1. The problem statement, all variables and given/known data Confirm Chadwick's statement that if the protons ejected from the hydrogen were due to a Compton-like effect, the incident gamma energy would have to be near 50 MeV, and that such gammas would produce recoil nuclei with energies up to about 400 keV 2. Relevant equations 3. The attempt at a solution To be honest I have no idea how to go about this question. I've read the papers in question where he gives these numbers, but he gives no indication of how he reached these numbers, other than with a "Compton-like" effect. I've tried putting various numbers into the Compton scattering equation, changing the mass of the recoil electron to the mass of a nitrogen atom, and solving for the energy after the interaction hoping to be able to verify that its momentum (or at least KE) will correspond to the velocity of the particle given in the paper (about 0.1c). But none of these things have worked (unless we've suddenly switched to a world where negative KE is allowed and no-ones told me so). And I have no ideas about what to do with this question. I'm planning on going to see the lecturer tomorrow some time, but in the mean time if any one has any hints or pointers for me then I'd be a much happier person (happier still if I could understand what I was doing with them!) Thanks for reading!