Monocapillary optics explained

Total-external-reflection mirror optics have been used with X-rays since Roentgen, who also noted their ability to reject higher-energy X-rays as a function of incident grazing angle, providing a rudimentary form of monochromator before the discovery X-ray diffraction by crystals. Full-complement toroidal mirrors were later used with many types of X-ray source, including the first rotating-anode developed by Siemens in the late 1940s. Toroidal mirror technology for lab sources reached its zenith in the 1970s with toroidal gold mirrors of several mm bore-diameter, capable of resolving small molecules in diffraction crystallography. Large paraboloidal toroids are now used in space-based X-ray telescopes. Part-complement or section-toroidal mirrors are commonly used in synchrotron beamlines.

Monocapillaries using total-external-reflection were first suggested in the 1960s as a potentially simple microfocussing element for X-rays. Unlike toroidal mirrors, these were envisaged as "light funnel" or "beam-condensing" optics in which X-rays are repeatedly reflected within a tapering-bore to emerge in an intensified beam having the diameter of the exit aperture of the monocapillary. However, limited ability to control the taper profile of early monocapillaries limited their usefulness to microbeam production using synchrotron sources.

Multiple-reflection monocapillary "funnel" optics suffer from several disadvantages:

  • The narrowest beam spot produced is right at the exit aperture of the optic.

  • The random multiple-reflection ray paths produce complex angular intensity distributions which may not be acceptable for some analysis methods.

  • The full angular divergence in the output beam is at least twice the critical angle for reflection at the X-ray energy used.

  • Unconstrained multiple-reflection designs are extremely inefficient for divergent X-ray sources such as electron-bombardment types.

  • Despite these limitations low-cost funnel monocapillaries have found wide use where very small beam sizes are required and beam structure is unimportant, as traditionally the only alternative has been the use of beam-collimating apertures in these cases.

    In the 1990s the development by AXCO of controlled-taper monocapillaries and accurate codes for optimal monocapillary design permitted the convergence of toroidal mirror and monocapillary technology to produce single-reflection "focussing" monocapillary optics. Traditionally these can be classified as either ellipsoidal (for point-to-point focussing or confocal geometry) or paraboloidal (for point-source to parallel beam or parallel beam to point-focus). In practice, finite source size or incident beam divergence lead to optimal taper profiles which depart from these classical Cartesian geometries for maximum flux concentration. The major advantages produced by the adoption of a single reflection are:

  • High flux transmission efficiency - up to 100% has been achieved.

  • Focussing at fixed macroscopic distances from the optic.

  • Designed use of the taper profile to aid monochromation via critical-angle rejection of unwanted X-rays.

  • Beam convergence/divergence constrained by taper design to specification.

  • Simple output beam angular intensity distribution, suitable for diffraction analysis.

  • Efficient flux collection from all types of X-ray source.

  • The state-of-the-art in monocapillary optics now permits the production of beams of specified intensity and divergence/convergence distributions at fixed positions using available X-ray sources optimally at close to theoretical maximum efficiency.

    Other popular types of X-ray focussing optics developed in the last few decades include multilayer mirrors and polycapillaries. The latter find ready use where large focal convergence angles, up to several degrees, are acceptable and beam structure is unimportant. Polycapillaries cannot however provide the maximum possible flux within a beam of narrow angular divergence limits suitable for diffractometry. Multilayer optics are individually flat, requiring pairs of optics to achieve focussing in two dimensions, with two successive reflections. Together with the requirement of long focal lengths producing considerable aberration sensitivity, the observed efficiency of multilayers remains lower in equivalent application comparisons to focussing monocapillaries.

    (c) copyright 2004 AXCO Pty. Ltd.