Bonse-Hart camera (USAXS)

The lower q-range of the pinhole SAXS camera in EH2 extends well into the USAXS region. However, a Bonse-Hart camera has several advantages and extends the low q limit to 8 x 10^-4 nm^-1. The setup consists of two Si-220 triple-bounce 'channel-cut' crystals as conditioning monochromator and horizontal analyzer, and a Si-111 double reflection vertical analyzer. The second analyzer is maintained at the Bragg angle and the crossed-analyzer configuration provides the scattering profiles without smearing effects (point geometry). The detector is a high dynamic range (~5 x 10⁷ cps) avalanche photo diode used in photon counting mode.

(a) Schematic scattering pattern: first and second analyzer crystals cut
out orthogonal slices from the two-dimensional scattering profile.
(b) and (c) Rocking curves recorded with the first (horizontal)
and second (vertical) analyzer crystal, respectively.

The camera is optimized to an operating energy of 12.4 keV, however provision is made to work up to 16 keV. Typically, the rocking curve spans more than 11 orders of magnitude in intensity. Pneumatically controlled attenuators are used to automatically optimize the counts on the detector during a scan. From the measured empty rocking curve widths of the horizontal and the vertical analyzer crystals, the scattered intensity can be directly converted to an absolute scale without any scaling factor or secondary standards.
The analyzer crystal can be scanned in linear or logarithmic steps or continuously. Because of the high flux, low beam divergence, and high dynamic range detection, fast scan over the entire USAXS region can be performed in ~ 10s, permitting limited time-resolved USAXS measurements. Note that dynamics is slower at small wave vectors and this time-resolution is adequate in many cases to study kinetics at these large length scales.

The switching between the Bonse-Hart configuration and the SAXS/WAXS setup in EH2 can be done within a span of few minutes. Therefore, both cameras can be used in a complementary manner.

 
Rocking curve recorded without sample together with the scattering
of a suspension of spherical silica particles (R ~ 300 nm).

Optimization of the Bonse-Hart instrument by suppressing surface parasitic scattering:
see: M. Sztucki, J. Gorini, J.-P. Vassalli, L. Goirand, P. van Vaerenbergh and T. Narayanan, J. Synch. Rad. 15 (2008).

A significant improvement of the Bonse-Hart USAXS setup was achieved by reducing the parasitic scattering background in the wings of the rocking curve by more than an order of magnitude.
Defects at the diffracting surface of the conditioning crystals were found to be at the origin of the observed diffuse scattering. In order to improve the performance of the instrument, the monolithic channel-cut crystals were replaced by pairs of separate polished and deeply etched crystals. These crystals are mounted on a mechanical stage to allow very precise parallel alignment of the crystals to within a tiny fraction of the rocking curve width (sub-μrad range).

Significant improvement of the signal-to-background ratio and the available wavevector range of the instrument make it suitable for studying the microstructure and dynamics of dilute and weakly scattering soft-matter systems.

         
Reduction of parasitic scattering background by more than an order of magnitude 
using double-crystal setups of highly polished and deeply etched single crystals.

Further Information

• Back to the ID2 start page
• Back to the Beamline layout (overview)
• Overview of detectors for SAXS/WAXS and USAXS
• Sample environments
• Procedures for data acquisition and online reduction
• Scientific and industrial applications
• Recent publications at ID2