1. The problem statement, all variables and given/known data 1)Determine the maximum possible speed of the daughter nucleus in negative beta decay of a He(6,2) nucleus which is initially at rest. 2. Relevant equations 3. The attempt at a solution I solved in the following way: He(6,2) --> Li(6,3) + electron + antineutrino The daughter nucleus will have maximum speed when the electron and antineutrino has zero kinetic energy. Applying mass-energy equivalence, M(He)c^2 = M(Li)c^2 + K(Li) Where M(He)=atomic mass of helium isotope, M(Li)= atomic mass of Lithium isotope, K(Li)=Kinetic energy of daughter nucleus On simplification I get, K(Li)=3.52 MeV (1/2)M(Li)v^2=3.52 MeV On simplification I get, v=1x10^7 m/sec But the answer given in my book is 1x10^5 m/sec. Could someone please tell me where I have gone wrong?