ng from and to the loading and unloading stations 12 and 14.
As can be seen in FIG. 2B, wherein the shields 20, 22 and 24 are removed, the explosive detection system is positioned over a central portion of the conveyer belt 16. Specifically, the explosive detection system includes a cavity structure 26 through which the conveyer belt 16 passes. As shown in FIGS. 2A and 2B, various articles of luggage and parcels may be positioned on the loading station 12 and may then be carried thorough the cavity 26 to the unloading station 14 by the conveyer belt 16.
The cavity is formed by external wall members including a top wall 28, side walls 30 and 31 and a bottom wall (not shown) which is positioned below the conveyer belt -6. Extending through the wall members are neutron sources, such as source 32 positioned at the top of the cavity, and as shown in FIG. 3, neutron source 34 spaced from the neutron source 32 and positioned at the bottom of the cavity. Also as shown in FIGS. 2 and 3, detector structures are positioned to form two C-rings of detectors having their opened ends facing the neutron sources. This may be seen in FIG. 3 wherein the side detector structures 36 and 38 together with the bottom detector structure 40 are all associated with the neutron source 32. Similarly, side detector structures 42 and 44 together with the top detector structure 46 are all associated with the neutron source 34.
As shown in FIG. 3, the side detector structures may be provided by two sets of four detectors located in each side detector structure 36 and 38. The bottom detector structure 40 includes two sets of seven detectors. The detectors associated with the neutron source 34 similarly include two sets of four detectors located in each side detector structure 42 and 44 and with two sets of seven detectors located in the top detective structure 46. The detectors associated with the neutron source 32, therefore, form a C-ring of detectors having the opened portion of the C facing upward. In an opposite fashion the detectors associated with the neutron source 34 form a C-ring with the opened portion of the C facing downward. The combination of the two sets of C-ring detectors thereby provide for the detection of a complete ring around the object under inspection to produce a better three dimensional profile of the nitrogen distribution within any particular object passing through both sets of detectors.
While the invention is described with reference to the use of two C-ring detector structures and with each C-ring including two sets of parallel rows and columns of detectors, it should be appreciated that only a single C-ring structure may be used with only a single row and column of detectors. The use of the additional parallel sets of detectors improves a visualization of the profile of the concentration of nitrogen, but a simpler system could be used with a single C-ring and single rows and columns of detectors. It is also to be appreciated that more or less detectors could be used. A full or partial ring of detectors could also be placed out of the plane of the source, around the cavity.
The actual structure of the neutron source and its environment, such as the structures 32 and 34, may be of any type. For example, the neutron source may be a radioisotope (such as 252 Cf) or an electronic neutron source (such as (D,D), (D,T), or other generators). By collisions, mostly with the nuclei of the selected materials surrounding the source the neutrons are slowed down to create a cloud of neutrons within the cavity. The neutrons specifically interact with the variety of nuclei in the luggage or parcel. The interaction of the neutrons produces characteristic high energy gamma rays which are detected by the external rows and columns of the detector(s).
Each detector in the rows and columns preferably are formed of inorganic scintillators. Specifically, all of the detectors, such as represented by a detector 48, may be formed of an inorganic scintillator material, such as sodium iodide (NaI). Other inorganic materials may be used, for example, i