1. The problem statement, all variables and given/known data Apologies if this is in the wrong section, it seemed too basic to place in the advanced physics bit. I'm just trying to calculate the Q-value for the reaction process n + 14N → 12C + 3H 3. The attempt at a solution The Q-value is just the difference in the nuclear masses on each side of the equation, I do some re-arranging to find out it's actually the difference in the mass excess of each atom on both sides of the equation. I'm using this site for the mass excesses; http://www.nndc.bnl.gov/amdc/masstables/Ame2003/mass.mas03 [Broken] For the neutron, 14N, 12C and 3H (tritium) the mass excesses are 8071.31710, 25251.164, 24926.178 and 14949.80600 keV respectively. I plug this in, and get the Q-value as -6.55 MeV, but I know the answer is meant to be -4.015MeV. Indeed, I've been told it is and also this Q-value calculator; http://www.google.co.uk/url?sa=t&rc...j4GoBg&usg=AFQjCNE3wKiIsOpfqB2dWJ8vcUsk4qJtSA Confirms it for me. So what could I be doing wrong? I'm definitely not typing it in to my calculator incorrectly! Edit: Aside note, I'm told that the neutron must have the threshold energy of 4.015MeV in the CM frame for this process to occur. Is there any reason it's specific to the CM frame? Is it to do with the fact that the total energy of the system in the CM frame is the minimal energy as seen from all possible inertial frames?