Neutrons originated from radioactive elements in the ambient have kinetic energies up to a few MeV. We want to detect such neutrons with a TPC containing ⁴⁰Ar. If the neutron energy is low enough, the internal structure of the nucleus is not resolved and it appears as a single object to the neutron (coherent scattering). Assuming a nuclear radius R_A=4 fm, what is the minimum neutron kinetic energy needed to resolve the nucleus structure? What is the maximum recoil energy of the nucleus in a collision with the limit initial kinetic energy [m_Ar=37.2 GeV]?