Examination of poly-capillary lenses for their use in micro-XRF spectrometers

The availability of poly-capillary lenses opens up a wide variety of possibilities for the analysis of small sample areas because a large part of the tube radiation can captured and focused on the sample. Propagation in the glass capillaries is achieved by external total reflection on their internal surface. At first the intensity gain of the lenses and their spot size were the most interesting parameters. However, by using these lenses in commercial instruments, other parameters became more important for characterizing their performance for spectroscopic use, e.g. the energy dependence of the spot size and of the transmission efficiency and their influence to the tube spectrum. In this paper, the examination of various lenses from different manufacturers is described. It is shown that for the main parameters spot size and intensity gain, the differences are negligible. However, other parameters can be influenced by the change in geometric parameters of the lens such as the diameter of the single capillaries and bending radius of the lenses. These changes need high perfection in lens manufacturing. It seems that at present only one of the manufacturers is able to control the process in this way. However, there is also another requirement for the implementation of lenses for commercial instruments, namely the uniformity of the outer dimensions. Only in this case can these components be used without special adaptation in a routine instrument. This also requires the complete Control of the manufacturing process. Because the use of poly-capillary lenses is in its early stages, it seems necessary to define more exactly the parameters that characterize the x-ray optical components. Only in this case will it be possible to define the performance of the optics with high precision and compare lenses from different manufacturers. Copyright (C) 2003 John Wiley Sons, Ltd.