The invention uses a series of combined passive neutron and gamma ray sensors systematically placed along a path of commercial traffic, for example an airport runway, combined with a pulsed source of low energy protons and deuterons. The pulsed source produces monoenergetic gamma rays and low energy (60 keV) neutrons. This pulsed source uses a pinch-reflex ion diode operated on a pulsed power generator to produce proton beams. These beams bombard a PTFE (Teflon) target to produce characteristic gamma-rays. These gamma rays would induce a fission reaction in any fissile material creating gamma rays and neutrons. The passive sensors located in the path of commercial traffic would sense both the resultant gamma and neutron products of the reaction. The short interrogation pulse, approximately 50 nanoseconds, provides a timing marker which can be used to turn on the passive detectors after the interrogation pulse so as to avoid saturation and to enable measuring the spectrum of the fast neutrons using time of flight. The decay rate of the intensity of fast and slow (thermalized by interaction with the runway materials) neutrons provides an unambiguous signature of the presence of fissile material.