KVPY Sample Paper KVPY Stream-SX Model Paper-7

  • question_answer
    A free neutron decays into a proton and an electron as, \[_{0}^{1}n\xrightarrow{{}}_{1}^{1}p+_{-1}^{0}e+v\] neutron-hydrogen atom mass difference is \[840\mu u\]. then the maximum possible kinetic energy of electron will be\[\left( 1u=932MeV \right)\]

    A) \[0.783MeV\]                 

    B)  \[0.840MeV\]

    C) \[0.589MeV\]                 

    D) \[0.687MeV\]

    Correct Answer: A

    Solution :

    Electron's energy is maximum if no neutrino is emitted. Then,
    \[{{K}_{\max }}=\left( {{m}_{n}}-{{m}_{p}}-{{m}_{e}} \right).{{c}^{2}}\]
    \[=\{{{m}_{n}}-\left( {{m}_{p}}+{{m}_{e}} \right)\}.{{c}^{2}}\]
    As, \[{{m}_{p}}+{{m}_{e}}=mass\,of\,H-atom\].
    We have,
    \[{{K}_{\max }}=\left( {{m}_{n}}-{{m}_{H}} \right).{{c}^{2}}=\left( 840\mu u \right).{{c}^{2}}\]
    \[=840\times {{10}^{-6}}u\times 932\frac{MeV}{u}=.0.783MeV\]


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