Will reduced radiation damage occur with very small crystals?

The primary event which occurs when an X-ray photon of energy less than 30 keV is absorbed in a protein crystal (or other organic material) is the production of a photoelectron with a similar energy to that of the absorbed photon. The electron then scatters inelastically off the surrounding material losing energy in the process. This reduction in energy takes place over track lengths of a few mm for 20 keV electrons. The vector distances between the initial and final positions of the photoelectrons are less than the track lengths owing to the non-linear tracks followed by the electrons. For crystals with smaller dimensions than the vector distances, a significant proportion of the energy could leave the crystal with the high-energy electrons. This could provide an advantage in terms of reduced radiation damage. In order to estimate the possible benefits, calculations of the electron tracks are given, initially using the continuous slowing-down approximation. A Monte Carlo approach is then used to provide more accurate values of the vector distance travelled by electrons inside a protein crystal. The calculations indicate that significant reductions in radiation damage could occur for crystals of a few mm in size. The benefits would be greater when operating at higher energies. In addition, a scheme for realising the possible benefits in a practical situation is described. This could then form the basis of trial experiments.