e presence of any explosive material.
It thereby follows that any system of checking luggage or parcels must have a very high probability of detection (PD) in order to be effective. Because of the large number of parcels processed, a high throughput is necessary for practicality. In addition, any detection system, because of the large number of scanned items, is bound to occasionally give a false alarm. The probability of these false alarms (PFA) must be minimized in order to provide for an effective explosive detection system. This is because when an alarm occurs it is not known, at that time, whether it is true or false. This means that each time an alarm occurs, a passenger or parcel must be detained for further investigation. If false alarms are significantly high the nuisance level and the delays could be unacceptable to the public. It is, therefore, important that any explosive detection system must have a very high probability of detection (high PD), a high throughput rate, and yet at the same time have a very low probability of false alarms (low PFA). These conflicting criteria have hampered efforts in the past to build a reliable and usable system.
In general, prior art systems have not met the desired characteristics of having a high probability of detection (PD) with a low probability of false alarms (PFA) at acceptable throughput rates. As an example, one such prior art system is shown in U.S. Pat. No. 3,832,545. This patent provides for a system for the detection of nitrogen, which is generally present in the explosive materials to be detected. As described in the referenced patent, a rough two-dimensional profile of the nitrogen content within the object being inspected is provided. This profile is then used in an attempt to determine whether explosive materials are present. Unfortunately, however, because of the types of detectors used by the invention described in the '545 patent (liquid or plastic scintillators), the processing of the detector signals is quite inefficient. Moreover, the two-dimensional 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 