Multilayer monochromators and supermirrors for neutron protein crystallography using a quasi Laue technique

摘要

The major problem in neutron protein crystallography is the low flux of present neutron sources. The collection of a data set using a conventional rotation technique and a 2D detector takes many weeks. One way to increase the flux at the sample is to increase the wavelength bandwidth. The conventional technique using a standard monochromator like Cu or Be has a typical bandwidth of 1%. A multilayer monochromator can however be tailored to give bandwidth up to 25% or even larger and have an angular acceptance angle of(approximately) 3%. This can be achieved by a multiple spacing multilayer. In this case the diffraction geometry is such that the reflection in diffraction condition is ''scanned'' by the (Delta)(lambda) and not by rotating the reciprocal lattice point through the Ewald sphere. Wavelength bandwidth of 10% satisfy this condition. It is important to tailor the bandwidth to the size needed by the extend of the reciprocal lattice ''point'' to minimize background. The intensity of the reflection is proportional to the (Delta)(lambda)r that just covers the width of the reciprocal ''point'' while the background is proportional to the full wavelength spread (Delta)(lambda) that hits the crystal. The conventional Laue technique is not suitable since it increases the background by the ratio of (Delta)(lambda)(Delta)(lambda)r, a ratio that can easily reach multiples of 10.