in a way to help determine the presence or absence of explosives, and this has not yet been accomplished.
In addition to high detection sensitivity and low false alarm, the detection of the explosive should be independent of the specific configuration and must be non-intrusive in order to protect privacy. The detection equipment, of course, must be non-hazardous to the contents of the checked items and to the operating personnel and environment. Other more general criteria are that the system must be reliable, easily maintained and operable by relatively unskilled personnel and that the cost must be low enough so as to be non-burdensome to airlines and airports. Finally, it is desirable, when all other requirements are achieved, that the size of the system be relatively small so that the system may be useful in a wide variety of environments.
In addition to the nuclear based systems described above, non-nuclear systems have also been investigated. These systems have occasionally achieved relatively high efficiencies of detection for some types of explosives, but generally have relatively high false alarm rates and have long screening times. These type of non-nuclear systems, therefore, by themselves cannot achieve the desired results. However, some features of such non-nuclear systems may advantageously be combined with a nuclear system as described herein, to significantly improve the overall detection capabilities of the system.
In order to develop a proper explosive detection system, an understanding is required of the properties of the various explosives relevant to the specific techniques to be used. Although there are a large number of explosive types, a general classification into six major groups with minor variations, has been proposed. The proposed classification scheme includes the following types of explosives: (1) nitroglycerine based dynamites, (2) ammonium nitrate based dynamites, (3) military explosives, (4) homemade explosives, (5) low order powders, and (6) special purpose explosives.
In general, all of these explosive types contain a relatively high amount of nitrogen ranging from nine to thirty five percent by weight. The nominal density of these explosives is typically 1.6 g/cm3, with ranges between 1 to 2 g/cm3 or more. These physical properties demonstrate that the most unique signature of explosives is the high concentration and density of the nitrogen content. Also the presence of other elements in combination with the presence of nitrogen can be considered. Physical factors may also help identify explosives. One physical factor, for example, is a minimum propagation thickness or diameter in order for most explosives to be effective. This minimum propagation thickness requires a sizeable contiguous body of explosives in the other two dimensions. This information is useful to the detection of explosives without making a specific assumption of the actual shape of the explosive.
It can be seen, therefore, that a nuclear detection technique can provide for the detection of the nitrogen content, which can provide an indication as to the presence of an explosive. However, the frequent occurrence of nitrogen in non-explosive materials limits the level of detection sensitivity and merely detecting the presence of absence of nitrogen alone is generally not sufficient. Therefore, additional information is required beyond simply sensing the presence of the nitrogen. The present invention provides for this additional information.
The present invention provides a nuclear-based detection system and method that efficiently detects explosives in checked airline baggage or other parcels with a high probability of detection (PD) and a low probability of false alarms (PFA). The detection system advantageously combines several detection criteria to improve the speed and performance with which the detection system is able to discriminate between objects (parcels, suitcases, packages, etc.) with explosives and objects without explosives.
The first and foremost criteria used by the present invention to detect explosive