Neutron based detection

The following sections present a summary of the physics that underlies the nuclear detection technologies, a survey of neutron-based detection approaches, and an overview of non-neutron-based nuclear detection technologies. 

As neutron-based technologies have evolved, the newer approaches are often refinements or extensions of the older approaches. The following survey of approaches is structured to show how the neutron-based detection technologies have evolved. 

Survey of Non-Neutron-Based Nuclear Detection Methods... 

The following subsections present background material on the physics and nomenclature used to address the nuclear detection technologies. The subsections are divided into neutron- and non-neutron-based nuclear detection methods. 

Signal-to-noise considerations make most neutron-based explosive detection approaches very difficult to implement. The basis for combining multiple detection approaches (FNA, along with thermal gamma detection and neutron transmission spectroscopy) in a FNAP application that preserves the small volume advantage of a APSTNG remains to be established. There are distinct advantages associated with the API approach, but the concomitant reductions in available neutron flux, issues of tube lifetime, and the intrinsic poor spatial resolution must be taken into consideration for potential applications. 

The most significant issue with the fielding of a neutron-based explosive detection technology is pubfic perception. Most people have difficulty putting into perspective low probabihty high consequence events. This has resulted in the word nuclear being associated with health risks and environmental issues even when detailed analysis shows that there is no issue. 

Due to several factors that have been discussed, using neutron techniques for transportable systems for the detection of explosives and other contraband remains only a goal at this time. The attraction of using neutrons lies in both their penetrating ability and the ability to use them to detect elemental composition. These properties are unique to neutron-based systems and have led to continued interest in their use, and multiple systems have been investigated as can be seen in Table 1 [44], At the present time, the use of neutron techniques as a complement to other inspection methods appears to be the most likely near-term mode of operation. 

Much like X-rays, the interactions of neutrons with matter are atomic in nature. The difference is that neutrons are sensitive to nuclei directly, whereas X-rays interact with electrons. Hence, while X-rays are unsuitable to detect light elements because of the low atomic electron count, neutron scattering factors depend on the properties of the nucleus [206]. The most relevant consequence in the context of this discussion is that neutron-based tools are better suited for the detection of H and Li than X-rays, as H and Li are among the most highly neutron-absorbing atoms, and that they offer isotope resolution capability. In principle, they are also nondestructive. 

Investigations based on equation (a) are indirect. Direct structural studies using diffraction techniques (X-ray or neutron), or electron microscopy, while they cannot detect the low concentrations of defects present in NiO or CoO are indispensible to the study of grossly non-stoichiometric oxides like FeO, TiOj, WOj etc., and particularly electron microscopes with a point-to-point resolution of about 0.2 nm are widely used. The first direct observation of a point defect (actually a complex of two interstitial metal atoms, and two oxygen atoms in Nb,2029) was made" using electron microscopy. 

Although following similar nuclear reaction schemes, nuclear analytical methods (NAMs) comprise bulk analysing capability (neutron and photon activation analysis, NAA and PAA, respectively), as well as detection power in near-surface regions of solids (ion-beam analysis, IB A). NAMs aiming at the determination of elements are based on the interaction of nuclear particles with atomic nuclei. They are nuclide specific in most cases. As the electronic shell of the atom does not participate in the principal physical process, the chemical bonding status of the element is of no relevance. The general scheme of a nuclear interaction is ... 

A proposal has been made for a PFNA-based system designed to detect illicit materials for air cargo inspection (ACI). This system is referred to as PFNA ACI. The PFNA ACI is a proprietary design that utilizes a tandem Van de Graff accelerator to accelerate deuterons. The neutron beam is collimated in a scan arm and focused on the front surface of the container. Testing has been conducted on aircraft LD3 container as well as truck cargo trailers. 

Nuclear detection approaches that use radioactive isotojjic sources (e.g., Cf for spontaneous fission and asociated neutron emission or ° Co for gamma emission) will have to obtain state and federal hcenses to field the equipment and abide by apphcable health and safety regulations. The Hcensing process takes some time to put into place and may restrict the easy movement of the detection equipment to new locations. This impacts the abffity to rapidly re-locate equipment based up inteUigence estimates of the behavior of smugglers. The use of fixed pre-licensed sites can help to some extent. 

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