ividual detectors 48, forming the row 40 and columns 36 and 38, provide for a cross sectional profile of any material containing nitrogen. The C-ring of detectors thereby provides for a two dimensional slice or plane of the nitrogen concentration and with a three dimensional profile built up by the successive slices produced as the luggage moves through the C-ring of detectors.
The two dimensional plane provided by the detector structures 36, 38 and 40 has less resolution at the upper end since the C-ring is not complete. Although a detector structure could also be provided along the upper surface of the cavity such detector structure could interfere with the production of the thermal neutrons by the source 32 of neutrons. A more efficient way of completing the ring is to have a second C-shaped group of detector structures provided downstream of the first group so that the luggage moves from the first C-ring of detector structures to the second C-ring of detector structures and with the open ends of the C-rings in the first and second sets being opposite to each other. The information from the two sets of C-rings of detector structures may be merged electronically in a computer to provide for a complete picture. As indicated above, this picture forms a three-dimensional image of the container such as the luggage and its contents by building up the successive slices or planes of information.
FIG. 4 illustrates in general the detection of the information by any one of the individual detectors 48. As shown in FIG. 4, neutrons from the sources, either 32 or 34, are thermalized and impinge on a piece of luggage or parcel as represented by the block 50. The individual detectors 48 forming the C-ring detector structures, each detect the production of gamma rays. The reaction between the thermal neutrons and the nitrogen in the explosive or other material, is as follows:
14 7 N+01 nth →15 7 N+γ
The first factor in the above equation is the nitrogen in the explosive or other material within the luggage. For example, wool, 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