The CALIBRANT Command

The CALIBRANT button allows the fitting of the powder pattern from a standard calibrant sample. The known D-spacings of the diffracting planes allow the beam centre and tilt to be defined with greater accuracy than with an unknown sample, and if reasonably high angle data is present allow the sample to detector distance, and the wavelength to be refined independently. This has been verified to agree to within the accuracy that may be obtained from scanning an absorption edge using a Ge detector on ID-30 at the ESRF.

Clearly for best results a very high quality powder sample should be used, and a well exposed image should be obtained.

First the SELECT CALIBRATION SAMPLE choice menu appears. This is shown in Figure 38.

Figure 38: The SELECT CALIBRATION SAMPLE Choice Menu

The following commands and choice of calibrants is available, and includes the commonly used samples from the American National Institute of Standards and Technology:

CANCEL: Cancel CALIBRANT command and return to POWDER DIFFRACTION interface menu.

ALUMINA (Al2O3: Alumina powder sample

CERIUM DIOXIDE: Ceria powder sample e.g. NIST standard

?: Outputs list of choices and short explanation of each choice.

LANTHANUM HEXABORIDE: $LaB_6$ powder sample e.g. NIST standard

PARAFFIN WAX: Macromolecular crystallographers ``standard'' calibration wax. PARAFFIN WAX is included since many crystallographers use this for distance calibration. However don't expect high accuracy, and if only low angle data is available don't try to refine both wavelength and distance together.

HELP: Outputs help text explaining in greater detail the command and available choices.

SODIUM CHLORIDE: NaCl Common salt (beware of moisture)

SILICON: Silicon powder sample e.g. NIST standard

USER DEFINED: Allows an arbitary series of D-spacings to be input from a text file. The file must contain only one D-spacing value in Angstroms per line. This allows for calibration samples which are not in the available choices, but also allows non-standard D-spacings to be used e.g. for samples at non-standard temperatures, or allows certain D-spacings to be delibrately missed, owing to problems of contamination etc. Mixed samples may also be treated this way.

After the appropriate calibrant sample has been selected, the CALIBRANT PATTERN REFINEMENT control form appears. This is shown in Figure 39.

Figure 39: The CALIBRANT PATTERN REFINEMENT Control Form

The following "buttons" are available:

DISTANCE: The sample to detector distance in millimetres.

WAVELENGTH: The radiation wavelength in Ångstroms.

X-PIXEL SIZE: The horizontal (as displayed) size of detector pixels in microns.

Y-PIXEL SIZE: The vertical (as displayed) size of detector pixels in microns.

ANGULAR SECTIONS: This the number of angular sections around 360 degrees of the powder rings which are used to calculate the centre of the rings. The beam centre and tilt are fitted to these positions on each ring. If the data is very noisy or shows poor powder averaging then a smaller number may be better. (No theoretical criterion exists to set an optimum value, so trail and error is recommended. Clearly, this value should not be too small.)

REJECT OUTLIERS: This option allows badly fitting ``ring'' positions to be rejected, and the data re-fitted. This is to make the fit procedure more robust and to allow for erroneous positions owing to contaminating Bragg peaks, etc.

REJECT LIMIT: If REJECT OUTLIERS is YES then this is the number of standard deviations away from the best fit predicted position after which ``outliers'' are rejected.

FULL INFO: YES for terminal screen output of information relating every stage of the fitting of the powder rings.

REFINE BEAM X/Y: YES to fit the beam centre position, as well as the tilt angles. NO to refine only the tilt angles (if variable). This is usually used when the beam centre has been determined from a direct beam mark.

REFINE DISTANCE: YES to refine the sample to detector distance, from the angles of the calibrant powder rings.

REFINE WAVELENGTH: YES to refine the radiation wavelength from the angles of the calibrant powder rings.

REFINE TILT: YES to fit the detector non-orthogonality to the beam (the tilt).

EXTRA ITERATIONS: Sometimes the initial values are not good enough to find higher angle rings. If YES an extra step is introduced which finds intermediate angle rings and refines using them, prior to trying to use all available rings. This should normally not be necessary.

As can be seen it is possible to refine, or not, all the different parameters. It is possible to refine all parameters together, but normally only the sample to detector distance or the wavelength should be refined. However, with high quality, high angle data is possible to refine both together, and get results as good as those obtained by scanning edges.

Set the required parameters and click O.K. to continue.

The user is prompted to define a ring of the calibrant pattern, by clicking on the ring 3 or more times. Normally the inner ring is requested. This defines the inner ring, and an initial beam centre.

This should be last user intervention, provided the initialisation is not too inaccurate. If the position of the ring does not correspond within 10% of the given distance and wavelength, then a WARNING message will be output and the operation will terminate. In such a case check the initial values and change as required.

When the initialisation is good enough, the routine will successively search for more ring positions and refine the refinable parameters as requested. The coordinates use and the rings predicted are drawn on the image as the command proceeds.

(See Section 11.2.1, Page  for the definition of the tilt angles.)