A collision of particles wherein the resultant sum of kinetic energies of the particles remains the same after collision as before. In well logging, the elastic collision between a fast [[neutron]] and [[nucleus]] of a hydrogen atom ([[proton]]) is the predominant means by which the neutron loses energy to reach epithermal or thermal energy levels. Elastic collision and the resulting elastic scattering is important in neutron-logging methods involving neutron-neutron interactions. See also [[epithermal neutron]] and [[thermal neutron]]. Compare [[inelastic collision]].