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There are essentially two principal possibilities for nuclear or radiological terrorism: detonation of nuclear weapons or devices, and radiological attacks, which release radioactive materials as a result of either assaults on nuclear installations, the use of "dirty" bombs, or placement of radioactive sources in public places. Whereas the consequences of nuclear detonation are potentially catastrophic in terms of loss of life, and political and economic destruction, the likely consequences of most forms of radiological attacks would be more psychological than physically destructive. Opportunities for the physics community to help counter either form of terrorism or to abate its impact might include improving the detection and interdiction of illicit weapons and special nuclear materials, and helping to educate the public about radiation protection and about the limited risks associated with radiological devices.
Technical means will need to be developed to detect the movement of special nuclear materials, especially highly enriched uranium, at cargo-container ports in the U.S. and abroad, and other "choke points," such as entry points into major U.S. cities. Whereas plutonium emits both neutrons and moderately penetrating gamma rays, uranium-235 emits very few neutrons and only weakly penetrating gamma rays. In the absence of intervening materials, detection ranges of tens of meters are practical, in tens of seconds, with appropriately sized passive neutron and gamma-ray detectors. In the presence of intervening materials, practical detection ranges for U- 235 can drop to a few meters or less, and thus current passive sensors will be most effective primarily for plutonium, and for U-235 in situations where only a small amount of intervening material is possible. In situations where the containers are large enough to provide substantial shielding from the detectors, more highly sensitive passive detectors will need to be developed, while smaller passive detectors need to be developed for other applications.
Alternatively, "active" sensor systems may need to be employed to probe the inspected volume for U-235 or to detect the presence of either highly attenuating materials or the fissile material itself. This is the case for cargo containers widely used on container ships, trains and trucks. Highly attenuating materials might be detected by gamma-ray transmission measurements. Fissile material can be detected by similar transmission measurements, as these materials are very dense, or more specifically by neutron interrogation - an area requiring further R&D. Cost, complexity and radiation safety are other important issues which may limit the applications of current active sensor technologies.
Detection methods need to be developed which will limit the number of false hits that would be received from legitimate transport of commercial radioisotopes. It has been recommended that large numbers of small simple passive counters be combined with smaller numbers of spectroscopic instruments to identify specific isotopic signatures in order to differentiate signals of interest from the background of naturally occurring radioactivity and medical and industrial radioisotopes. New R&D has been recommended to develop low-cost instruments with spectroscopic capability and improved sensitivity and selectivity.
The physics community may also be able to play a role in educating the public about radiation. It has been recommended that public education efforts be carried out well in advance of a possible nuclear or radiological attack in order to provide the affected public with basic information on protection against radiation and, in the case of nuclear attack, against fallout. With regard to radiological dispersion devices, the public fear of radiation and radioactive materials appears to be disproportionate to their actual hazards. It has also been recommended that prepackaged kits of written materials on basic radiation science and effects should be developed for the media and national state and local leaders to help them respond appropriately to radiological attacks.