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XRD1 Beamline Description

Overview

The X-Ray Diffraction 1 (XRD1) beamline has been designed (in collaboration with the Istituto di Cristallografia - CNR) primarily for macromolecular crystallography, but the characteristics of the beamline permit to perform a wide variety of experiments: the beamline hosts small molecules, protein crystallography, powder diffraction, high pressure physics and solid-state experiments.


Insertion Device

The insertion device of the XRD1 beamline is shared and used simultaneously with the SAXS beamline. The insertion device is a hybrid multipole wiggler, composed of three sections for a total length of 4.5 m, with a fixed working gap of 22 mm and B=1.6T. The white beam resulting from the source is cut to 4 keV with a series of cooled graphite layers and is then monochromatized.

Beamline Layout



XRD1 has the traditional layout used in case of important horizontal X-ray beam divergence,with a Collimator Cylindrical Mirror (CCM) faced up, a Double Crystal Monochromator (DCM) dispersing in the vertical plane and a bendable Focusing Toroidal Mirror (FTM) faced up. The horizontal acceptance is 1.5 mrad limited by the FTM sagittal radius and the vertical acceptance is 0.18 mrad.Also in this case the vertical acceptance is limited by the lengths of both the mirrors due to the grazing angle of 3.2 mrad.
A mask determines the angular acceptance and stops low-energy radiation from the wiggler. After about 22.3 m from the source the beam is collimated in the vertical direction by means of a platinum coated cylindrical mirror with a radius of curvature of 14.8 km. At 24.5 m from the source, a Si 111 double crystal monochromator permits to select a specific energy. At 28.0 m from the source, a bendable toroidal mirror, with 55 mm and 9.3 km of sagittal and tangential radius, permits to focalize the monochromatic beam with dimensions at the sample of 0.7 x 0.2 mm. Acting on the focusing mirror tilt, a rapid feedback loop using a downstream 4-quadrant beam position monitor keeps the vertical position of the beam fixed in space.


 


Monochromator

A Si(111) double-crystal fixed-exit monochromator, in parallel non-dispersive mode, is used to select a specific energy in the range between 4 and 21 keV, with a resolution of few eV. Crystals are cryogenically cooled by means of a LN2 closed loop system (Bruker). Parallelism of the surfaces of the two crystals is guaranteed by a piezoelectric stage acting on the second crystal: a rapid feedback loop using a downstream 4-quadrant ionization chamber maintains the flux intensity at the maximum. The monochromator is routinely calibrated via the absorption edge from fluorescence signal of reference materials (routinely iron/selenium/molibdenum edges) and the diffraction from NIST standards (usually LaB6).


Last Updated on Monday, 20 July 2020 16:50