[en] The work has focused on developing low-cost methods for bulk detection of explosives using two criteria: 1) explosive materials have higher density than that of most common organic materials, but less than that of soil and metals, and 2) nitrogen-based explosives have a higher nitrogen and oxygen content than most common materials. An attempt to directly use fast neutrons to detect explosives was empirically done by monitoring fast-neutron transmission, together with the scattering, at many directions, of slowed-down neutrons, from an object exposed to californium-252 neutrons. Mapping on these measurements against each other indicated that the count rates tended to cluster within a characteristic region in the map when an explosive-like material (fertilizer) was present in the object. Another approach is to use photon absorption (the photoelectric effect) which is strongly dependant on the atomic number, while photon scattering (the Compton effect) is dependant on the electron density; which is in turn dependant on the mass density. Therefore a combination of the two effects can meet the two criteria designated above. The results of some preliminary work are promising and demonstrate the feasibility of the concept, but more work is needed for development of a detection system