The NMM probe contains a slow neutron detector and a radioactive source (Americium Berellium 241AmBe) that emits fast neutrons. These are absorbed, deflected and thermalised (slowed) by the surrounding material. Hydrogen (H) thermalises neutrons most effectively as the atoms' similar size results in an energy losing elastic collision. Oxygen (O), nitrogen (N) and carbon (C) are also relatively efficient thermalisers, however, roughly 6-8 times more collisions are required than hydrogen.

Consequently, the number of thermalised neutrons deflected back to the NMM is directly proportional to the amount of water (H2O) present in the surrounding soil (but also depends on the prescence of trace elements, tubing material, and soil density). The sphere of influence of the NMM depends on the wetness of the soil and varies from 15-50 cm in wet and dry conditions respectively (50% vol/vol and 2% vol/vol).