[en] Fast neutrons are scattered by the nuclei of heavy atoms. When they collide with hydrogen nuclei that have about the same atomic weight, they also lose speed. The number of slowed-down or thermal neutrons is approximately proportional to the number of hydrogen atoms per unit volume. A nuclear moisture probe consists of a source of fast neutrons with a detector of thermal neutrons. The method allows a continuous non-contact measurement and gives representative moisture values from a large amount of material. The measuring indication can be calibrated directly in weight-% moisture, if the dry bulk density is constant. Moisture/density probes are equipped with a γ source in addition to the neutron source. An electronic division of the moisture in volume-% and of the density allows also calibration in weight-% moisture in such cases. The volume of material measured by the nuclear method is dependent on the range of neutrons. The diameter of the spherical volume amounts to about 140 cm at 1 volume-% moisture and is diminished to about 48 cm at 25 volume-%. The probes must be mounted in such a way that the active measuring volume includes the flowing material. Also the moisture value of the laboratory sample is only coincident with the nuclear indication when a homogeneous moisture distribution exists. So-called bunker probes and surface probes are available. At the same moisture value and the same intensity of the neutron source, bunker probes are about 5 times more sensitive than surface probes. (author)