Neutron-Based Techniques

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. 

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Concerning the spatial resolution of NR images the present IP are not adequate to the best film based direct neutron imaging techniques. However, with time-and the development of new IP technology there are good possibilities to improve the inherent unsharpness of the IP systems to the level even better than with Gd/film combination ... 

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. 

Henderson et al. [223] presented a detailed pattern of the structure of bacteriorhodopsin using high-resolution cryoelectron microscopy. Using X-ray and neutron diffraction techniques, Dencher et al. [224—227] could decode the secondary and tertiary structure of bacteriorhodopsin during the photocycle. Nevertheless, we should emphasize that the resolution still shows transitions in the active site (protonation of counterions, deprotonation of Schiff base, and reprotonation of counterions), leading to a metastable state of the protein. 

A major goal of fundamental research aiming to rationalize the interplay of structure, dynamics, and chemical reactivity, is to determine multidimensional potentials for nuclei in various environments. On the one hand, potential surfaces can be calculated with quantum chemistry methods at various levels of approximation. On the other hand, from the experimentalist viewpoint, vibrational spectroscopy techniques can probe dynamics of atoms, molecules and ions, in various states of the matter. However, there are fundamental and technical limitations to the determination of potential hypersurfaces from vibrational spectra of complex systems, and the confrontation of experiments with theory is far from being free of ambiguities. Consequently, the interpretation of vibrational spectra remains largely based on experiments. Recent progress in neutron scattering techniques have revealed new dynamics, specially for... 

Using nucleonic level detectors These usually employ gamma ray absorption or neutron backscatter techniques, but may also use gamma ray backscatter (Sec. 14.5). A case where a gamma ray absorption indicator solved a level control problem at the base of a column has been described (71). 

Two elemental analyzer systems have been developed, the "Continuous On-line Nuclear Assay of Coal", CONAC, (Science Application, Inc., Palo Alto, CA) and "The Elemental Analyzer" (MDH Industries, Inc., Monrovia, CA). Both of these units are based upon the measurement of prompt gamma rays that are emitted from a nucleus following the capture of a neutron. This technique is commonly known as prompt neutron activation analysis, PNAA. 

Structures of powdered P-rhombohedral boron and amorphous boron were investigated with pulsed neutron diffraction techniques (Delaplane et al. 1988). To avoid intensive neutron absorption by °B nuclei, samples were "B isotopically enriched up to 97.1% and 99.1%, respectively. Earlier neutron diffraction studies based on nuclear reactor data did not permit the derivation of a meaningful radial distribution of atoms in amorphous material due to limited range of the neutron wave vector (<10.8 A" ). The obtained static structural factor and derived radial distribution function supported a structural model of amorphous boron based on building blocks of B,2 icosahedra resembling those found in p-rhombohedral boron, but with disorder occupying in the linking between ico-sahedral subunits. The intensity data indicated that amorphous samples contained 5% of a mixture of crystalline a- and p-rhombohedral boron. 

Time resolved SAXS/SANS allow a structural observation of kinetic processes on the nanoscale (1-100 nm) on a time scale ranging from milliseconds to hours. This allows micellar kinetics to be followed in real time, giving direct structural information of the process and its evolution. Synchrotron SAXS can reach smaller time scales and exhibits better resolution compared to neutron-based methods. However, SANS offers the possibility for contrast variation via simple H/D exchange chemistry, which opens up a world of possibilities for the investigation of kinetics in soft matter systems, in particular transport and exchange processes that otherwise would be invisible in scattering experiments. As most of these techniques have become available over recent years with advancements in both instrumentation and sample environments, there is a need for an overview of the development and the possibilities that are now available in the field of soft matter in general and micellar systems in particular. 

The thermal neutron activation technique has been applied by Borsaru and Mathew (1980) to find the concentration of AI2O3 in coal measuring the 1.78 MeV y-ray produced from the reaction Al(n, y) A1. In another study, these authors (Borsaru and Mathew 1982) applied the thermal neutron activation technique to bulk samples (=11 kg) of Australian black coal for the determination of alumina, silica, and ash. The determination of alumina was based on the reaction Al(n, p) Mg and coimted the 0.844 MeV peak (ti/2 = 9.4 min). Silica was determined by means of the reaction Si(n, p) Al and counting the 1.78-MeV peak ti/2 = 2.3 min) applying correction for the interference from alumina. 

There are several MS-based techniques that can provide chemical information for thin and thick layers [12]. For very thin layers (sub to 1-2 monolayers), nondestructive techniques such as static SIMS [13], ion scattering MS [14], or MS of recoiled ions [15] are suitable. These techniques are also the best adapted for examining surface contamination. They are all based on surface interactions of an ion beam with the solid surface. For depth profiling of thin and thick layers, MS is associated with a destructive source of neutrals or ions dynamic SIMS, secondary neutron mass spectroscopy (SNMS), glow discharge mass spectroscopy (GD-MS), matrix-enhanced SIMS, laser desorption/ionization MS, and desorption electrospray ionization (DESI) MS [16]. Ions are either desorbed from the solid surface or generated by postionization of neutrals sputtered off the surface. 

Neutron activation analysis (NAA) is the general term used to describe a nuclear-based technique in which a solid or liquid sample is irradiated with neutrons. Capture or absorption of a neutron excites the nuclide that returns (promptly or after a delay) to ground state by emission of an energetic photon (gamma ray) and/or other particles from the nucleus (Figure 1.20). 

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