t of the wool article is quite high, the concentration does not reach the levels of explosive material.
FIG. 5(c) illustrates an explosive material in a sheet form along one side or edge of the luggage and again, the concentration of nitrogen and high readings relative to the lower readings indicates the presence of something having a relative high concentration of nitrogen, plus a relatively high density for this concentration. This again would typically be a profile of an explosive material. The computer 58, therefore, may be programmed to identify such specific profiles and provide for an alarm such as through an alarm 62 so that the luggage or parcel may be subjected to a more thorough inspection.
Referring next to FIG. 6, a typical thermal neutron activation gamma ray spectrum is illustrated. The vertical axis represents the gamma ray count; the horizontal axis represents the particular channel number through which the count is detected. As each channel is set up to measure a particular energy level, the horizontal axis is thus proportional to the various energy levels of the detected gamma rays. A good description of the types of instruments and measurement techniques used to detect nuclear radiation, such as gamma rays, may be found in, e.g., Nuclear Instrumentation, Section 1.5, pp. 4-18 (McGraw-Hill Series in Nuclear Engineering).
In FIG. 6, it is seen that the gamma ray peak 126 corresponding to nitrogen, labeled "N (10.8 MeV)", has a smaller amplitude than does the surrounding background noise. Thus, in order to better detect the gamma rays occurring at 10.8 MeV, the present invention contemplates using a spectral correlation method (SCM), or similar technique, to reduce the background noise. Essentially, the SCM determines the background noise at the desired location of the spectrum i.e., the nitrogen peak, by correlation with other spectral regions such as near the chlorine peak 128 (8.6 Mev). Interferences from weak peaks in the desired region can be determined by measuring an associated strong peak elsewhere in the spectrum and subtracting the appropriate fraction from the region of interest. As the sources of background noise in the system are fundamentally related over the entire spectrum, i.e., the noise at one location is related to the noise at another location, this background subtraction technique has the desired effect of removing much of the background noise near the spectral peak of interest.
FIG. 6 also shows additional gamma ray peaks which correspond to other elements, e.g. hydrogen, chlorine. Gamma rays detected in these peaks can be used to measure the amount of elements other than nitrogen present in the object under investigation. Measurements of these elements are used to further enhance the detectability of contraband, e.g. explosives, in the object. For example, explosives typically exhibit hydrogen in combination with nitrogen. Thus, the presence of a characteristic nitrogen signal without a characteristic hydrogen signal is not typical of most explosives, whereas the presence of a characteristic nitrogen signal in combination with a characteristic hydrogen signal is typical of most explosives.
Referring to FIG. 7, a schematic representation of the principal hardware components that may be included in a further embodiment of the present invention is illustrated. Neutrons 132 from a source of neutrons 130 are directed to a parcel or object 134, the contents of which are to be investigated. If desired, a collimator 136 may help to further direct the neutrons 132 so as to irradiate a desired cross sectional shape at the point where the neutrons enter the object 134. As shown in FIG. 4, for example, the cross sectional shape of the irradiated portion may assume that of a narrow rectangle 138.
The object 134 is carried past the source 130 of neutrons 132 within a shielded chamber 135 on a conveyor belt 140, or equivalent parcel-carrying mechanism. The conveyor belt 140 is driven by a motor 142 in a continuous or step-wise fashion. The conveyor belt 140 continues to carry the object 134 thro