1. The problem statement, all variables and given/known data Hi everybody, I was studyng a photon disintegrating in one electron and one positron in the presence of a heavy atomic core; I want to find the final momentum of the atomic core 2. Relevant equations Conservation of energy: Eγ=2me+Sqrt(Mn2+Pn2) Conservation of momentum Pγ=Pn Relation between the energy and momentum of a maseless particle Pγ=Eγ 3. The attempt at a solution From the conservation of energy: (Eγ-2me)2=Mn2+Pn2; and since Pn=Eγ, operating after developing (Eγ-2me)2, I obtain Pn=4me-(Mn2/me). If I introduce the values in units of atomic mass, I obtain a negative value of Pn, the momentum of the atomic core. Can anyone please tell me what am I doing wrong? Thanks.