Elimination approach:
"Normalization" and "Radiation" are not standard terms describing neutron energy loss in a medium, they are simply not the physics vocabulary used here, so they can be discarded. "Cooling" is a loose, non-technical word that does not describe a specific physical mechanism. The precise nuclear-physics term for fast neutrons repeatedly colliding elastically with surrounding atomic nuclei, such as hydrogen in soil moisture, until their kinetic energy matches the thermal energy of the medium is thermalization. This principle is, in fact, the basis of neutron soil-moisture probes, which count thermalized neutrons to estimate water content.