Abstract: Measurement of the level of materials in bins, hoppers, and the like is effected by a neutron monitoring technique. A sensor is located to move substantially vertically in a guide located in or near the bin or hopper. The sensor contains a fast neutron source, a detector of thermal neutrons, and a moderator-containing material in the vicinity of the detector. In a preferred form of the sensor, the neutron source and thermal neutron detector are separated by lead and cadmium screening, and the moderator-containing material is a sheath of polyethylene around the detector. With sensors of this type, material levels may be sensed whether the material contains a neutron moderator or not.