The minimum crystal size needed for a complete diffraction data set

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, WilsonBfactor, 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.

Performance of a confocal multilayer X-ray optic

In recent years, several companies have developed the technique of arranging two multilayer mirrors in confocal optics for monochromatizing X-rays. In this study, a focusing device of Osmic Inc., with a source-to-focus distance of 1005 mm, has been used. The goal was to measure the homogeneity of the beam, the cross section at various distances from the focus and the efficiency of the optic when it is operated with vacuum and with air in the beam path. A small crystal sphere set at various distances is used to compare the intensities and the widths of reflections with those of a flat graphite monochromator. In a standard diffraction experiment (crystal size 0.25 mm), the gain factor with respect to graphite is roughly ten at a position where the beam plateau is 0.5 mm. The suppression of the CuKβ radiation and of higher harmonics ofKα is very good.