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 .