Beamline: BW1

X-Ray Reflectivity and Grazing-Incidence-Diffraction from Films at Liquid Surfaces

The BW1 liquid surface diffractometer with (right to left) white beam entering He-filled box with Be monochromator, monochromatic beam striking the liquid surface at a shallow vertical angle, PSD for horizontally diffracted beam and NaI detector for specularly reflected beam.

The main scattering plane of the monochromator - and of the rest of the diffractometer - is horizontal, allowing for heavy sample environments (cryostats, etc.) and in particular for studying surfaces of liquids. By tilting the monochromator, the beam can be deflected downwards to an angle αi ≤ 10° onto the horizontal liquid surface.

The instrument comprises some twenty stepper-motorized movements. For the monochromator, horizontal and vertical translations, horizontal and vertical aperture, crystal rotation, tilt and 2θM. One meter from the monochromator, the sample stage has horizontal θand vertical translations, rotation, and a goniometer with two crossed arcs and two translations. Two independent 2θ-movements (2_U and 2θ_L) allow measurement of X-rays specularly reflected from the liquid surface and, simultaneously, of X-rays laterally diffracted by the two-dimensional 'crystals' in a monolayer at the liquid surface. The (horizontal) 2θ_M, 2θ_U and 2θ_L arms are X-95 profiles. Optical benches (PI system) before and after the sample carry slits and detectors and can be made to follow the incident and exit beams in the vertical plane (αi, αf; two movements for each).

The instrument is controlled by Risø´s TASCOM program , running on the BW1 Linux PC. A live display of motor positions (in millimeters or degrees) is available, as well as a device allowing for manual movement of motors by means of a joy stick.

Applications

Grazing incidence diffraction and specular reflectivity from Langmuir monolayers on liquid surfaces.
Heavy sample environments

Literature on Instrumentation

J. Als-Nielsen, "X-ray reflectivity studies of surfaces", pp. 471-504 in: Handbook on Synchrotron Radiation, Vol. 3, eds. G. Brown and D.E. Moncton, North Holland, Amsterdam 1991

Instrument Specification


Source (4.5 GeV)	127-pole undulator with a variable magnetic gap (B_max = 0.46 T with 15.6 mm gap, 3rd harmonic at 9.1 keV)
Mirrors (usual optional)	1st cylindrical, Au-coated, 2nd flat, Au coated with mirror bender (glancing angles 6.5 mrad, cut-off energy = 11 kev)
Monochromator	One Beryllium crystal (0.25 mm thick; 0.2 mrad mosaic spread) reflecting from the (002) planes in Laue (transmission) geometry (Alternatively a diamond Laue crystal may be used.)
	The Beryllium crystal is water-cooled at the edges and also by Helium gas. Water-cooled variable-size apertures immediately in front of the crystal serve to control the beam size.

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