Abstract: A process measurement system for measuring levels, interfaces and other characteristics related to products positioned between a fast neutron source and a detector that directly measures the transmitted fast neutrons. The system far exceeds the limitations of neutron backscatter, thermal neutron transmission and gamma techniques while avoiding problems associated with them. As shown, a fast neutron source is placed on one side of a vessel that is being monitored, and a detector is placed on the opposite side. Fast neutrons have excellent penetration properties and thus measurements through thick walls and across substantial distances are possible. A direct measurement of the fast neutrons transmitted through the vessel and/or product is made. This arrangement provides an output that is a direct function of the transmitted fast neutrons between the source and detector, and is not dependent upon inferential measurement where errors are encountered.