A four-circle diffractometer on a focused, tuneable synchrotron radiation source: mechanical design, computer control and evaluation of system performance

An Enraf-Nonius CAD-4 diffractometer has been specially adapted and mounted on a computer-automated alignment carriage for use with the synchrotron radiation X-ray source at SSRL. A separated-function focusing monochromator system is used [Hastings, Kincaid & Eisenberger (1978). Nucl. Instrum. Methods, 152, 167–171] to provide radiation which is rapidly tuneable from 3–9 keV. Software for automatic alignment of the goniometer for wavelength calibration and for monitoring the decay of the stored beam have been developed. Integrated reflection intensities were 40 times higher than for the same reflections recorded using a fine-focus X-ray tube (Ni-filtered Cu Kα, 40 keV, 30 mA), for SPEAR conditions of 3.7 GeV and 20 mA with colliding beams. For crystals of low mosaic spread, the reflection widths were around 0.015°. For such narrow reflections, integrated intensities can be measured to 2.7% accuracy. For wider reflections, 2.1% accuracy has been obtained. A comparison of data collected with synchrotron radiation and a sealed-tube source gave an R-factor based on F of 1.8%. Thus, accurate structure factors are easily obtained from the synchrotron radiation data. The use of a highly mosaic monochromator crystal resulted in intensity gains at the expense of wider scan widths. The apparatus makes possible anomalous scattering experiments close to absorption edges, rapid data collection from weakly diffracting samples and possible increases in signal-to-noise ratios from samples of low mosaicity.