 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 through a chamber of an X-ray system 150 wherein a source of X-rays 152 and a corresponding detector (not shown) are used to produce an ordinary electron density image of the object and its contents.
An array 144 of gamma ray detectors are selectively positioned around the object 134 near the region where the object is irradiated by the neutron cloud or beam 132 within the shielded chamber 135. It is to be appreciated that while only one array 144 of such detectors is shown positioned above the object 134, several such arrays may be used, e.g., one above the object as shown, one below the object, and one behind the object, thereby forming a C-ring that effectively encircles the object 134 as it is irradiated, as shown in FIG. 3. In addition to the array of gamma ray detectors 144, at least one neutron detector 146 is placed behind the object 134 opposite the source of neutrons 130. This neutron detector 146 captures some of the neutrons that pass through the chamber 135 and the object 134. Additional neutron detectors may be placed at other locations in the cavity.
Appropriate control circuits 148 interface with the above-described components. The control circuits, in turn, are monitored or driven by a suitable computer 154. The computer 154 includes conventional input/output devices, such as a keyboard 156, a terminal display screen 158, and/or a printer (not shown). A non-volatile memory storage device 160, such as a disk or tape memory, may also be coupled to the computer 154, as required, in order to retrieve or track the large amounts of data that may be needed to examine a large quantity of objects for specified contraband.
In operation, the object 134 is irradiated by the neutrons 132 in sections or slices as the object moves past the beam. The gamma rays resulting from the interaction of the neutrons with atomic nuclei in the object are detected in the gamma ray detector array 144. Neutrons that pass through the chamber 135 and object 134 are detected in the neutron detector by each detector 146. The number o