Neutrons anomalous scattering

Table 9.3. A list of almost all the structures that have been solved using neutron anomalous scattering methods.

This discussion brings out an important principle that will reappear time and again in this chapter. If we can assign the absolute direction of the hand vis-a-vis the polar axis b (Figure 2), that is, establish whether the wrist or the fingers of the hand point up, then the absolute chirality of the hand is fixed as well. This determination of absolute polarity can be achieved in several ways other than anomalous scattering of X-rays or neutrons, for example, by an external means such as a solvent, a chemical reagent, or a physical measurement (e.g.,... 

The next speech was by Chadwick who treated (in 32 pages) the anomalous scattering of the a particles, the transmutation of the elements, and the evidence for the existence of the neutron he had discovered in 1932.45... 

G.H. Lander, Neutron elastic scattering from actinides and anomalous lanthanides 635 Author index 711... 

Anomalous scattering also occurs in neutron diffraction, several nuclei (" Cd, Eu, Gd, and Pu) showing... 

Although the physical processes responsible for X-ray and neutron resonance scattering are vastly different a unified approach of resonance (or anomalous) scattering can be given on the basis of the famous optical theorem ... 

The generation of a first-order difference by the use of anomalous scattering methods (either X-ray or neutron) offers the advantage that only a single sample is required. However, little use has been made of this possibility so far. On the other hand, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate, in a limited way, gMo( ) and the existence of inner-sphere complexing 69). 

The quantity b is the neutron coherent scattering amplitude. In the entry for Li, i = y/—l. The anomalous factor for Li involves a phase shift of about 8° [(0.025/0.18) = tan8°] relative to most other nuclei. 

Any reasonably heavy atom can serve as an anomalous scatterer if the appropriate wavelength of X rays is used. For neutrons, however, there are fewer atoms that scatter anomalously (for example, Li, B, Cd, Sm, Eu, Gd) and their high absorption may limit their use. If X-ray diffraction intensity data are sufficiently carefully measured, it is possible to determine absolute configurations even if there are no heavy atoms in the structure, although values of A/" are very small for the light elements. Results of determinations of absolute structure must come from laboratories with an excellent reputation for careful intensity measurement. For example, Hakon Hope has been able to assign absolute... 

Some isotopes also scatter neutrons anomalously. 

Employing information other than Cu Ka x-ray diffraction data, e.g. anomalous scattering near the K-absorption edge of one of the metals, neutron diffraction data (see below), and/or spectroscopic results. 

These NCS experiments [Chatzidimitriou-Dreismann 1997 (a) Chatzidimi-triou-Dreismann 1999 Karlsson 1999], which were motivated by the theoretical work of C. A. Chatzidimitriou-Dreismann [Chatzidimitriou-Dreismann 1991 Chatzidimitriou-Dreismann 1997 (b)] on protonic quantum entanglement in condensed systems and by the results of a an earlier Raman experiment on liquid H O / D O mixtures [Chatzidimitriou-Dreismann 1995], were followed by a series of other experiments on liquid and solid organic materials [Chatzidimitriou-Dreismann 2000 (b) Chatzidimitriou-Dreismann 2001 Chatzidimitriou-Dreismann 2002 (a)], various metallic hydrides [Abdul-Redah 2000 Karlsson 2002 (b) Karlsson 2003 (b)], liquid hydrogen [Chatzidimitriou-Dreismann 2004 (b)] and among others an ionic solid [Abdul-Redah 2004] using the same experimental technique, i.e., neutron Compton scattering. All these experiments confirmed the anomalous results found earlier and also revealed certain new aspects of the considered effect. 

The first term / (k) is the X-ray atomic scattering factor and the other two terms are the dispersion corrections. Similar anomalous scattering occurs for neutrons also and the neutron scattering factor is formulated in a similar way. 

The RMC method is more general than this simple algorithm in that any set(s) of data that can be directly related to the structure can be modelled. It can be applied to isotopic substitution in neutron diffraction, or equivalently to anomalous scattering in X-ray diffraction, to EXAFS (multiple edges) and possibly to NMR data. All data sets can be modelled simultaneously simply by adding the respective y1 values. 

Independent measurements on neutron-irradiated PbTe, Te metal, and Te02 (before and after thermal annealing) showed no evidence at all for anomalous effects [57]. The conditions used in this case generated only displacements by the thermal neutron capture process, suggesting that this was not the cause of the earlier observations which may have been made under conditions where fast neutron elastic scattering can also take place. 

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