
  2. jcsd
  3. Mar 2, 2007 #2
    In a fission reaction some rest mass energy is converted to kinetic energy. The products of the reaction get the kinetic energy.
    In this reaction the alpha particle starts with a large amount of kinetic energy which is then used to create the extra mass of the products. Because we start with large amounts of KE you need to subtract the mass of the reactants from the products to find the 'mass defect'.
    Remember there isnt really a mass defect. We get the mass defect because we use rest mass values. The mass of the moving alpha particle is higher than its rest mass value.
    The nitrogen and oxygen atom are assumed to have very little KE. We do not normally find atoms moving at very high energys. At room temperature their velocity due to temperature is about 500 m/s.
    It is the high energy of the alpha particle which makes it an ionising radiation particle, distinguishing it from just an ionised helium atom. The same is true of the difference between beta radiation and electrons. 1.89*10^-13 j may not sound high but it is enough to make the alpha particle move at nearly 8 million m/s.
  4. Mar 2, 2007 #3
    thank you,hope u will help me in future as well
  5. Mar 2, 2007 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    It is not true that in a fission event the mass of the original nucleus will always be greater than the masses of the products. In fact, in the fission of a U nucleus, this is not the case. The masses of the products are less than the mass of the original nucleus. If it were otherwise, energy would not be released.

    Whether energy is released in any nuclear reaction is determined by the difference in total nuclear binding energies of the original nuclei compared to the total nuclear binding energies of the products.

    Since these binding energies indicate the amount of energy required to break all the nuclear bonds, the greater the binding energies, the lower the mass per nucleon. If the total nuclear binding energies of the products is greater than the total nuclear binding energies of the original nuclei, there is a net release of energy.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook