K.A. Miller W.S. Charlton “An Inverse Transport Approach to Radiation Source Location for Border Security”, Proceedings of the 29th Annual Meeting of the European Safeguards Research and Development Association, Aix-en-Provence, France, May 22-24, 2007.
Radiation portal monitors are being deployed at border crossings throughout the world to help prevent against the smuggling of nuclear and radiological materials. Many of these borders have several lanes for vehicles, each equipped with a portal monitor. With the current technology, if the detectors are alarmed, border guards must stop traffic and search for the source. In some cases, it can take hours to get through a busy border crossing. If radiation detection equipment adds a mere twenty seconds per car, this wait can increase by more than an hour. Another problem with these systems derives from the fact that one source can set off detectors in multiple lanes. If the source is being shielded by a vehicle in its lane, it may set off detectors in adjacent lanes but not its own. The purpose of this research is to develop an algorithm for identifying the location of a radioactive source using a distributed array of detectors. To locate the source, some knowledge about the vehicles is needed. When a detector is alarmed, cameras installed in each lane will take a picture of the vehicles and a computer algorithm will build a cross section model of the traffic. The cross section model will be used in neutron and radiation transport calculations to determine the position of the radioactive material. There has been a lot of work done using inverse transport calculations to determine the material properties of an object, and this work uses some of the same techniques for source location. Forward transport calculations using a step-difference approximation are used to define an error functional describing the difference between the actual and calculated detector readings given an estimated source location. Adjoint transport calculations making use of a steepest descent method are used to minimize that error functional and thus identify the source location.