her elements representative of an explosive.
As indicated above, the currently preferred inorganic material is sodium iodide, but other inorganic materials may be used. For example, bismuth germinate has a high effective atomic number because of the bismuth and a higher density than the sodium iodide. The efficiency of a bismuth germinate scintillator is, therefore, higher than that of sodium iodide. However, bismuth germinate scintillators are inferior to sodium iodide in energy resolution and the cost for a bismuth germanate scintillator is much higher than that for sodium iodide, and it also has a background component that can interfere with the nitrogen signal. However, both of these inorganic structures are superior to the organic scintillators used in the prior art devices.
The main advantage of the prior art organic scintillators, which may be plastic or liquid, is their very fast response time permitting exceedingly high count rates. Because of the very high count rates, a high background or "pile-up" from overlapping pulses can be handled and thereby eliminate the need for a sophisticated cavity design. Another advantage of the organic scintillators is their relatively low cost and ease of manufacture. Even with these advantages, however, the use of inorganic scintillators, and specifically in the particular C-ring configuration of the present invention, provides for a higher resolution and thereby a more efficient detection of any explosive material. The organic scintillators are inefficient detectors for high energy gamma rays and their gamma spectroscopical qualities are poor. Organic scintillators thereby have poor energy resolution and make the separation between nitrogen and deleterious signals, such as occur with Cl, Fe, Cr or Ni, very difficult.
As can be seen in FIG. 3, any item to be scanned, such as a piece of luggage, passes through the cavity on the conveyer 16 and is subjected to the thermal neutrons produced by the neutron source 32. At successive positions of the piece of luggage, the individual 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,