KVPY Sample Paper KVPY Stream-SX Model Paper-14

  • question_answer
                                \[A{}^{238}U\] nucleus emits a 4.2 MeV \[\alpha \text{-}\]particle. If we take account of recoil energy of \[{}^{234}Th\] daughter nucleus formed in the process, then Q value (disintegration energy) of the reaction will be

    A) \[4.2\left( 1+\frac{4}{238} \right)MeV\]

    B) \[4.2\left( \frac{234}{238} \right)MeV\]

    C) \[4.2\left( \frac{238}{234} \right)MeV\]

    D) \[4.2\left( 1-\frac{4}{234} \right)MeV\]

    Correct Answer: C

    Solution :

    Uranium nucleus is initially at rest. Conservation of momentum gives
    \[0={{p}_{\alpha }}+{{p}_{Th}}\]\[\Rightarrow \]\[{{p}_{Th}}=-{{P}_{\alpha }}\]
    Also, \[Q={{K}_{a}}+{{K}_{Th}}\]
    \[\Rightarrow \]\[Q={{K}_{\alpha }}+p_{Th}^{2}/2{{m}_{Th}}\]
    \[={{K}_{\alpha }}+\frac{p_{\alpha }^{2}}{2{{m}_{Th}}}={{K}_{\alpha }}+\frac{2{{m}_{a}}\cdot {{K}_{\alpha }}}{2{{m}_{Th}}}\]
    \[={{K}_{\alpha }}\left( 1+\frac{{{m}_{\alpha }}}{{{m}_{Th}}} \right)=4.2\left( 1+\frac{4}{234} \right)\]
    \[=4.2\left( \frac{238}{234} \right)MeV\]


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