X17: A new force, or evidence for a hard $γ+γ$ process?

Abstract: It is investigated whether the "X17 puzzle" might be explained by a nuclear decay chain and a conversion of the two resulting highly energetic $\gamma$s into an electron-positron pair. It is found that the corresponding kinematics fits perfectly to the experimental result. Also the conversion rates of this process are reasonable. However, the assumed nuclear chain reaction is not favored in the established nuclear models and no explanation for the isospin structure of the signal can be given. Thus, it has to be concluded that the process studied in this paper does not give a completely satisfying explanation of the "X17 puzzle".