A Fieldable-Prototype, Large-Area, Gamma-Ray Imager for Orphan Source Search

We have constructed a unique instrument for use in the search for orphan sources. The system uses gamma-ray imaging to ldquosee throughrdquo the natural background variations that effectively limit the sensitivity of current devices. The imager is mounted in a 4.9-m-long trailer and can be towed by a large personal vehicle. Source locations are determined both in range and along the direction of travel as the vehicle moves. A fully inertial platform coupled to a Global Positioning System receiver is used to map the gamma-ray images onto overhead geospatial imagery. The resulting images provide precise source locations, allowing rapid follow-up work. The instrument simultaneously searches both sides of the street to a distance of 50 m (100-m swath) for millicurie-class sources with excellent performance as determined using source injection studies and receiver-operator-characteristic techniques.