Data Acquisition on a Synchrotron Powder Diffractometer

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Angle-Dispersive Mode

Data Acquisition

Current angle-dispersive laboratory powder data acquisition typically involves uses point by point step scanning. By contrast, the wheel has turned full circle with the latest state-of-the-art synchrotron powder diffractometers. Instruments such as ID31 at the ESRF, Grenoble, have such a high incidence flux from an undulator source that a step scanning approach would be to slow. Consequently the diffractometer is rotated at constant speed (say 1°/min) and the position and accumulated counts on the detectors are read out frequently (say at 1 ms intevals). Afterwards, the data is re-binned into 2θ channels with constant Δ2θ intervals so that it looks like step-scanned data. Some typical high-resolution silcon data may be seen here.

Choice of Wavelength

Angle-dispersive powder diffraction experiments at a synchrotron are similar to those in the laboratory, but with one big difference: the wavelength is an experimental variable. In virtually all cases, one should not choose a synchrotron wavelength equal to that of the standard copper X-ray tube used in the laboratory. This does not take full advantage of the range of wavelengths produced by the synchrotron. Certainly for the case of capillary samples, one should aim to use much shorter wavelengths than 1.54 Å so as to reduce absorption, since although the peaks will be squashed into a much small 2θ range, this is more than compensated for by the higher resolution of synchrotron diffractometers compared to the laboratory instruments.

Another point to consider is the possible existence of X-ray absorption edges in the sample. Absorption edges have been discussed in terms of their rôle as X-ray filters (see previous section); a table of values for absorption edges of the chemical elements is provided in the International Tables for X-ray Crystallography, Volume C. A few selected values for comparison are provided in the table below:

The values shown in red above correspond to K absorption edges, in contrast to those shown in mauve, which are for the L edge. For most routine powder diffraction studies, it is strongly advisable to choose a wavelength that is either slightly above or well below any of the absorption edges of elements in the sample. In special cases, it may be necessary to choose a wavelength that is close to the absorption edge of a particular element in the sample so as to exploit the so-called anomalous scattering effect. This effect will be discussed in a later section of the course material.

Energy-Dispersive Mode

Detector Energy Calibration and Choice of Detector Angle

The first step in any energy-dispersive diffraction (EDD) experiment is to calibrate the detector for its energy response. This is best achieved using a γ-radiation source, together with some X-ray K lines induced by fluorescence. A detailed description is provided. Once the detector has been calibrated for energy, then it is necessary to calibrate for angle. In practice it may be necessary to adjust the detector angle so that a suitable range of d spacings is covered by the detector. Again, a detailed description is provided.

The experimental objectives in a energy-dispersive diffraction (EDD) experiment are usually quite different from those in an angle-dispersive one. For example, EDD is often used for real-time in-situ experiments (which will be discussed later in the course material), while angle-dispersive experiments at a synchrotron are carried out for the determination of crystal structures (which also will be discussed later in the course material). A direct comparison between data sets obtained on a Bragg-Brentano laboratory diffractometer and on an energy-dispersive diffractometer at ESRF, Grenoble, is provided for the reader. While this comparison is useful for comparing instrument resolution, it fails to compare experimental acquisition times. Typically, EDD data sets are collected in anything from a few seconds to many minutes depending on the signal to noise ratio of the detector signal; this will be best with well crystallised samples which are not too absorbent. In certain respects, energy-dispersive diffractometers are analogous to angle-dispersive diffractometers employing large PSDs since in both cases the emphasis is on the collection of the whole diffraction pattern with a short acquisition time.

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