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Scattering Power (scattering + power)
Selected AbstractsEvaluation of double-crystal SANS data influenced by multiple scatteringJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000aroun Evaluation of small-angle neutron scattering (SANS) data is often complicated by multiple scattering effects if large particles of relatively high volume fraction have to be studied and dilution or contrast reduction is impossible. The use of pin-hole SANS instruments is often limited due to the contradictory requirements of high resolution and short wavelength needed to keep scattering contrast as low as possible. Double crystal (DC) SANS diffractometers of Bonse-Hart and bent-crystal type are useful alternatives in such cases, as they permit reaching very high resolution with thermal neutrons. A method for SANS data evaluation suited to DC instruments is presented. It includes the common scheme of the indirect Fourier transformation method, but takes multiple scattering into account. The scattering medium is described by the frequency function g(x) defined as the cosine Fourier transform of slit-smeared data. Although a simplistic model of polydisperse spheres is used to represent g(x), resulting g(x) function and some integral parameters are independent of this model. Tests on simulated data show, that the method reproduce well true values of microstructural parameters, though systematic errors are observed in the cases when the unscattered part of incident beam completely disappears. If the scattering power Fast two-dimensional detection for X-ray photon correlation spectroscopy using the PILATUS detectorJOURNAL OF SYNCHROTRON RADIATION, Issue 5 2009Fabian Westermeier The first X-ray photon correlation spectroscopy experiments using the fast single-photon-counting detector PILATUS (Paul Scherrer Institut, Switzerland) have been performed. The short readout time of this detector permits access to intensity autocorrelation functions describing dynamics in the millisecond range that are difficult to access with charge-coupled device detectors with typical readout times of several seconds. Showing no readout noise the PILATUS detector enables measurements of samples that either display fast dynamics or possess only low scattering power with an unprecedented signal-to-noise ratio. [source] The minimum crystal size needed for a complete diffraction data setACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2010James M. Holton In this work, classic intensity formulae were united with an empirical spot-fading model in order to calculate the diameter of a spherical crystal that will scatter the required number of photons per spot at a desired resolution over the radiation-damage-limited lifetime. The influences of molecular weight, solvent content, Wilson B factor, X-ray wavelength and attenuation on scattering power and dose were all included. Taking the net photon count in a spot as the only source of noise, a complete data set with a signal-to-noise ratio of 2 at 2,Å resolution was predicted to be attainable from a perfect lysozyme crystal sphere 1.2,µm in diameter and two different models of photoelectron escape reduced this to 0.5 or 0.34,µm. These represent 15-fold to 700-fold less scattering power than the smallest experimentally determined crystal size to date, but the gap was shown to be consistent with the background scattering level of the relevant experiment. These results suggest that reduction of background photons and diffraction spot size on the detector are the principal paths to improving crystallographic data quality beyond current limits. [source] ACORN2: new developments of the ACORN conceptACTA CRYSTALLOGRAPHICA SECTION D, Issue 9 2009E. J. Dodson The density-modification procedures incorporated in ACORN, available in the CCP4 package, have proved to be very successful in solving and refining high-resolution crystal structures from very poor starting sets. These can be calculated from a correctly positioned initial fragment containing between 1 and 8% of the scattering power of the total structure. Improvements of ACORN, reported here and incorporated in the program ACORN2, have lowered the size of the fragment required and examples are given of structures solved with only 0.25% of the scattering power in the fragment, which may be a single atom. Applications of ACORN2 to structures with space group P1 have shown the remarkable property that when the starting point is a pair of equal atoms, or even a single atom placed at the origin, the refinement process breaks the centric nature of the initial phases and converges to phases corresponding to one of the two possible enantiomorphs. Examples are given of the application of ACORN2 to the solution and/or refinement of a number of known trial structures and to the refinement of structures when phases are available either from MAD or from a molecular-replacement model. [source] Phase transition of triclinic hen egg-white lysozyme crystal associated with sodium bindingACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2004Kazuaki Harata A triclinic crystal of hen egg-white lysozyme obtained from a D2O solution at 313,K was transformed into a new triclinic crystal by slow release of solvent under a temperature-regulated nitrogen-gas stream. The progress of the transition was monitored by X-ray diffraction. The transition started with the appearance of strong diffuse streaks. The diffraction spots gradually fused and faded with the emergence of diffraction from the new lattice; the scattering power of the crystal fell to a resolution of 1.5,Å from the initial 0.9,Å resolution. At the end of the transition, the diffuse streaks disappeared and the scattering power recovered to 1.1,Å resolution. The transformed crystal contained two independent molecules and the solvent content had decreased to 18% from the 32% solvent content of the native crystal. The structure was determined at 1.1,Å resolution and compared with the native structure refined at the same resolution. The backbone structures of the two molecules in the transformed crystal were superimposed on the native structure with root-mean-square deviations of 0.71 and 0.96,Å. A prominent structural difference was observed in the loop region of residues Ser60,Leu75. In the native crystal, a water molecule located at the centre of this helical loop forms hydrogen bonds to main-chain peptide groups. In the transformed crystal, this water molecule is replaced by a sodium ion with octahedral coordination that involves water molecules and a nitrate ion. The peptide group connecting Arg73 and Asn74 is rotated by 180° so that the CO group of Arg73 can coordinate to the sodium ion. The change in the X-ray diffraction pattern during the phase transition suggests that the transition proceeds at the microcrystal level. A mechanism is proposed for the crystal transformation. [source]
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