sional limitation allows many materials to be positioned in the object being examined so as to defy detection, thereby providing an unacceptably low PD and high PFA.
Other types of prior art explosive detection systems depend upon the prior seeding of explosive materials with a tracer material, such as a radioactive tracer. Although this type of system could be very useful if all explosive material were manufactured with such tracer material, because of the large amount of explosive material which has already been manufactured and because of the difficulty of controlling the manufacture of all explosive material so as to contain such tracer material, this type of system is not practical. A useable system must be able to detect the presence of explosive material of a conventional type and of an unconventional type, whether disposed within an object either in its original manufactured form, or if deployed within the object so as to attempt to confuse or evade the detection system. The prior art systems have not met these various criteria and cannot produce the desired high probability of detection with the relatively low production of false alarms.
An acceptable response to the explosive threat to aviation, mails, or shipping requires detection techniques that are highly sensitive, specific, rapid and non-intrusive. The efficient detection of nitrogen, at this point, offers the best overall solution, although other elements, such as hydrogen and chlorine, could also be detected. It is, therefore, important that the detection of nitrogen be provided to give the maximum information of the physical parameters of the explosive, such as density and spatial distribution. The use of nuclear based techniques which subject the luggage or parcels to thermal neutrons can be the basis of a system to produce the desired results, but this system cannot be based on the prior art techniques. It is important that the intensity, energy and spatial distribution of the detected radiations from the object under observation be provided 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