 cotton, etc. all contain nitrogen. The nitrogen when bombarded with thermal neutrons, as shown by the second factor, produces nitrogen in a changed form (another isotope of nitrogen) plus gamma rays, of which approximately 14% are at 10.8 MeV. Each gamma ray as detected by a detector 48 produces an output from the detector as shown in waveform (a) in FIG. 4. As can be seen, the detector 48 produces an output signal having a height "h" and decaying exponentially to zero in approximately one micro second. The detectors 48 are supplied with a high voltage from a high voltage source 52. The height "h" and the area under the decaying signal are both proportional to the gamma ray energy.
The output from each detector 48 is passed directly through a preamp and amplifier 54 to produce an output signal as shown in waveform (b) in FIG. 4. It can be seen that the individual gamma ray is converted from the exponentially decreasing signal to a pulse signal having a height "H" which is proportional to the area under the signal shown in waveform (a). It is to be appreciated that each gamma ray received by each detector 48 produces successive signals whose amount represents the concentration of nitrogen.
The output from the preamp/amplifier 54 is passed through an analog to digital (A-to-D) converter 56 to produce a digital number representing the height "H" of the waveform (b) of FIG. 4. It can be seen, therefore, that the outputs from A-to-D converters 56 are a series of digital numbers representing the detection of gamma rays, whose number in turn represent the concentration of nitrogen. A small range of the digital numbers correspond to the gamma rays of interest. As more and more gamma rays are detected at each detector, the digital number from the A-to-D converters 56 at each point in time is counted. The counts of each digital number which occurs, which is proportional to the number of nitrogen gamma rays incident on the detector, are then coupled into a computer 58 for computation of a profile for each slice or plane of 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 conten