ted 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, inorganic scintillator materials such as cesium iodide (CsI), bismuth germanate (BGO-Bi4 Ge3 O12) or barium fluoride (BaF2). Solid state detectors also may be used to provide for the detectors, such as lithium-drifted germanium (Ge(Li)), high purity germanium (HPGe) or mercuric iodide (HGI2). The particular details of a specific detector structure do not form a part of the present invention, but the specific use of an inorganic scintillator with good energy resolution and efficiency to detect gamma rays produced by thermal neutrons provides for a unique detection of nitrogen and/or other elements do form a part of the present invention.
Although inorganic scintillators have been used in the past with thermal neutrons, this use was not for the detection of nitrogen in explosives, but rather to provide for the detection of chlorine, iron, chromium, etc. as a background component and not specifically for the detection of the nitrogen component and spatial distribution of the explosive material. Other uses of inorganic scintillators have been in combination with fast neutron sources so as to detect nitrogen, but this different type of neutron source provides for a different type of detection.
The present invention contemplates the combination of a thermalized neutron source with an appropriate scintillator, e.g. an inorganic scintillator, such as a sodium iodide detector. This specific combination provides for the capability of resolving closely spaced high energy gamma ray lines and specifically for detecting the particular gamma ray lines representative of the nitrogen content of explosives. These particular high energy gamma ray lines occur at 10.8 MeV. The inorganic scintillator detector is preferably used because it is a very efficient detector and because it provides acceptable features in a number of areas. These areas include level of total count rate, the shape of the detector, availability of detector, reliability and cost. It is to be appreciated that the inorganic scintillator may also be used to detect ot