Calibration Procedures

Summary of Calibration Procedures

In order to share good practice and information about different procedures the following page provides summaries as to how small-angle scattering instruments are calibrated as regards wavelength, intensity, momentum transfer and other quantities, We encourage people to fill in details and make comments on this page. In order to simplify finding information, we encourage everyone to use similiar sub-headings where appropriate.

This project to gather information is new and we hope to have some information soon.

Quokka - Bragg Institute, ANSTO

40 m Small-angle neutron scattering instrument

ESRF ID02 Time-resolved SAXS/WAXS/USAXS/ASAXS

Monochromatic X-ray pinhole camera with 10 m vacuum flight tube (distances 0.8 m to 10 m). Setup optimized for 12.6 keV.

see also http://www.esrf.fr/UsersAndScience/Experiments/SoftMatter/ID02/BeamlineDescription for more details.

(draft version)

Wavelength

Global calibration of the monochromator (over the whole energy range, the actually available monochromatic energy range is restricted to energies below 20 keV due to the mirror) at different absorption edges and least square refinement of dBragg and thetaOffset over all scanned edges: Fe K-edge, Cu K-edge, Au LIII-edge, Zr K-edge, Mo K-edge, Rh K-edge (error DeltaE/E < 10^-4).

Absolute Intensity

We are using the small angle scattering of water to determine a normalization factor that adjusts the online corrected intensity. This calibration factor supersedes any other calibration factor, e.g. the intensity calibration of the beam intensity monitors and the detector efficiency.

Detector Response/Flat Field

The detector uniformity is determined with fluorescence radiation. The detector is placed in 1 m - 2 m distance from the fluorescent sample. The remaining non-uniformity of the field (<0.25% for a 10 cm x 10 cm active area) is corrected by normalizing the intensity pattern to the spherical angle (software). For calibrations around 12 keV we use the K-alpha radiation of bromine at 11.9 keV (solution of HBr in water filled into a 3 mm thick glass capillary). The average value of the resulting flat-field pattern is normalized to unity (for convenience).

Pixel Size

The pixel size is measured with a calibration grid in front of the detector. The grid is also used to determine and to correct image distortions, e.g. when using fiber optically coupled CCD detectors.

Sample to Detector Distance

SAXS

The movement of the detector is encoded. To calibrate the distance of a reference position from the detector a sample is placed there (e.g. silverbehenate or any other sample showing well defined rings, the d-spacing is not needed here). Then a series of scattering patterns is taken for different detector positions and the rings are extrapolated to zero diameter (accuracy <10^-3) which gives the position of the reference sample. The distance of any sample from the detector is measured relative to the reference position with a ruler (typically 1 mm accuracy). The center is determined with a scattering sample that is permanently present in the setup and that can be inserted into the beam path.

WAXS

PBBA (para-brome benzoic acid) is used as scatterer at the sample position (e.g. in a capillary or as powder on a film). Calibration of the beam center, distance and detector inclinations is done by a least square refinement. This refinement can also be used for SAXS.

Center

The beam center and/or point of normal incidence is determined with a reference sample showing a circular scattering pattern (see above).

Comments about advantages/problems

Usually, the standard calibration is sufficient. If needed, additional fine adjustments can be done with reference samples.