The Application of Gamma Spectrometric Techniques with Plastic Scintillators for the Suppression of “Innocent Alarms” in Border Monitoring for Nuclear and other Radioactive Materials

Border monitoring equipment for nuclear and other radioactive materials apply fixed installed, automated “portal monitors” at checkpoints such as road and rail border crossings, airports or seaports, to detect gamma and neutron radiation in order to alert the front line officer about the presence of nuclear and other radioactive materials. Although highly sensitive and reliable systems are available today, a major problem, particularly for truck and railroad monitors as well as for pedestrian monitors e.g. at airports are frequent “innocent alarms” caused by naturally occurring radioactive materials (NORM) and medical radioisotopes administered to patients crossing borders. Such “innocent alarms” require extensive investigation by the front line officer to verify the alarm and identify the radioisotope in order to prove the innocent nature of the event. In addition this usually requires a lengthy holdup of the persons and vehicles involved. New developments of gamma spectrometric techniques and advanced software can be applied to automatically suppress “innocent” alarms caused by commodities such as fertilizers, ceramics, colored glass, optical lenses or welding rods containing NORM, i.e. K, Th Ra and U, even with low energy resolution plastic scintillation detectors as required for truck and train monitors. In addition pedestrian monitors, based on NaI scintillation detectors can be designed to simultaneously detect and identify the most frequently used medical radioisotopes in real time and indicate the “innocent” nature of the alarm to the front line officer. The paper describes detailed investigations with novel prototype truck and pedestrian monitoring systems at the Austrian Research Center Seibersdorf and the Vienna airport to evaluate the possibility to suppress “innocent” alarms while still maintaining adequate sensitivity for the detection of illicit nuclear and other radioactive materials. Introduction Numerous incidents in the past years involved illegal movement of nuclear materials and other radioactive sources across State borders. Even more frequently radioactive sources out of regulatory control have entered the public domain, in particular in metallurgical scrap, and sometimes caused significant radiation exposure. This creates a potentially serious hazard to public health as well as a thread of nuclear proliferation and terrorist activities. In 1995 the IAEA started a program to combat illicit trafficking in nuclear and other radioactive materials, which includes the operation of an international database on illicit trafficking incidents, which is now collecting reports from about 85 Member States [1]. Nuclear smuggling involving nuclear proliferation or nuclear terrorism is considered today as a “prime national security threat” in the United States and many countries all over the world, particularly after September 11. It is likely that front line inspectors will be the first law enforcement personnel to encounter radioactive materials. Law enforcement officers have therefore assumed a new important responsibility to detect and properly respond to special nuclear material and weapons of mass destruction, interdict hazardous radioactive materials and to protect themselves, their fellow citizens, the public and the environment from radiation hazard. Illicit trafficking in nuclear and radioactive materials is not a new phenomenon. However, concern about a "nuclear black market” has increased remarkably in the last decade. In addition to the threat of nuclear weapons getting in wrong hands, radioactive materials are widely used in industry of medicine and much easier accessible for criminals than nuclear materials, which are generally under physical protection. However, combined with conventional explosives they create “radiation dispersion devices (RDDs)”, nowadays often called “dirty bombs”. Such technically quite simple devices, could lead to a