Hello, I looked around before and could not find an answer to this question. So given the reaction neutron + X -> Y + gamma and we assume that both initial particles are at rest. Then using momentum balance we can find that the energy of the gamma is: Egamma = -m_Y*c^2 + sqrt( (m_Y*c^2)^2 + 2Q*m_y*c^2). Here is what I don't get. The question says to assume that m_Y*c^2 >> Q. Using the binomial theorem and rearranging the above you get that in the limit, E_gamma -> Q. Why does the energy of the gamma approach Q? I originally thought that the energy of the gamma would approach 0 since the increasing rest mass of Y would cause more energy to be needed as binding energy in Y, thus leaving less energy for the gamma. Please help... ;) Thanks.