The optimum conditions to collect X-ray data from very small samples.

A previous paper [Nave & Hill (2005). J. Synchrotron Rad. 12, 299-303] examined the possibility of reduced radiation damage for small crystals (10 microm and below in size) under conditions where the photoelectrons could escape from the sample. The conclusion of this paper was that higher-energy radiation (e.g. 40 keV) could offer an advantage as the photoelectron path length was greater and less energy would be deposited in the crystal. This paper refines these calculations further by including the effects of energy deposited owing to Compton scattering and the energy difference between the incident photon and the emitted photoelectron. An estimate is given for the optimum wavelength for collecting data from a protein crystal of a given size and composition. Another way of reducing radiation damage from a protein crystal is to collect data with a very short pulsed X-ray source where a single image can be obtained before subsequent radiation damage occurs. A comparison of this approach compared with the use of shorter wavelengths is made.