ID31 Optics

The beam line has relatively simple optics.

The X-rays are produced by three 11-mm-gap ex-vacuum undulators, (standard ESRF design, each 1.6 m long, one with a magnetic period of 35 mm, two with a period of 32 mm) which cover the entire energy range from 5 keV to 60 keV (2.48 Å. to 0.21 Å in wavelength).

The beam is monochromated by a cryogenically cooled double-crystal monochromator, with a choice of Si 111 crystals (for standard operation) or Si 311 crystals (for higher energy resolution, though this option is rarely used). The first monochromator crystal is side cooled by copper blocks through which liquid nitrogen flows. The second crystal is cooled by thermally conducting braids that link to the first crystal, and has electrical heaters to maintain its temperature stable to -60 ± 0.05ºC. A piezoelectric feedback system compensates for changes in the Bragg angle of the second crystal caused by changes in the heat load on its supporting mechanics. Water-cooled slits define the size of the beam incident on the monochromator, and of the monochromatic beam transmitted to the sample, typically in the range 0.5 – 2.5 mm (horizontal) by 0.1 – 1.5 mm (vertical).

Powder diffraction is a technique that requires a relatively large beam to illuminate a sufficient volume of sample so as to give a good powder average. Thus there is no focussing; the highly collimated beam from the source is simply monochromated and passes unperturbed to the sample.

Experimental hutch

The diffractometer (picture on the left) is of a heavy-duty construction and is mechanically stable, accurate (nominally ±1 arcsec) and precise (sensitive to displacements of 5x10^-5°). It can accept spinning capillary or flat plate specimens. In routine operation, a bank of nine detectors is scanned vertically to measure the diffracted intensity as a function of 2theta. Each detector is preceded by a Si 111 analyser crystal and the detector channels are approximately 2° apart. The nine analyser crystals are mounted on a single rotation stage, so only a single adjustment of the crystals' Bragg angle needs to be made when changing the wavelength. This multianalyser stage [1], figure below, was conceived by the Laboratoire de Cristallographie at the CNRS, Grenoble, and manufactured as part of the collaboration that supported the construction of BM16.

Powder diffractometers at the new national synchrotrons have pushed this concept further, with the diffractometers at APS having 12 detectors; Soleil, 21 detectors; and Diamond, 45 (5 banks of 9). Having multiple crystals operating in parallel increases the efficiency of detecting the diffracted radiation, and for dynamic measurements means that on ID31 the detector arm needs to be scanned by no more than about 2.3º to measure an angular range of 18º in 2theta.

Mechanically and optically the exact offset between channels is not critical. However, to combine the data from different channels, the offsets need to be calibrated accurately. This is done by comparing those parts of the diffraction pattern measured by all of the channels. The offsets and channel efficiencies are computed so that the signals superimpose as closely as possible [2]. A well diffracting sample such as Si standard NIST 640c is usually used for this calibration step.

A diffracted X-ray must arrive on the analyser crystal at precisely the correct angle to be diffracted into the detector. Since the acceptance of a Si 111 crystal is very small (a few arcsec), an analyser crystal stringently defines the 2º angle of diffraction. Since the analyser determines a true angle of diffraction, (rather than inferring the angle from the position of a slit or a channel on a position-sensitive detector), the use of an analyser crystal renders the positions of diffraction peaks immune to aberrations, such as specimen transparency and misalignment of the sample with respect to the axis of the diffractometer, that affect conventional arrangements with a scanning slit or a PSD. Coupled with the excellent mechanical integrity of the diffractometer and the high collimation of the beam, not only are the peaks very narrow, with a nominal instrumental contribution to the FWHM of around 0.003º 2theta, but also their positions are accurate and reproducible to a few tenths of a millidegree. Narrow peaks (see figure on the right: peak from LaB₆ NIST 660a, lambda= 0.8  Å) and accuracy in the peak positions are essential for high quality powder diffraction measurements. Since for flat-plate samples there is no need to be in the theta/2theta condition for narrow peaks, (there being no parafocussing condition to fulfill), the analyser crystals also offer advantages for diffraction at grazing- incidence, and for reflectivity.

For crystallographic studies, the use of a spinning capillary mounted on the axis of the diffractometer greatly reduces preferred-orientation effects for all but needle-like samples that can align with the capillary axis. With the energy range available, the energy and capillary diameter can be selected to minimise absorption even for samples containing heavy elements. Hence the diffracted intensities can also be very accurate. Indeed, even in cases where a highly absorbing sample is unavoidable, such as when working at the absorption edge of an element in the sample, we would usually prefer to stick a fine layer of sample to the outside of a 1-mm-diameter glass capillary with silicone grease, rather than use a flat plate.

 
Measurements of the flux at the sample yield,  for example, 1.5x10¹² photons mm^-2 s^-1 at 0.43 Å wavelength and 6.1 x 10¹² photons mm^-2 s^-1 at 0.85 Å. This high intensity is excellent for a wide range of powder diffraction measurements. The increase in flux density on the sample - by one to two orders of magnitude depending on wavelength - coupled with a decrease in the overall detection efficiency (by using Si 111 analysers instead of Ge 111, and by reducing the width of the axial receiving slits from 10 – 15 mm to 4 mm, to improve resolution and peak shapes), mean that on ID31 radiation damage is a major consideration.

References:

[1] Nine crystal multi-analyser stage for high-resolution powder diffraction between 6 and 40 keV. Hodeau J.-L. Bordet P. Anne M. Prat A. Fitch A. N. Dooryhee E. Vaughan G. Freund A. SPIE Proceedings, 3448, 353–361 (1998)

[2] Merging data from a multi-detector continuous scanning powder diffraction system. Wright J. Vaughan G. Fitch A. IUCr Computing Commission Newsletter, No. 1 (2003) p. 92; on http://www.iucr.org/iucr-top/comm/ccom/newsletters/2003jan/index.htm