with an unusual disease or syndrome and to then establish the nature of the pathogen by standard laboratory diagnosis. Physical sensors that screen for aerosolized particles, and molecular probes that establish the nature of the organism, would complement the classic process and permit quicker analyses. There is also the possibility of symptomatic surveillance—real-time screening in hospital emergency rooms of syndromes such as flulike illness, diarrhea, and rashes and spots. By feeding such data into sophisticated computer models, it may be possible to detect subtle fluctuations in symptomatic admissions, suggesting that something above the background rate of illness, such as a bioterrorist attack, is occurring.

One of the most exciting possibilities for early detection of a biological outbreak is preclinical diagnosis. With the elucidation of the DNA sequence of the human genome, it may be possible to examine selective patterns of gene mutation induced by different biological agents in humans long before the actual organism has been detected. As we learn more about the pathogenesis of different agents and the specific bodily responses mounted against them, it may turn out that each pathogen induces a unique molecular signature in the host gene-expression response. Thus, using DNA chips, it may someday be possible, without ever having to culture suspected agents, to know what type and perhaps what species we are encountering—and to commence focused and rapid treatment accordingly. (See Chapter 3 for more on detection of biological outbreaks.)

An important line of defense in a layered system of homeland protection is the detection and interdiction of illicit nuclear weapons and special nuclear material (SNM), as well as the detection and disruption of illicit weapons development programs. Sensors and sensor networks can contribute to this defense effort by providing technical means for detecting the movement of SNM, especially highly enriched uranium (HEU), either in weapons or as contraband, through border transit points and around critical U.S. assets such as ports, cities, and other high-value facilities. A national detection network could consist of several types of sensors: large numbers of simple counters that indicate the presence of radiation, backed up by smaller numbers of spectroscopic instruments to identify specific isotopic signatures. The technical challenge for the deployment of both types of sensors is the differentiation of signals of interest from the background of naturally occurring radioactivity and medical/industrial radioisotopes.

The presence of certain types of penetrating radiation is a signature of most (but not all) SNM. Passive detection of gamma rays and/or neutrons can be an effective technique in some circumstances for revealing the presence of illicit SNM or improvised nuclear devices (INDs), though passive monitoring of these materials would require large-area detectors for acceptable sensitivity. In other cases, active interrogation methods using neutron detectors and pulsed neutron sources may be required. Active systems are more complex and costly than passive detectors. Additionally, some materials (those with high atomic number) can be detected indirectly by gamma radiography. While shielding of SNM can



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