ugh 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 of gamma rays at specified energy levels detected by each detector in the array provides a measure of the particular element, or combination of elements, present within the irradiated slice or section of the object characteristic of that level of gamma ray emission. By combining this information from all of the detectors, as well as by considering the number of neutrons that pass through the object without interacting with atomic nuclei (as sensed by the neutron detector 146), a more accurate density distribution of the elements within the irradiated section of the object may be obtained. By combining such density information for all of the slices of the object that are created as the object moves past the neutrons 132, a three-dimensional density image of the contents of the object can thus be formulated.
Advantageously, the nuclear density information obtained using the gamma ray and neutron detectors as above described can be further enhanced by combining it with the electron density information obtained from the X-ray system 150. If such combined density information suggests the presence of contraband, the object is flagged for further investigation (e.g., diverted off of the conveyor belt for a manual search).
Referring back to the embodiment of the invention in FIG. 4, it is seen that the outputs of the A/D converters 56 are directed to a computer 58. It is one of the main functions of the computer 56 to analyze the data (e.g., the number of gamma rays at a particular energy) sensed by each of the detectors 48 in order to accurately and rapidly classify an object as one with or without explosives. A similar function must be performed by the computer 154 shown in the embodiment of FIG. 7. (Other functions, such as record keeping and miscellaneous housekeeping functions associated with the explosive detection system may also be performed by the computers 58 or 154, but such functions are viewed as conventional functions and will not be described herein.)
There are a number of data analysis techniques that can 