THE DETECTION OF THE ILLICIT MOVEMENT OF NUCLEAR MATERIALS BY MEANS OF HIGH-RESOLUTION GAMMA-RAY SPECTROMETRY

Four classes of malevolent acts my be detected and interrupted by measurement of gamma rays emitted from nuclear materials: 1) The removal of stolen fissile materials from U.S. territory; 2) The introduction of fissile materials for later assembly into a destructive device; 3) The introduction of an explosive device intended for the dispersal of radioactive materials; 4) The introduction of a nuclear explosive device. In response to these possible threats, a large number of radiation detectors are being installed at various locations. These detectors produce a large number of “innocent” alarms through the detection of radioactive materials present as a result of legitimate activities. This includes radioisotopes used in medical practice or radiography, old radium dial instruments, natural radioactivity in fertilizer, etc. These alarms make heavy demands on responders and interfere with legitimate activities. The solution to this problem is the rapid acquisition and analysis of high-resolution gamma-ray spectra which can immediately identify the materials in question and distinguish legitimate activities from malevolent ones. In the latter case the spectra may provide forensic data which will aid in the investigation of the event. We are investigating three approaches to this problem: 1) The use of a HPGe spectrometer equipped with Compton suppression, which will provide increased detection sensitivity and analyzing power and which will provide design parameters for a similar portable instrument; 2) The use of a simple and rapid method for the separation of radium isotopes from uranium samples and their subsequent analysis by gamma-ray spectrometry, which can provide forensic evidence on the history of the uranium; 3) The utilization of Canberra Industry’s ISOCS gamma-ray spectrometry system for the rapid detection and analysis of various radioactive materials of interest. Procedures will be developed which will enable the instrumentation to be used effectively by non-specialists. The status of these investigations will be reported.