High Resolution Detection Systems for Interdiction of Nuclear Material Trafficking

The interdiction of nuclear materials during illegal transportation requires both detection of radiation and identification of radionuclides in order to distinguish among the various categories of nuclides and their implied threat. Interdiction regimes, based upon available technologies, were developed rapidly after the tragic events of 9-11. In general, a low resolution or "no-resolution" portal monitor, based on scintillators, provides initial detection, followed by a search and identify phase. A major problem with these systems is that of innocent alarms or false positives, where, for example, a naturally radioactive substance triggers a portal monitor. An equally severe problem is false negatives whereby nuclear material can be masked or hidden in an otherwise innocent shipment. Recent experience has shown that in general, 90% of innocent alarms at airports are due to medical isotopes in the bodies of travellers, while in cargo freight, the major problem is caused by natural emitters. Ultimately reliable nuclide identification must be made. For logistical reasons, such as to preserve traffic flows, it is necessary to make a positive identification in as short a time as possible. Germanium detectors (HPGe) have been the choice for high-resolution, gamma-ray spectroscopy for many years because of their high resolution and efficiency or stopping power especially at energies above 2 MeV. These detectors are considerably less vulnerable to the problems of false negatives and positives, and are able to identify nuclides with a high degree of certainty. A new portable nuclide identifier based upon the latest HPGe technology will be described.