 the object under observation and for the production a three dimensional representation of the concentration of nitrogen of the object. Waveform (c) illustrates the profile of the spectrum received by the detectors 48 and with the space 60 between the two lines representing the area of interest, more specifically the gamma rays representing nitrogen.
An alternative technique (and a preferred technique in most instances) for processing the detector signals, and in particular for accurately counting the signals at a high rate, is described in Drndarevic, Ryge & Gozani, "A Signal Processor for High Counting Rate gamma ray Spectroscopy with NaI(T1) Detectors," IEEE Proceedings of Nuclear Science (Feb. 1988), which article is incorporated herein by reference. The processor described in the referenced article advantageously addresses the problem of "pile up" and allows very few valid pulses to be lost while minimizing interfering background noise.
FIGS. 5(a), (b) and (c) illustrate typical profiles for explosive material in a block form; nonexplosive materials, such as a wool coat or jacket; and explosive material in sheet form. As can be seen in FIG. 5(a), which represents the detection from one column of detectors at four successive planes as the object moves past the detectors, the high readings of 12 in two successive planes and 8 in the third successive plane, represent a high concentration of nitrogen rich material probably representative of a block of explosive material. The detectors in the other column and along the row would confirm the presence of such block material. The large difference between readings in the profile of FIG. 5(a) show an unusual density of nitrogen material not typical in other types of items which contain nitrogen.
For example, FIG. 5(b) illustrates an item such as a wool coat or suit which may contain a relatively high amount of nitrogen, but with the nitrogen spread out in a diffuse pattern which would not be representative of an explosive material. Although the overall nitrogen content 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