Richard A paper from some time back which describes this is Increased resolution data from a large unit cell crystal collected at a third-generation synchrotron X-ray source Authors: W R Wikoff, W Schildkamp, J E Johnson Acta crystallographica. Section D, 56(Pt 7):890-3.
By focusing the bending magnet beam (small source size) on the detector they had two advantages 1. Spreading out the beam on the largish crystal meant they got more images per crystal. 2. Sharper spots on the detector meant they had a better signal to background ratio and improved resolution. Focusing on the crystal is obviously a good idea if the crystal is small or one wants to pick out the best small parts of a larger crystal. There are also more subtle effects being discussed (e.g. Stern et. al. Acta Cryst (2009). D65, 366-374). There might also be advantages in using a highly parallel (coherent) beam on a large crystal but this is even more speculative. Colin -----Original Message----- From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Richard Gillilan Sent: 19 June 2009 18:37 To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] sharp beam focus and radiation damage? I know that sometimes people like to defocus the x-ray beam at the sample so that the intensity profile is not sharply peaked. I think the rationale is that the sharp peak will cause damage, but contribute few photons to the overall diffraction pattern. Does anyone know of a reference where this phenomenon has actually been tested (or even just used)? Richard -- Scanned by iCritical.
