Fig. 8
Evolution of collimator and beamstop designs. a Collimator and beamstop design typically used at protein crystallography beamlines at second generation synchrotron sources. A fraction of a relatively large X-ray beam is selected by using a 0.5 mm inner diameter collimator tube extending 5 mm close to the sample. After interaction with the sample, the direct beam is blocked with a 2 mm outer diameter metal cylinder with a blind hole placed about 30 mm behind the sample. The total path of the direct beam in air is about 35 mm. b Beamstop design currently used at most third generation synchrotron protein microfocus crystallography beamlines where much smaller samples can be investigated. Owing to the significantly smaller X-ray beam waist, the outer diameter of the beamstop can be one millimeter or less. Placed at a distance of again 30 mm low q-reflections can be now recorded up to a half opening angle of 16.7 mrad. The total path of the X-ray beam in air is again 35 mm. c Low background collimator and beamstop concept used for the present work. The path of the direct beam in air is reduced to 8 mm. X-ray photons scattered by air within the capillary are absorbed by the capillary walls. Compared to previous beamstop concepts a, b the path of the direct X-ray beam in air is shortened to 8 mm—reducing the background scattering level by a factor of about 4. At the end of the capillary the direct beam is blocked with a solid absorber. d Future beamstop concept: already 4 mm behind the sample the direct beam is fully enclosed by a 0.25 mm outer and 0.15 mm inner diameter metal capillary. The direct beam can be either blocked with a solid absorber or the tube can be extended through a hole in the detector and then blocked behind (favorable for XFEL applications). For data collection at cryogenic temperatures, collimator and post-sample beam-capillary would require heating, to prevent ice deposition caused by the cold-stream
