Neutron Methods

The advanced experimental fuel counter (AEFC) is used for the characterization of spent fuel from research reactors stored underwater (Menlove et al. 2007). The AEFC can be operated in 

The spent fuel neutron counter (SFNC) is a prototype neutron-detector system that verifies closely packed spent fuel assemblies stored in a spent fuel pond (Ham et al. 2002). The system contains a fission chamber moderated by a polyethylene cylinder housed in a watertight stainless steel enclosure. The SFNC measures total neutron signals from long-cooled spent fuel assemblies while in their storage position, without requiring them to be moved. The technique can detect a missing fuel assembly. These measurements are performed underwater in a gap between four assemblies. 

The ratio of neutron to gamma ray data, when combined with other complementary information, is used to characterize a particular type of fuel. To simplify the verification of spent fuel, a technique involving gross gamma ray and neutron measurements can be used. 

The SMOPY device can also be operated in active mode using an AmLi source. This has been successfully demonstrated for the underwater verification of canisters containing residues of irradiated HEU. This application is based on total neutron counting and detects the difference between active background and active background plus induced fissions. 

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NAA cannot be used for some important elements, such as aluminum (in a Si or Si02 matrix) and boron. The radioactivity produced from silicon directly interferes with that ftom aluminum, while boron does not produce any radioisotope following neutron irradiation. (However, an in-beam neutron method known as neutron depth profiling C3J be used to obtain boron depth profiles in thin films. ) Another limitation of NAA is the long turn-around time necessary to complete the experiment. A typical survey measurement of all impurities in a sample may take 2-4 weeks. 

The development of electron diffraction and microscopy was parallel but separate. Although there are many obvious similarities between the theories, they are usually expressed in different notations and it is common that workers trained in the one have little knowledge of the other. This is particularly unfortunate in that the techniques are almost always complementary, and many investigations will benefit by the use of both electron and X-ray (or neutron) methods. The emphasis in this book is on the X-ray methods. Electron techniques have been amply covered in a number of excellent publications we have not... 

The review by Teller and Bau11 is the key reference in the important area of structural studies. This covers both X-ray and neutron methods, illustrates all the main structural types, and tabulates all the relevant structural data. In view of the availability of this review, we will only sketch the main features of the topic and refer the reader to it for the detailed data. 

The approaches and methods described here are selected from a much larger set of neutron methods, which have been proposed and, in some cases, experimentally tested. At the time of this writing (2006), neutron methods are still not in common use for contraband, explosive or land mine detection, despite their clear potential for obtaining more detailed information than X-ray methods. The reasons for this are not immediately clear from the physics however, it is important to examine the practical issues that have arisen with neutron systems. The issues with larger accelerator-based systems are usually... 

This is a method for determination of soil moisture status in situ without disturbing the system In the neutron prohe method number of hydrogen neuclei present per rmit volume of soil is measured. Since hydrogen neuclei have a marked property for scattering and slowing neutrons, the same is exploited in the neutron method for measuring soil water content. 

This particular deficiency is not possessed by the neutron method. Molecular librations have the effect of seriously distorting the distribution function for the atomic position away from the gaussian function expected for harmonic motion and shifting the apparent center of gravity of the atom, so that the interatomic distances calculated on the simple model can be short by as much as hundredths (more often, thousandths) of an angstrom. 

The above illustrations and discussion lead us to several general conclusions concerning the use of neutron spectroscopy in the study of torsional vibrations (and other large-amplitude modes) in molecular systems. First, the neutron technique-since it involves the interaction of neutrons with vibrating nuclei and is especially sensitive to large amplitude motions—can for appropriate molecules be an ideal complement for optical spectroscopy. Neutron spectroscopy, however, is hampered somewhat by the available instrumental resolution ( 10 cm-1) and by the inherent recoil resolution broadening in fluid-phase spectra. In addition, present accessibility of instrumentation for the neutron method (for low k molecular spectroscopy) is limited. For example, there are only a few reactors in the United States where appropriate instruments and intensity exist for such measurements (neutron sources and instrumentation amenable to the study of crystal and liquid structure and interatomic and intermolecular dynamics are more accessible). These factors make it imperative that studies of molecular systems be chosen with some care. 

Abstract Neutron scattering was first used to derive the self-diffusivities of hydrocarbons in zeolites, but transport diffusivities of deuterated molecules and of molecules which do not contain hydrogen atoms can now be measured. The technique allows one to probe diffusion over space scales ranging from a few A to hundreds of A. The mechanism of diffusion can, thus, be followed from the elementary jumps between adsorption sites to Lickian diffusion. The neutron spin-echo technique pushes down the lower limit of diffusion coefficients, traditionally accessible by neutron methods, by two orders of magnitude. The neutron scattering results indicate that the corrected diffusivity is rarely constant and that it follows neither the Darken approximation nor the lattice gas model. The clear minimum and maximum in diffusivity observed by neutron spin-echo for n-alkanes in 5A zeolite is reminiscent of the controversial window effect . 

Fourier analysis of neutron diffraction data (time-of-flight method) was also employed by Etherington et al. (1984b) to probe the structure of the barium dizirconate glass and the combination of the X-ray and neutron methods allowed an assignment of the different peaks in the total correlation function with a good degree of... 

In the case of X-ray and neutron methods, the reflectivity is measured as a function of incident angle. The reflectivity data is then modeled as being made up of a number of layers varying in refractive index to determine the concentration profile (15). In the case of neutron reflectivity, the contrast can be provided using deuterated solvents. X-ray reflectivity is also most commonly used to study the thickness of dry films or adsorption at the air-liquid interface. 

A small maximum is observed on the Fermi surface, which is formed mainly by the p-states of tin. Boulet et al. (1982) have measured the de Haas-van Alphen effect, the Fermi surface was constructed and the band structure was calculated. The conduction band is formed by the 5p-state of Sn and 5d-state of La. The results obtained on the single crystal by the polarized neutrons method agree with the theoretical calculation of the spin magnetic form factor. It is supposed that the Fermi level is formed mainly by the 5p-state of Sn. 

The maximum neutron flux from research reactors Is limited by heat dissipation in the moderator. The optimum performance of a moderator assembly can therefore be Improved by pulsed neutron methods. The first phase in the development of pulsed neutron sources was based on electron linear accelerators which produce fast neutrons in a heavy metal target by (Y,n) and (y>f) processes C3 ] The problems of heat dissipation again limit the flux but the use of an incident proton beam overcomes this difficulty. The Spallation Neutron Source (SNS) C35] Is based on a 800 MeV proton... 

Even in the case of crystals, the direct determination of the structure by means of vibrational spectroscopy is impossible the structure must be known in advance by radio-crystallographic or neutron methods. Spectral analysis is based on group theory [14] applied to isolated molecules or to crystal lattices triply periodic. In disordered lattices, such an approach is impossible one must proceed by successive approximations. 

A delayed neutron method, a cover gas method and a tagging gas method are used to detect and locate fuel failure. 

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