Experiences and expectations of a novel X-ray microsource with focusing mirror. I.

Data are presented from a novel microfocus X-ray generator installed with a choice of ellipsoidal specularly reflecting mirrors. Diffraction data from proteins show the useful flux from this low-power device to be approaching equivalence with that from many far more powerful generators. Intensity measurements show that for small crystals the brilliance is now restricted by the performance of the mirror, which appears to be limited by imperfections in the figure of its surface rather than by a low reflectivity. Suitable choices of ellipsoidal mirror enable the size and divergence of the X-ray beam to be altered readily to match the different requirements of successive samples and appropriate designs are proposed. Alternative types of mirror are expected to be advantageous, especially for the smallest crystals. For crystals of sizes 300 microm or less, which need a small well collimated beam with low divergence, the output from this X-ray tube running at 24 W provides a usable flux similar to that available from rotating-anode generators. The relative performance of this tube and mirror combination becomes increasingly advantageous with the study of ever-smaller crystals.