NOTE: Beamline is in the commissioning phase and will be open to users during 2012
XALOC (BL13) is the only macromolecular crystallography beamline included in the initial beamline portfolio of ALBA, a state-of-the-art 3-GeV storage ring built in Barcelona. The photon source is an in-vacuum undulator optimised to deliver the highest flux at the Se K edge while keeping full tunability in the 5-21 keV range. The optics consist of a crystal monochromator and a pair of focusing mirrors. The End Station includes:
- A high-accuracy single axis diffractometer (MD2M, EMBL-ESRF design) complemented by a removable mini-kappa mount.
- An automated mounting robot that can work with both cryogenic samples and crystallisation plates (Irelec CATS).
- A photon-counting Pilatus 6M detector with a large sensitive area (431×448 mm2), a fast framing rate, a large dynamic range (20 bits, >106), and negligible dark current noise. The detector can be placed at any distance between 80 mm to 1300 mm from the sample.
The optical design foresees three main operation modes:
Focused mode: The mirrors focus the beam to 50×7 μm2 FWHM (H×V) on small crystals, while keeping a small vertical divergence (0.1 mrad).
Unfocused mode: one or both mirrors are removed from the beam path. Beam size increases to 300 μm in the unfocused dimension, but reduces the beam divergence down to 0.03 mrad vertically or 0.1 mrad horizontally. This mode can be useful for medium-sized crystals with large unit cell parameters.
Defocused mode: The mirrors allow variable focusing to match the size of the X-ray beam to the dimensions of the crystals or to focus at the detector. In this case, the beam size at the sample position can range from 50×7 μm2 to 300×300 μm2 (H×V). To avoid X-ray beam deformations caused by the optics, slope errors of the mounted mirrors have been reduced to 70 nrad rms.
To fulfill the needs of standard multiple wavelength anomalous diffraction experiments, the beamline will deliver over 3×1012 ph/s in the 5-21 keV energy range, which covers all the common K and L3 absorption edges, and an energy resolution of ΔE/E ~2×10-4. Finally, a major design objective has been to optimise the beam stability at the sample position to improve successful data collection.