A free neutron spontaneously decays into a proton, electron, and antineutrino. G

Question

A free neutron spontaneously decays into a proton, electron, and antineutrino. Given that mn = 939.6 MeV/c2, mp = 938.3 MeV/c2, me = 0.511 MeV/c2, m? ? 0 MeV/c2, and assuming that the antineutrino carries away a total mass-energy of 0.189 MeV, what is the approximate kinetic energy of the electron?
(A) 0.3 MeV.
(B) 0.5 MeV.
(C) 0.6 MeV.
(D) 0.8 MeV.
(E) 1.3 MeV.

Explanation / Answer

The reaction is           neutron ------> proton + electron + anti-neutrino     Energy of electron and anti-neutrino is          (939.6 Mev/c^2 - 938.3Mev/c^2 )c^2 = E + 0.189 Mev         E = 1.111MeV       E is the total energy of the electron (K + 0.511MeV)            K + 0.511 = 1.111MeV      kinetic energy of the electron          K = 1.111MeV - 0.511MeV    = 0.6 MeV

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