Security systems are presently limited in their ability to detect contraband, weapons, explosives, and other dangerous objects concealed under clothing. Metal detectors and chemical sniffers are commonly used for the detection of large metal objects and some kinds of explosives, however, a wide range of dangerous objects exist that cannot be detected with these devices. Plastic and ceramic weapons developed by modern technology increase the types of non-metallic objects that security personnel are required to detect. The alternative of manual searching of subjects is slow, inconvenient, and would not be well tolerated by the general public, especially as a standard procedure in, for example, airports.
Known prior art X-ray systems suggested for detecting objects concealed on persons have limitations in their design and method which prohibit them from achieving the low dose and high image quality which are prerequisites of commercial acceptance.
Radiation exposure is an important consideration in X-ray concealed object detection systems. The United States National Council on Radiation Protection (NCRP), in NCRP Report No. 91, "Recommendations on Limits for Exposure to Ionizing Radiation", 1987, addresses this issue. In this report, the NCRP states that a radiation exposure of less than 1000 microRem per year in excess of environmental levels is negligible, and efforts are not warranted at reducing the level further. Persons employed in security facilities or frequently traveling by airlines may be subjected to many hundred security examinations per year. A yearly radiation exposure of 1000 microRem permits a single scan exposure within the range of 1 to 10 microRem for the general public. In accordance with the NCRP recommendations, radiation levels significantly higher than this present a non-trivial health risk.
An inspection system which operates at a low level of radiation exposure is limited in its precision by the small number of X-rays that can be directed against a person being searched. X-ray absorption and scattering further reduces the number of X-rays available to form an image of the person and any concealed objects. In prior art systems, this low number of detected X-rays has resulted in unacceptably poor image quality.
Radiant energy imaging systems have been proposed to detect concealed objects. The system disclosed in U.S. Pat. No. Re. 28,544 of Stein et al. projects a scanning pencil beam of X-rays through the subject's body where the beam will be transmitted or absorbed depending upon the concealed object, if any. A detector is scanned vertically behind the subject to collect transmitted X-rays. Stein et al. comment that, "although the detector 25 is shown behind the object being scanned for responding to the radiant energy transmitted through the object being scanned, it is within the principles of the invention to position the detector in the region between the radiant energy source and the object being scanned to respond to the scattered energy." However, the X-ray tube potential is set at up to 150 Kilovolts and is specifically chosen to transmit X-rays through the person being examined. Operation of the X-ray tube at 150 Kilovolts or even at 100 Kilovolts would negate benefits of imaging by backscatter detection, e.g., low dose scanning. This technique requires the subject to be exposed to a substantial radiation dosage, especially if the subject is scanned often, e.g., a frequent flyer.
U.S. Pat. No. 4,839,913 of Annis et al. teaches a flying spot scanning system for baggage inspection which uses X-ray-induced fluorescence to permit detection of concealed objects. Since fluorescence is dependent upon atomic number, each object emits a fluorescent radiation line which is unique to its atomic number Z. Scattered, transmitted and fluorescence signals are generated to distinguish objects from the background. The energy level of the source must be sufficient to excite a selected fluorescence radiation line so that the fluorescent radiation line has sufficient energy to escape the object. Thus, it may be necessary to expose the object to relatively high X-ray energy in order to detect certain materials, which would be unacceptable for personnel inspection systems.
Another baggage inspection system is disclosed (U.S. Pat. No. 4,799,247 of Annis et al.) which has detectors for both transmitted and backscattered X-rays to independently produce signals from the same incident beam. The separate signal may then be used to enhance each other to increase the system's accuracy in recognizing low Z materials. Clearly, with the incident beam being of sufficient energy to provide both transmitted and backscattered signals, the X-ray energy must be relatively high, similar to that of Stein et al., making such a system undesirable for personnel inspection. A similar technique is described by Glockmann et al. (U.S. Pat. No. 4,884,289) for baggage inspection where both transmission and backscatter is used.
Systems of the foregoing prior art that apply to body scanning are designed to detect radiant energy that has been transmitted through the body, scattered from the body, or emitted from the body. In the prior art systems, images are produced by characteristics of the subject's body and any object concealed under the subject's clothing. The system operator then inspects each image for evidence of concealed objects.
For various reasons the prior art systems do not adequately detect plastics, ceramics, explosives, illicit drugs, and other non-metallic objects. One reason in particular is that these materials share the property of a relatively low atomic number (low Z). Low Z materials present a special problem in personnel inspection because of the difficulty in distinguishing the low Z object from the background of the subject's body which also has low Z. Detection of low Z materials is addressed in the Annis '247 patent, but only for baggage. As mentioned above, this technique exposes a live subject to radiation levels which are excessive.
It would be desirable to provide an X-ray inspection system for detecting concealed objects carried by a person which is capable of detecting low Z materials as well as metals, but which does not expose the subject to radiation doses significantly higher than normal environmental radiation levels. It is to such a system that the present invention is directed.