norganic 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 other 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 ind