Active neutron interrogation for the detection of fissile material

Active neutron techniques are being used worldwide for improving sensitivity of fissile material detection for security as well as in nuclear industry for nuclear material accounting. The advantage of this method is that it can detect very small quantities of fissile materials even in presence of high neutron and gamma background and it is much more sensitive compared to passive neutron and/or gamma techniques. An active neutron interrogation technique based on differential die-away analysis has been developed at Purnima Labs. This technique works by exposing a medium to pulses of neutrons. An external fast neutron detector measures the time dependent fast neutron flux resulting from the irradiation. When no fissile material is present in the inspected medium, the detector should only measure a signal representing the diffusion of the thermalized neutrons in the detector body (detector neutron ''die-away'' time). If fissile material is present, the detector will show an additional signal decaying with the die-away time of the inspected medium. If the latter is significantly longer than the former it will dominate the decay curve at later times and unequivocally establish the presence of fissile material in the inspected object. The amplitude of the signal and its time constant is determined by several factors, among them the quantity and location of fissile material present in the inspected medium