Neutron reflection data collection

Early studies, which did not include many high-order reflections, revealed systematic differences between spherical-atom X-ray- and neutron-temperature factors (Coppens 1968). Though the spherical-atom approximation of the X-ray treatment is an important contributor to such discrepancies, differences in data-collection temperature (for studies at nonambient temperatures) and systematic errors due to other effects cannot be ignored. For instance, thermal diffuse scattering (TDS) is different for neutrons and X-rays. As the effect of TDS on the Bragg intensities can be mimicked by adjustment of the thermal parameters, systematic differences may occur. Furthermore, since neutron samples must be... 

The neutron data collected at 4.5K and room temperature show that the symmetry is no longer cubic, in agreement with Bursill, et al.(5). This is most clearly seen by inspection of the (880) reflection, which is found to be asymmetric and considerably broadened with respect to the (12,0,0) reflection. Although (12, 0,0) in fact occurs in combination with (884), the latter has a negligibly small intensity and thus the true symmetry may be assumed to be rhombohedral, as confirmed by subsequent profile analysis. 

With time, improvements in instrumentat-ion26,27,33,41-45 jjj jjg yhich resulted in reduced sample size and/or data collection time. An especially significant step (following developments in X-ray data collection) involved the use of area detectors to collect dozens to hundreds or thousands of reflections simultaneously. The details of various area detectors used are too varied to warrant a detailed discussion here, and may not be of interest to many readers of this article. Suffice it to say that most detectors rely on efficient neutron-absorbing components (such as He, Li, °B, or Gd), which then produce other particles (typically, photons or electrons) that are actually counted. 

Chapter 1 summarizes methods for the stabilization of artificial lipid membranes. They include synthesis of new types of polymerizable lipids and polymerization of membranes. Creation and characterization of novel poly(lipid) membrane systems, as well as their functionalization for biotechnological applications, are also described. Chapter 2 addresses experimental studies on the design and characterization of lipopolymer-based monolayers at the air-water interface. Thermodynamic and structural data collected with X-ray and neutron reflectrometry, infrared reflection absorption spectroscopy, and sum frequency generation spectroscopy provide... 

Data collection on a point-detector diffractometer consists of scanning the reflections (at predetermined positions) in succession, by one of the scan modes described above, as well as measuring the background on both sides of every reflection. The ty/20 scan mode is preferable for good crystal with sharp peak profiles, (o mode for poorer quality crystals. In neutron experiments, a compromise ty/xt scan is used, where x equals 1 or it is variable. In order to attain the same statistical accuracy (e.g. 1%) for all reflections, weaker reflections are scanned more slowly than the strong ones. For a good structure determination, the diffraction data should extend at least to sin 9/X = 0.6 A , that is, a complete set of... 

The ribosome has been a favorite nucleo-protein for study by neutron scattering (References in (3)). We shall mention briefly a recent experiment which made use of the scattering curve I(Q) and [H2O] [ D2O ] variation in the solvent (3). Electron micrographs of 16S RNA had been obtained and showed it to have a Y shaped structure. Is this structure maintained within the 30 S ribosome particle The scattering curve of the model was calculated and compared with neutron data. In 42% D2O, the excess scattering mass of protein is negligible, so that data collected from the 30 S in this buffer should only reflect the RNA structure within the particle. The... 

Neutron reflection has been used to stu die interfaces between the melt phases of ciystallisable polymers as well as real time interdiffusion of polymers and oligomers. Both systems are experimentally demanding and have required the use of specialised cells and data collection procedures. The interfacial widths for a number of polymer systems have been determined and the Flory Huggins interaction parameters obtained. In addition, the interdiffiision process has been followed for a polystyrene-polystyrene system above its Tg and also for a polystyrene-oligostyrene in-situ in real time using very rapid reflectivity scans. 

With current neutron sources the times of data collection for each partial reflectivity scan will remain of the order of minutes. However, with the advent of the next generation of high intensity neutron sources being planned or built around the world significant advances can be predicted for these real time measurements. Time will tell as the saying goes. 

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