D2AM, a Beamline with a High-Intensity Point-Focusing Fixed-Exit Monochromator for Multiwavelength Anomalous Diffraction Experiments

D2AM is a french CRG beamline installed at the ESRF (European Synchrotron Radiation Facility) in Grenoble, with half of the time dedicated to biological macromolecule crystallography and half to materials science studies (diffraction, wide-angle and small-angle scattering). It is constructed at the front-end BM02 of the ESRF storage ring, using the X-ray beam from a 0.8 T bending magnet. D2AM entered into routine operation at the end of 1994, and is used either for single-wavelength or for multiwavelength anomalous diffraction studies. The beam is monochromated by an Si[111] two-crystal monochromator with a resolution of about 2 × 10⁻⁴. The first crystal is water cooled. The X-ray photon energy covers the range between 6.5 keV (λ ≃ 1.9 Å) and 17 keV (λ ≃ 0.7 Å), a domain of energy with many K or L absorption edges of heavy atoms of interest for biological macromolecules studies and in materials science. The X-ray beam is focused in the vertical plane by two long curved mirrors and in the horizontal plane by the second crystal of the monochromator which is given an adjustable sagittal curvature. A spot size of 0.3 × 0.1 mm (FWHM) is measured at the sample position. Both mirrors are cut out of a 6"-diameter 1.1 m-long Si single crystal, polished and coated with a 400 Å Pt thin film. The rugosity is better than 4 Å r.m.s. and the longitudinal slope error is better than 5 × 10⁻⁶ rad r.m.s. The first mirror is water cooled, the second is not. The beam intensity on the sample is about 10¹¹ photon s⁻¹ on a 0.3 × 0.3 mm focus area at 100 mA in the storage ring of the ESRF. The harmonic rejection ratio obtained with the two mirrors is better than 10⁻⁵ for λ/3. The combined optical system, mirror/monochromating-crystals/mirror, used on D2AM constitutes altogether a high-intensity point-focusing fixed-exit monochromator, which has the additional property that the energy resolution is not dependent on the beam divergence in use. Its stability and resolution are perfectly adapted to multiwavelength anomalous diffraction studies. The alignment of the mirrors and the monochromator is fully automated, taking 5 min, with the exception of the adjustment of the sagittal focusing. During multiwavelength diffraction experiments the wavelength is changed by a fast single monochromator rotation. Neither realignment of the mirrors nor readjustment of the beam focusing are necessary. The stability and reproducibility of the selected X-ray photon energy is better than 0.5 eV.